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Despite widespread occurrence of seasonal sociality among animals, little is still known about the social drivers and population-level social implications of seasonal grouping behaviours, especially in birds. Here, we studied the combined effects of ecological and social factors on seasonal grouping patterns in a sedentary population of Egyptian vultures living on the Eastern Canary Islands. We focussed on the social significance of large-scale gatherings taking place outside the breeding season at a highly preferred feeding station and a nearby temporary roost. Group sizes at this feeding patch followed a strong seasonal pattern characterized by distinct monthly changes in group composition, according to sex, age and territorial status. In between reproductive periods, vulture numbers at the feeding station may reach up 50% of the total population on a single day. GPS-tracking showed that this increase in vulture numbers was in part due to a shift in foraging range towards the centre of Fuerteventura by low-ranked territorial birds breeding in remote areas. During this period, vultures may spend on average 30% of their monthly time in a social gatherings context, depending on social status. We show that seasonal grouping patterns are shaped by the complex interplay between ecological factors (reproductive constraints, resource seasonality, food predictability), age-specific traits and social competitive processes, while social attraction may be an important additional component. We propose that for facultative social foragers living in highly despotic territorial systems, collective foraging may be of particular relevance regarding the development of hierarchical social relations and maintenance of population-level social cohesion. Significance statement We show that seasonal grouping patterns in a sedentary island population of a territorial vulture are shaped by the complex interplay between ecological factors (reproductive constraints, resource seasonality, food predictability), age-specific traits and social competitive processes. We argue that vultures visit large gatherings also for social purposes. Group foraging events may be of special interest to vultures, given opportunities for rank maintenance/improvement, but also for conspecific evaluation and mate-seeking, and play an important role in the social structuring of populations. Vultures may serve as important model species to test the role of scavenging and fission-fusion social dynamics in driving the evolution of avian social cognition, or avian social complexity in general.
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Seasonal grouping dynamics in a territorial vulture: ecological drivers
and social consequences
Thijs van Overveld
&Laura Gangoso
&Marina García-Alfonso
&Willem Bouten
&Manuel de la Riva
José Antonio Donázar
Received: 14 August 2019 /Revised: 9 January 2020 /A ccepted: 17 January 2020
#Springer-Verlag GmbH Germany, part of Springer Nature 2020
Despite widespread occurrence of seasonal sociality among animals, little is still known about the social drivers and population-
level social implications of seasonal grouping behaviours, especially in birds. Here, we studied the combined effects of ecological
and social factors on seasonal grouping patterns in a sedentary population of Egyptian vultures living on the Eastern Canary
Islands. We focussed on the social significance of large-scale gatherings taking place outside the breeding season at a highly
preferred feeding station and a nearby temporary roost. Group sizes at this feeding patch followed a strong seasonal pattern
characterized by distinct monthly changes in group composition, according to sex, age and territorial status. In between repro-
ductive periods, vulture numbers at the feeding station may reach up 50% of the total population on a single day. GPS-tracking
showed that this increase in vulture numbers was in part due to a shift in foraging range towards the centre of Fuerteventura by
low-ranked territorial birds breeding in remote areas. During this period, vultures may spend on average 30% of their monthly
time in a social gatherings context, depending on social status. We show that seasonal grouping patterns are shaped by the
complex interplay between ecological factors (reproductive constraints, resource seasonality, food predictability), age-specific
traits and social competitive processes, while social attraction may be an important additional component. We propose that for
facultative social foragers living in highly despotic territorial systems, collective foraging may be of particular relevance regard-
ing the development of hierarchical social relations and maintenance of population-level social cohesion.
Significance statement
We show that seasonal grouping patterns in a sedentary island population of a territorial vulture are shaped by the complex interplay
between ecological factors (reproductive constraints, resource seasonality, food predictability), age-specific traits and social compet-
itive processes. We argue that vultures visit large gatherings also for social purposes. Group foraging events may be of special interest
to vultures, given opportunities for rank maintenance/improvement, but also for conspecific evaluation and mate-seeking, and play an
important role in the social structuring of populations. Vultures may serve as important model species to test the role of scavenging
and fissionfusion social dynamics in driving the evolution of avian social cognition, or avian social complexity in general.
Keywords Egyptian vultures .Feeding stations .Social interactions .Seasonal sociality .Social complexity
Seasonal changes in social behaviour are widespread among
animals and often linked tocyclic variation in food availability
and phenology (e.g. reproduction) (Sueur et al. 2011; Silk
et al. 2014). A prominent example of this is found in the
formation of large aggregations around scarce and patchily
distributed food resources during the non-breeding season
(Krause and Ruxton 2002; Møller and Laursen 2019). While
the role of ecological factors in shaping seasonal groupings is
well-established, still, little is known about the extent to which
Communicated by C. R. Brown
Electronic supplementary material The online version of this article
( contains supplementary
material, which is available to authorized users.
*Thijs van Overveld
Department of Conservation Biology, Estación Biológica de Doñana
(CSIC), Américo Vespucio 26, 41092 Sevilla, Spain
Theoretical and Computational Ecology, IBED, University of
Amsterdam, Science Park 904, 1098
XH Amsterdam, The Netherlands
Behavioral Ecology and Sociobiology (2020) 74:28
social factors also contribute (Sueur et al. 2011; Silk et al.
2014). However, it is increasingly recognized that many birds
and mammals live in social systems that are structured by
long-term affiliative and agonistic relations (Massen 2018;
Kappeler 2019). Recent work suggests that seasonal aggrega-
tions may fulfil a crucial role in the development and mainte-
nance of social relationships (Fishlock and Lee 2013; Smith-
Aguilar et al. 2016; Uhl et al. 2018) and importantly contrib-
ute to the social structuring of populations (Aureli et al. 2008;
Sueur et al. 2011; Boucherie et al. 2019). To date, however,
detailed studies investigating the combined effects of ecolog-
ical and social factors underlying seasonal aggregations are
still scarce, hampering a more comprehensive understanding
of the social significance of this phenomenon.
While considerable attention has been given to the structure
and dynamics of mammalian social groupings (Kappeler et al.
2019), information about the relative complexity of avian so-
cial systems is still limited and largely restricted to research
performed on corvids (Boucherie et al. 2019). Members of
this family are renowned for their sociality and well-
developed socio-cognitive skills (Emery and Clayton 2004;
Emery et al. 2007). Most species live in social systems that
are structured by clear dominance relations, often consisting
of social alliances among young subdominant birds (Dall and
Wright 2009; Boucherie et al. 2016). They maintain their
strong social relations by providing each other social support
(Fraser and Bugnyar 2012), by complex forms of conflict
management (Fraser and Bugnyar 2011) and through sophis-
ticated waysof social reasoning, allowing a fine-scaled assess-
ment of the dominance rank of conspecifics as well as their
own social position within groups (e.g. Paz-y-Miño et al.
2004;Massenetal.2014). Their habit of living in open
groups(Richner 1989; Marzluff and Heinrich 1991), charac-
terized by high degrees of fissionfusion dynamics (Aureli
et al. 2008), is assumed to be a key driving force behind the
evolution of corvid cognition and social complexity
(Boucherie et al. 2019). Especially outside the breeding sea-
son, the composition of these groupings may be socially com-
plex, consisting of a mixture of new visitors (i.e. dispersers,
fledglings) and breeders that meet again after a period of re-
production (Loretto et al. 2017; Uhl et al. 2018). Repeated
interactions with conspecifics at these groupings may promote
the development of new social relationships but also select for
enhanced cognitive skills needed to reorganize social struc-
tures and/or to re-establish existing relationships (Ramos-
Fernandez et al. 2018; Boucherie et al. 2019). So far, however,
little is known about levels of social complexity in other
groups of bird displaying similar forms of fissionfusion
A group of species that show strong parallels in socio-
ecology with corvids are vultures. Both groups of species
are scavengers, with vultures being obligate carrion con-
sumers that rely almost exclusively on ephemeral,
unpredictable food resources (Campbell 2015). Like corvids,
vultures are well known for their complex grouping dynamics.
Although foraging often occurs solitarily, or in small, highly
dispersed groups, individuals frequently merge to form large
gatherings around carcasses, communal roosts or resting areas
(Campbell 2015). Vultures show a marked diversity in breed-
ing systems, ranging from strict territoriality to various forms
of social breeding, characterized by distinct species-
differences in group foraging, hierarchical social structures
and mating behaviours (reviewed in TvO et al. unpublished
data). Although vulture grouping dynamics have been exten-
sively studied from a social foraging perspective (e.g. Buckley
1997; Dermody et al. 2011; Cortés-Avizanda et al. 2014;
Harel et al. 2017), other aspects of their social living remain
largely unexplored. In fact, despite the highly threatened sta-
tus of many vulture species (Ogada et al. 2012;Buechleyand
Sekercioglua 2016), their behavioural and socio-ecology still
remains among the least understood of birds.
The Canarian Egyptian vulture (Neophron percnopterus
(Fuerteventura and Lanzarote), provides an excellent model
species to study vulture social dynamics and complexity. They
are long-lived and highly social birds that aggregate when
feeding and roosting but display high levels of territoriality
during the breeding season. Insular populations of Egyptian
vultures are sedentary (Donázar et al. 2002a; Porter and
Suleiman 2012), probably resulting in a higher frequency of
repeated interactions among individuals and more intense so-
cial conditions compared to migratory mainland populations.
In previous work, we showed that their social system is char-
acterized by strong female-dominance social structures, with
non-territorial and young individuals of both sexes having a
lower social position that increases with age (van Overveld
et al. 2018). Dominant females rely more on predictable food
provided at a feeding station located in the centre of
Fuerteventura, and breed closer to this site, while subdominant
females and dominant males rely more on farms as their main
source of food. However, outside the breeding season, indi-
viduals form large gatherings at this specific feeding patch,
which is visited by both low- and high-ranked individuals,
despite the presence of other feeding stations on both islands
(van Overveld et al. 2018). In the same period, vultures also
form a large temporary roost located at ca 5 km distance from
the central feeding station.
Here, by combining different sources of data (visual obser-
vations of individually marked vultures, GPS-tracking and
behavioural observations), we assess the different ecological
and social drivers underlying their seasonal grouping patterns.
Our main aims were twofold: (1) to quantify different aspects
of seasonal social dynamics, in particular temporal variation in
group size and structure (i.e. the number and composition of
vultures visiting the feeding station) and temporal variation in
socializing activities (i.e. time-activity patterns at both the
28 Page 2 of 13 Behav Ecol Sociobiol (2020) 74:28
feeding station and mountain roost) and (2) to extend previous
findings on the interplay between ecological factors (breeding
activities), individual traits (sex, age, territorial status, social
rank) and social competition underlying seasonal changes in
grouping habits.
Our predictions were as follows: First, we hypothesise that
seasonal variation in vulture numbers at the feeding station is
associated with distinct seasonal changes in group composi-
tions. In particular, we predict a decrease in the number of
adult breeders during the breeding season, but also in the
number of unpaired birds because of information gathering
on adult breeding activities (van Overveld et al. 2018).
However, we expect the number of young birds (immatures)
to remain relatively constant throughout the year given their
poor foraging skills. By contrast, we predict non-breeding
gatherings to be visited by the majority of vultures within
the population (including most recently fledged juveniles
and vultures from the nearby island of Lanzarote), despite
increased levels of competition for food and the availability
of carcasses at other feeding sites. Second, we hypothesise
that seasonal changes in activity patterns at the feeding station
(i.e. vulture numbers and time spent at this site) are part of an
overall shift of vulture activities towards the centre of
Fuerteventura (i.e. reflecting broad-scale seasonal fission
fusion dynamics at the population-level). We expect that adult
breeders increasingly abandon their territory during the non-
breeding season, but that territorial structures remain intact
throughout the year, given the high value of this resource,
especially for high-ranked breeders. In addition, we expect
the mountain roost to fulfil a specific role as a gathering place
for unpaired birds and low-ranked breeders that are less at-
tached to territories. Lastly, we provide descriptive informa-
tion about antagonistic (ritualized dominance displays, esca-
lated fights) and affiliative behaviours (non-reciprocal
allopreening) displayed at the feeding station, to show that
vulture gatherings are more than simple aggregations at food
Study population
The endemic Canarian subspecies of Egyptian vulture was
once abundant throughout the Canarian archipelago, but its
range is currently restricted to the eastern islands (between
27.62°29.42°N and 13.33°18.17°W) (Donázar et al.
2002a). The species almost went extinct during the second
half of the twentieth century with only 20 occupied territories
present on Fuerteventura and Lanzarote in the mid-nineties
(Donázar et al. 2002b). Extensive research and the application
of several protection measures have resulted in a current esti-
mated population size of ca. 350 birds in 2018 (TvO et al.
unpublished data). Fuerteventura(1662 km
) is the stronghold
of the population (68 breeding pairs in 2018), while Lanzarote
(845.9 km2) and adjacent islets are home to six breeding pairs.
Yearly ringing of fledglings and trapping of adults (through
cannon-netting) has resulted in over 90% of the total popula-
tion being marked allowing individual identification in the
As part of the speciesrecovery plan, three feeding stations
were created on the islands and are maintained all year-round
by local administrations. One feeding station is located in the
centre and one in the north of Fuerteventura (created in 1999
and 2008 respectively, and separated by approximately 40 km
distance) and one on the East coast of Lanzarote (created in
2016, distance ca. 60 and 100 km from the other two feeding
stations; Fig. 1). Although each of the three feeding stations is
provided with goat and sheep carcasses by local farmers, the
central feeding station is also provisioned with slaughterhouse
remains (± 200 kg per week consisting of pig heads and vis-
cera). In the past, slaughterhouse remains were also provided
at the northern feeding station, but this was stopped because
few birds visited this site (JAD and MdlR, pers. obs.).
Vultures may use small roosting sites (fewer than five
birds) throughout the islands but concentrate in four large
communal roosting areas throughout the year (usually with
more than 30 birds) all on electricity pylons located near sites
with a predictable food supply. They also gather at two sites
located in the mountains in the centre of the island at respec-
tively 5 and 8 km distance from the central feeding station
(Fig. 1;Fig.S1, distance between sites 6 km). Note that these
sites are used both as a diurnal and nocturnal roost.
Seasonal dynamics of social gatherings
To quantify seasonal variation in gatherings at the central
feeding station, we used data on visual observation of ringed
birds from a hide (using telescopes) between February and
September 2016. We included only observations made on
the first day of fresh food supply. We focussed on this specific
year because we performed additional extensive observations
on agonistic interactions (> 500 h, see below), in addition to
the standard monitoring work. This period covers the entire
reproductive season (MarchJune), the months when the tem-
porary roost is formed (AugustSeptember) and the pre-
breeding period (February) during which many pairing activ-
ities can be observed. We calculated the proportion of adult
unpaired and paired birds, immatures and juveniles (newly
fledged birds) visiting this feeding station relative to the total
population composition, the latter being quantified by using
all available visual observations of ringed birds collected
throughout the year at feeding stations, roost-sites and breed-
ing territories (see Table 1for details).
We used GPS-tracking data of 45 birds (UvA-BITS www., The Netherlands, n= 26 and e-obs Digital
Behav Ecol Sociobiol (2020) 74:28 Page 3 of 13 28
Telemetry, Grünwald, Germany, n= 19) tagged between October
2015 andDecember 2017 to assess between-year consistency in
patterns of gathering at both the feeding station and resting area.
Sample size slightly differed between study years: 2015, n=43;
2016, n= 45; 2017, n= 42, and some trackers were re-used in
case individuals died. GPS-tags were attached to the bird using
backpack harnesses. The total weight of the system varied be-
tween 31 g (UvA-BiTS) and 54 g (e-obs), representing 1.42.4%
of the bird body mass, which is well below the percentage as-
sumed to be harmless to the individual (Sergio et al. 2015).
Given the difficulty in observing vulture numbers and iden-
tities at the mountain resting sites, we could only use GPS-
tracking data to quantify seasonal variation in vulture num-
bers. An estimate of actual vulture numbers was made by
extrapolation of the relationship between vulture numbers
and GPS-tracked birds at the central feeding station. Note that
these extrapolated numbers represent a rough estimate since
the relationship between GPS-tagged birds and total vulture
numbers at both sites is unclear.
Table 1 Overview of the population structure and age-composition of
birds marked with PVC-rings (n= 277) and GPS-trackers (n= 42) in
Category NMedian Range NMedian Range
Males 44 10 522 9 6 510
Females 56 10.5 420 13 9.5 513
Males 45 4 316 10 5 37
Females 33 6 319 10 6.5 37
Males 39 2 12
Females 21 2 12
Males 21 0
Females 18 0
Fig. 1 Overview of the study area, Lanzarote (top) and Fuerteventura
(bottom) and the location of the feeding stations (squares), garbage
dumps (dots) and temporary mountain roost (triangles). The movements
of four territorial females in May 2016 (chick-rearing period, panel a)and
September 2016 (social gathering period, panel b) are shown (same
colour lines correspond to the same individual). Note the shift in foraging
range towards the centre of Fuerteventura associated with the abandon-
ment of territories. Also note the clear preference to forage at the centrally
located feeding station, despite the presence of other supplementary feed-
ing patches
28 Page 4 of 13 Behav Ecol Sociobiol (2020) 74:28
Monthly activity budgets
To construct monthly activity budgets, we used GPS-fixes be-
tween sunrise and sunset, excluding data from night-time, and
distinguished between time allocated to flight vs. non-flight
behaviour (including resting and foraging) using a threshold
ground speed 3 m s
(following similar procedures as in van
Overveld et al. 2018). We used data from paired and non-paired
males and females tracked over a 12-month period between
June 2016 and May 2017 (details in Table 1). GPS-loggers
were programmed to record locations every 1 to 5 min, but data
were re-sampled to a resolution of 10 min (range 911 min, R-
function developed by D.S. Viana, see Suppl. Mat.) to allow
direct comparison between individuals and both GPS devices.
GPS locations with intervals exceeding this 10-min resolution
(i.e. owing to battery and/or satellite reception issues) were
removed from the dataset. Year-round, non-flight behaviour
was further subdivided into time allocated at the mountain
roosts (daytime only, see above), feeding stations and the nest
location by using the number of GPS locations within a buffer
zone of 300 m, 75 m and 1 km around the centre of each place,
respectively. Note that in the latter case, behaviour may not
include only resting but also feeding at natural available carrion
resources. We excluded time spent at the territory and mountain
roost of one female with a territory in close proximity to the
mountain roost (< 50 m).
Social interactions at the central feeding station
Data on social dominance were collected in February 2016
(pre-breeding season) and AugustSeptember 2016 (post-
breeding season). We recorded all agonistic displacements
between ringed individuals around baits between sunrise and
sunset. In total, we observed 4593 interactions between 141
individuals being involved in more than 20 displacements
(average 65.1 ± 3.0 SE, range 20175). Rank scores for each
individual were determined using Davids score corrected for
chance of encounter and thus independent of group size or
visiting rate (Gammell et al. 2003) using the ds.R function
within the competepackage (Curley 2016)forRsoftware
(R Core Team 2017). Rank was scaled between 0 and 1 with 1
being the most dominant bird. Rank order relationships ac-
cording to sex, age and territorial status are presented in Fig.
In addition, we provide descriptive information on agonis-
tic and affiliative behavioural displays. Although we quanti-
fied social competitive interactions according to intensity on a
scale between 1 and 3 (1 = push/non-contact displacement,
2 = peck or kick and 3 = more elaborate physical contact),
most (90%) of interactions were scored as 1s or 2s.
Although during initial crowd-foraging for food (in case of
viscera) interactions can be slightly more intense and difficult
to assess, overall levels of aggression during food exploitation
(pig heads/goat carcasses) are typically low. Therefore, we
provide video examples of their ritualized dominance displays
and rare occasion of escalated fights (see discussion) which
we consider highly illustrative and informative for under-
standing low levels of aggression and their hierarchical way
of living. We also provide information about allopreening be-
haviours displayed outside a pair-living context between indi-
viduals of known sex, age and, in most cases, degree of kin-
ship. Note that these observations are not an indication of the
frequency of the behaviour, as it was not the main focus of
behavioural observations. Also, we only include information
of the behaviour in case where we were able to read the ring of
at least one individual. It was not possible to record data blind
because our study involved focal animals in the field.
Data analyses
Patterns of activity of GPS-tagged birds were analysed using
SAS 9.4 software (SAS Institute Inc., Cary, NC). First, we
assessed whether seasonal time-activity patterns at the central
feeding station (June 2016 to May 2017) differed between
individual categories using a linear mixed model (LMM).
We included sex, territorial status, social rank and month as
fixed terms plus all interactions. Since seasonal time-activity
patterns varied in a non-linear manner, we included month
and a four-way interaction term (month
× sex × territorial sta-
tus × social rank) to test for trait-specific differences in season-
al time activities. Second, we assessed whether time spent at
mountain resting area (September/October 2016) differed be-
tween categories, using a similar model, but without testing
for seasonal effects. Lastly, we assessed whether time activi-
ties differed between sites, using a similar model, but includ-
ing a categorical variable site(feeding station or mountain
resting area) as a fixed term. We also tested whether birds that
did not visit the mountain resting area (n= 12) also spent less
time at the feeding station, by including a categorical variable
presence(yes or no). We performed additional analyses on
territorial birds to test for dominance-specific seasonal chang-
es in time spent at territories (June 2016May 2017). We fitted
separate models for males and females and included month
and a two-way interaction (month
× dominance rank) in both
models. Note that month was included as a continuous vari-
able in models on time spent at the feeding station and terri-
tories, but as a categorical variable in models on time spent at
the mountain resting area. All dependent variables (percentage
of monthly time) were square-root transformed to attain nor-
mality of residuals. Bird ID was included as a random term in
all models.
Data availability
The datasets generated and/or analysed during the current
study are available in the [Figshare] repository, [Egyptian
Behav Ecol Sociobiol (2020) 74:28 Page 5 of 13 28
vultures seasonal sociality:
Social gatherings at the central feeding station
GPS-tracking revealed a clear seasonal pattern in the number
of individuals visiting the central feeding station (Fig. 2), with
lowest numbers during the breeding season (JanJun) and
highest numbers outside this period (JulDec). Data on visual
observations confirmed this pattern, showing an increase in
vulture numbers towards the end of the breeding season, with
a peak in September, when approximately half of the total
population visited this feeding patch on days of fresh food
supply (Fig. S3). Vulture numbers also showed a clear daily
pattern, with large numbers of feeding birds in the morning,
and an increase in resting vultures in the afternoon, reaching
up to 65 birds in September (Fig. S4). The highest proportion
of unpaired males and females (relative to the total population
of unpaired birds) was reached in February (82% and 67%,
respectively) and August/September (73% and 63%, respec-
tively) (Fig. S5; see also Fig. S6 for an example of large-scale
explorative flights made by males during the breeding sea-
son). The proportion of paired birds increased from March
onwards reaching maximum numbers in September (> 65%
in both sexes; Fig. S5). The number of immature birds was
relatively consistent throughout the year (ca. 60% of both
sexes; Fig. S5), but also increased after the breeding season
(> 80%). A sharp drop in the number of female immatures
occurred in JuneJuly. The total number of birds observed
between 11 Aug 2016 and 29 Sep 2016 was 204 (ca.74% of
the total population), which included five birds (out of five
pairs) with a territory on Lanzarote (two females and three
males, see also Fig. 1; see Fig. S2 for an example of the
movements of a breeding female from Lanzarote to the
centre of Fuerteventura). Nearly all newly fledged juveniles
were observed at this site (92%), including all three juveniles
fledged at Lanzarote. By contrast, vulture numbersrecorded at
the feeding station in the North (17 observation days between
FebruaryJuly) ranged between 2 and 23 individuals,
consisting of four males and five adult females (territory dis-
tance < 20 km from this site, but including one male from
Lanzarote), 38 and 16 unpaired males and females respective-
ly and five and two immature males and females respectively.
Social gatherings at the mountain resting area
The existence of a mountain roost was first observed in early
fall 2002, when 35 individuals of different age classes were
recorded resting on a mountain cliff (corresponding to ca.
30% of the population at that time). A new mountain roost
was located in autumn 2003 at 6 km from the feeding station,
which has become their preferred roosting site in the last
5 years. GPS data revealed that vultures typically form gath-
erings at this site outside the breeding season, showing a peak
around SepOct in all years (Figs. 2and 3). The number of
different GPS birds visiting the resting area in these months
was 27/27 in 2016 and 27/30 in 2017 (> 60% of total tagged
birds). Extrapolating from the relationships between number
of visually observed birds and GPS-tagged birds at the central
feeding station (Pearson correlation, r = 0.84, p<0.001, n=
69, equation: y = 3.95x 3.08), the estimated daily number of
birds at the resting area in September/October in 2016 was ca.
35 (range 180) and ca. 31 (range 175) in 2017. Irregular
visits at this site at the end of August in 2018 seem to confirm
these numbers (ranging between 24 and 44, mostly adults;
TvO, pers. obs.). Although the occurrence of this resting area
overlaps with temporal increases in numbers at the central
feeding station, the daily numbers of GPS birds at both sites
were not correlated (r = 0.11, p=0.24, n= 115 days).
Time-activity patterns at the mountain resting area,
feeding station and territories
The average monthly percentage of time spent at the mountain
resting area (during daytime) and feeding station in
September/October 2016 (pooled) was 14.6% ± 10.0 SD and
13.9% ± 10.8 SD, respectively (details in Table 2). During this
period, the majority of territorial birds also spent a consider-
able amount of time within < 1 km of their nest (average
percentage of monthly time 12.9% ± 12.4 SD; details in
Tab le 2, see also Fig. 3).
Fig. 2 Average monthly variation (error bars represent the standard
deviation) in GPS-tagged birds visiting the feeding station (solid line)
and mountain resting area (dashed line) between October 2015 and
November 2017. Data based on 45 GPS-tagged birds (2015, n= 43;
2016, n= 45; 2017, n=42)
28 Page 6 of 13 Behav Ecol Sociobiol (2020) 74:28
Seasonal time-activity patterns at the feeding station dif-
fered strongly among individual categories and in relation to
social rank (month
× sex × territorial status × social rank,
= 4.66, p= 0.0011; Fig. 3). Further analyses revealed
that within non-territorial males and females, there was no
correlation between social rank and time activities at the cen-
tral feeding station (p> 0.3), nor were there significant sex-
differences in time spent at this site (p> 0.1). However, in
breeding adults, dominant females spent more time at the cen-
tral feeding station throughout the year (p=0.0273;Table3;
Fig. 3), while no main effect of social status was found in
males (p=0.89;Table3; Fig. 3). Outside the breeding season,
subdominant territorial birds of both sexes increased the time
spent at the central feeding station (rank × month
β=0.03 ± 0.01, p= 0.0061; males: β=0.07 ± 0.02,
p< 0.0001; Table 1), while an opposite pattern was found
for time spent at territories (rank × month
; females: β=
0.04 ± 0.01, p= 0.0038; males: β= 0.06 ± 0.02, p< 0.0108;
Tab le 3; see also Figs. 1and 3).
There were no significant differences in time spent at the
mountain resting area between territorial and non-territorial
males and females (sex: F
=0.41, p= 0.52; territorial sta-
tus: F
=2.92, p= 0.10, interactions: p>0.29 for all).
However, non-territorial birds spent more time at the moun-
tain resting area compared to the feeding station (territorial
status × site: F
= 14.99, p= 0.0002; a similar but non-
significant trend was found for birds visiting both sites:
= 3.52, p= 0.0652). For territorial birds, no effect was
detected of distance between the nest-site and mountain rest-
ing area on the time spent at this site (p> 0.8). There were six
females and six males (consisting of four territorial and two
non-territorial birds in each sex) that did not visit the resting
Fig. 3 Average monthly time-budgets of GPS-tagged Egyptian vultures
(n= 42) on Fuerteventura between June 2016 and May 2017. Individuals
were categorized according to breeding status and social rank (BFB dom-
inant female breeders, n= 7), SFB subdominant female breeders (n=5),
DMB dominant male breeders (n= 4), SMB subdominant male breeders
(n= 5), UF unpaired females (n= 10), UM unpaired males (n=10).
Monthly time activities were quantified by time spent at the feeding
station (red), mountain roost (green) and territories (purple). The remain-
ing time was categorized as flight (dark blue), non-flight (light blue) and
unexplained (i.e. no signal, orange). The high percentage of time without
signal between March and May is due to birds incubating eggs on nest in
caves or on cliffs with bad reception
Behav Ecol Sociobiol (2020) 74:28 Page 7 of 13 28
area. Vultures not visiting the resting area also did not spend
much time at the feeding station (F
effect of sex and/or territorial status). However, dominant
males did not visit the mountain resting area (β=13.2 ±
5.27 SE, F
=6.25, p=0.0456; Fig.3).
Allopreening behaviours
Pairs visiting the feeding station often engage in mutual
allopreening, most commonly involving preening of the neck
and head feathers (Fig. 4a). However, we observed 17 cases of
allopreening (involving at least one individually marked bird)
outside a pair-living context (Feb (2×), June (2×), Aug (5×),
Sep (5×), Oct (2×), Nov (1×), summarized in Table S1). As
opposed to allopreening between pair members, this behaviour
was usually non-reciprocal (video 1 van Overveld 2019a). In
half of these cases, allopreening behaviour occurred between
young, unpaired birds (median age 4 years, range 27, n=7),
with both males and females acting as initiators or recipients
(Table S1). One unpaired female was observed in allopreening
with three different unpaired males within a 1.5-year period
(Fig. 4b;Fig.S7). In three cases, paired birds were preening
other paired or unpaired birds (Table S1;Fig.4c;Fig.S8). In
another three cases, allopreening was observed among same-
sex birds (femalefemale (video 2 van Overveld 2019b), male
male (video 3 van Overveld 2019c)). We twice observed an
Table 3 Results of GLMM on seasonal effects and social status on time
spent at the central feeding station (June 2016May 2017), mountain
resting area (September/October 2016) and time spent < 1 km from the
nest location for territorial female (n= 12) and male (n= 9) Egyptian
vultures tracked with GPS-loggers on Fuerteventura. Statistically signif-
icant associations (p0.05) are denoted in italics
Feeding station (JuneMay) Mountain resting area (Sep/Oct) Territory (JuneMay)
fixed effects β(SE) F
NUMdf, DENdf
pβ(SE) F
NUMdf, DENdf
pβ(SE) F
NUMdf, DENdf
Territorial females
Social rank 4.20 (1.71) 6.06
0.0273 0.95 (2.17) 0.19
0.67 2.38 (5.27) 0.00
Month 0.25 (0.11) 5.13
0.0250 0.25 (0.28) 0.80
0.39 0.22 (0.12) 3.29
0.06 (0.11) 25.67
<0.0001 n/a n/a n/a 0.05 (0.01) 18.12
*Rank 0.03 (0.01) 7.77
0.0061 n/a n/a n/a 0.04 (0.01) 8.67
Territorial males
Social rank 0.40 (2.79) 0.02
0.89 13.2 (5.27) 6.25
0.0465 1.48 (3.57) 0.21
Month 0.10 (0.11) 0.89
0.35 0.38 (0.19) 3.94
0.0876 0.31 (0.15) 4.58
0.03 (0.01) 8.84
0.0038 n/a n/a n/a 0.05 (0.01) 15.64
*Rank 0.07 (0.02) 16.89
<0.0001 n/a n/a n/a 0.06 (0.02) 6.79
Table 2 Summary of time spent at the mountain resting area and
feeding station for territorial and non-territorial birds and time spent with-
in territories (< 1 km from nest location) by territorial birds. Data from
September and October 2016. The coefficient of variance (CV) refers to
variation in time spent at each location
September October
Days Hours ± SD %time CV Number days Hours ± SD CV %time Number
Time at the mountain resting area
Territorial females 13.8 ± 11.1 47:29 ± 55:16 12.8 1.17 8 13.4 ± 6.3 36:14 ± 17:34 0.49 10.2 7
Non-territorial females 17.1 ± 7.2 79:10 ± 43:20 21.4 0.54 7 14.4 ± 4.8 49:54 ± 22:15 0.44 14.0 7
Territorial males 22.0 ± 4.7 69:40 ± 37:16 18.9 0.54 4 10.8 ± 7.9 35:44 ± 35:20 0.99 10.1 5
Non-territorial males 16.3 ± 10.6 52:16 ± 40:50 14.1 0.77 7 12.6 ± 6.9 40:39 ± 27:21 0.67 11.4 7
Time at the feeding station
Territorial females 16.8 ± 6.8 61:25 ± 33:59 17.2 0.56 13 17.1 ± 7.7 57:12 ± 37:51 0.67 16.5 14
Non-territorial females 15.8 ± 5.8 58:42 ± 23:53 15.9 0.40 8 15.9 ± 9.5 55:08 ± 42:29 0.77 15.5 9
Territorial males 16.4 ± 5.5 38:41 ± 21:44 10.5 0.58 8 11.8 ± 6.9 28:38 ± 20:19 0.70 8.1 9
Non-territorial males 14.8 ± 8.5 36:23 ± 27:09 9.9 0.75 9 14.7 ± 5.4 32:46 ± 17:17 0.53 9.2 7
Time spent within territories
Territorial females 15.1 ± 9.4 44:05 ± 37:38 11.9 0.86 9 10.6 ± 9.0 21:42 ± 12:07 0.55 6.1 11
Territorial males 19 ± 12.2 70:57 ± 57:36 19.2 0.81 9 19.5 ± 9.6 58:09 ± 47:28 0.82 16.4 8
28 Page 8 of 13 Behav Ecol Sociobiol (2020) 74:28
adult bird preening immature birds (Fig. 4d). Note that
Canarian Egyptian vultures do not provide post-fledging care
(LG, unpublished data). In none of the cases, allopreening in-
volved family members as based on pedigree data.
Despite the widespread occurrence of seasonal sociality
among animals, detailed information about the ecological
and social mechanisms driving seasonal variation in social
dynamics is still scarce, especially for avian social systems
(Silk et al. 2014; but see Uhl et al. 2018). Our study provides
novel insights on how seasonal grouping patterns result from
the complex interplay between ecological and individual-
based social factors and highlights the need of combining
different sources of data (i.e. modern GPS-tracking and basic
behavioural observations) to understand this complex
As predicted, group sizes at the central feeding station
followed a strong seasonal pattern characterized by distinct
monthly changes in group composition. The number of adult
breeders visiting this feeding patch was typically low at the
start of the breeding season, but their numbers gradually in-
creased from the egg-laying phase onwards, most probably as
a result of nest failure and fledging of young (only 40% of
breeders raise young, unpublished data see also García-Heras
et al. 2013; van Overveld et al. 2018). The low number of
unpaired/non-territorial birds observed during the breeding
season confirms results obtained from GPS-tracking, showing
that these birds, and males in particular, make large-scale ex-
plorative flights in this period, often covering the entire island
of Fuerteventura (van Overveld et al. 2018; see also Fig. S6).
Since unpaired birds were largely absent throughout the entire
breeding season, this seems to suggest that they may not only
search for vacant territories, but also gather information about
the quality of existing territories and their owners (Valone and
Tem ple ton 2002). Lastly, as predicted, a relatively large pro-
portion of immature birds was present at the feeding station
throughout the year, which at least in part could be linked to
their poor foraging skills and limited environmental knowledge,
as shown in many other bird species (e.g. Sanz-Aguilar et al.
2015). Similar factors may also explain why nearly all newly
fledged birds within the population can be found at this site.
While reproductive activities, information gathering and
age-specific foragingconstraints may importantly explain sea-
sonal variation in group size and composition, it is striking
that vultures show such a clear preference for the centrally
located feeding station. In between reproductive periods, vul-
ture numbers at this feeding patch (and a nearby temporary
roosting site) may reach up to 3050% of the total population
on a single day consisting of ca. 80% of the total population of
each individual category. During this period, unpaired and
paired vultures, depending on social status, may spend on
average 30% of their monthly time in a social gathering con-
text. These gatherings are particularly remarkable since vul-
tures largely neglect a feeding station that is located at only
40 km distance from this site (see also van Overveld et al.
2018), while both adults and fledglings from the nearby island
of Lanzarote prefer to spend their summer in the centre of
Fuerteventura, even though a feeding station is also present
on the former island (Fig. 1). Furthermore, although the
mountain resting area may be linked to the use of the central
feeding station, i.e. this roost provides an overview of the
plain where the central feeding site is located, this site is typ-
ically not used year-round, and visited by both local and non-
local breeders (and unpaired birds), although not by high-
ranked territorial males.
The general preference for the central feeding station, and
the formation of a temporary roost nearby, seems importantly
Fig. 4 Allopreening birds at the
central feeding station on
Fuerteventura showing a
members of a pair from
Lanzarote, ban unpaired male
preening an unpaired female (note
that a previously preened female
by the same male is sitting next to
him), ca paired female preening a
male of a different pair and dan
adult bird preening a 2-year old
male. Photos: T. van Overveld
Behav Ecol Sociobiol (2020) 74:28 Page 9 of 13 28
linked to its location near traditional breeding grounds
(Donázar et al. 2002b). Although the population has steadily
increased and expanded over the past 20 years (Badia-Boher
et al. 2019), the highest density of breeding pairs can still be
found within this area. This feeding station was the first
established at the start of the study period in 1999, suggesting
that apart from its specific geographic location, historical fac-
tors and habituation effects potentially also play a role (e.g.
socially inherited routines, Valenzuela et al. 2009; Andrew
2017), especially when taking into consideration that almost
all individuals within the population have been feeding at this
site from an early age.
The increase in vulture numbers at the feeding station also
seems to reflect an overall shift in foraging activities towards
the centre of Fuerteventura (Fig. 1). It is possible that vultures
partly abandon their territories because of a seasonal reduction
in the availability of natural carrion resources (Medina 1999).
By gathering in the centre, vultures may reduce searching time
for natural available goat carcasses and those provided at farms
(i.e. through local enhancementeffects Cortés-Avizanda et al.
2014), while taking advantage of food dumps at the central
feeding station. On the other hand, however, the high density
of vultures in the centre may increase the cost of food exploi-
tation, in particular for individuals from remote areas which are
typically lower ranked (van Overveld et al. 2018). Since there
are many goat farms distributed throughout the island and given
the semi-predictable character of this resource type, the social
foraging benefits for lower-ranked birds are currently not fully
understood. In addition, if food availability would truly be lim-
ited in summer (e.g. driving social foraging tactics), we would
also expect that other feeding stations would be used more
frequently, which is not the case.
As a last explanation, we propose that this feeding patch
may also be visited for social purposes. Besides foraging,
Egyptian vultures display a wide variety of activities at this
site, most notably conflict-settlement (between neighbouring
territorial or between unpaired birds), allopreening (among
and between paired and unpaired birds, see below) and exten-
sive copulation activities (in the pre-breeding period). Social
gatherings may be of particular relevance for territorial vul-
tures that are facultative social foragers, which social struc-
tures are characterized by strong age- and sex-dependent hier-
archical relationships, with social rank also being linked to
territorial status (van Overveld et al. 2018; this study).
Within such highly despotic societies, rank maintenance and
improvement may form an integral part of the daily activities,
beginning as soon as young fledge (Fig. S9). Group foraging
events, where many other individuals can be found in a com-
petitive context, may attract special attention, given opportu-
nities to strengthen/improve social rank, to gain knowledge
about the social rank of conspecifics and potentially also to
update information about their own relative fighting ability/
social status within the population (Rutte et al. 2006). Apart
from food exploitation, feeding sites may importantly contrib-
ute in optimizing decision-making about future interactions
(e.g. to avoid costly conflicts) and potential mates.
Although we acknowledge the inherent difficulty in
assessing whether individuals actively join gatherings for so-
cial purposes, extensive behavioural observations on agonistic
and affiliative interactions nevertheless point towards the ex-
istence of well-developed social communication skills in this
species. Despite the complex fissionfusion dynamics charac-
terizing their group feeding, with individuals coming and go-
ing at different times and rates during the day (or so called
open groups: Boucherie et al. 2019; see also introduction),
Egyptian vultures rarely fight extensively over food and com-
petitive displacements are often subtle. Competitive interac-
tions and aggression levels may be regulated by body-size
differences, but probably also through information on age
and social rank (e.g. Paz-y-Miño et al. 2004;Massenetal.
2014). During feeding, individuals often first look around
and specifically select a lower-ranked bird of the same sex
and age to be displaced, which suggests the existence of an
active avoidance/attack strategy based on individual recogni-
tion. Furthermore, escalated fights, although rare, typically
attract muchattention and are carefully watched by bystanders
(Fig. 5; video 4 Mulet 2019). Social conflicts may also be
settled through ritualized dominance displays (e.g. duels,
whereby birds erect their head and neck feathers, and parade
face to face, until one or both birds attacks, or retreats (video 5
van Overveld 2019d). Although threat displays are common
in birds (Hurd and Enquist 2001), we are currently not aware
of other (non-vulture) bird species that evolved such a distinct
and elaborate agonistic repertoire.
Apart from antagonistic displays, vultures also frequently
engage in affiliative interactions through non-reciprocal
allopreening outside a pair-living context. We speculate that
this behaviour may primarily serve a function in mate-seeking
Fig. 5 Example of an escalated fight between two females outside the
breeding season (August). Note that these fights are carefully watched by
other birds. On some occasions, these bystanders may start to peck on the
defeated individual. Photo: T. van Overveld
28 Page 10 of 13 Behav Ecol Sociobiol (2020) 74:28
or mate-sampling, since the behaviour is displayed mostly by
unpaired birds at the age of recruitment (e.g. 37 years).
Allopreening among paired and (un)paired birds may be
linked to their complex mating strategies, with the frequent
occurrence of both polyandrous and polygynous trios on
Fuerteventura (authors unpublished). However, occasional
observations of the behaviour in more complex social setting,
such as among same-sex birds and unrelated adults and im-
matures, may point towards alternative social functions such
as stress reduction and/or conflict avoidance (Rabenold 1986;
Bertran and Margalida 2003; Radford and du PLessis 2006;
Lewis et al. 2007). An intriguing question is whether these
specific interactions reflect the existence of secondary rela-
tionships (Boucherie et al. 2019) and/or specific post-
conflict repair for social bonding purposes (Fraser and
Bugnyar 2011). Although Egyptian vultures do not form
distinct social alliances during foraging as observed in
some social corvids (Fraser and Bugnyar 2012), it is possi-
ble that hierarchical social relations include more complex
sublayers linked to specific individual strategies for rank
To conclude, our findings show that seasonal grouping pat-
terns are shaped by the complex interplay between ecological
factors (reproductive constraints, resource seasonality and food
predictability), individual traits (age, sex and rank within hier-
archy) and social competitive processes. We argue that vultures
visit large gatherings at food resources also for social purposes.
Group foraging events may be of special interest to vultures
given opportunities for rank improvement/maintenance, but po-
tentially also for mate-seeking/evaluation, and in general act as
important places where young birds become integrated within
local population social structures. An important, but challeng-
ing, goal for future work will be to determine whether (or
which) individuals actively join social gatherings for social pur-
poses and the potential gains in fitness to fully assess its poten-
tial role as a meeting place. In addition, future studies should
also reveal whether social gatherings fulfil a similar role in
mainland populations and/or to what extent sedentary island
conditions promote the formation of more complex social rela-
tions and/or stronger dominance hierarchies. Lastly, we provid-
ed examples of the agonistic and affiliative repertoire of
Egyptian vultures, further adding to other unique behaviours
displayed by the species such as stone throwing to open eggs
(van Lawick-Goodall and van Lawick 1966; Carrete et al.
2017), mud-bathing (van Overveld et al. 2017) and coprophagy
(Negro et al. 2002). We propose that vultures, like corvids, may
serve as important model species to test the role of scavenging
and fissionfusion social dynamics in shaping the evolution of
avian social cognition, or avian behavioural complexity gener-
ally. Overall, our work highlights the need for more in-depth
knowledge about the rich social life of vultures, which could be
invaluable for understanding vulture social responses to current
environmental changes.
Acknowledgments We thank Ana Trujillano, Carmen Díez, Marcos
Mallo, Walo Moreno, Toni Mulet, Juan Ramírez and Julio Roldán for
their invaluable work in monitoring the Guirrepopulation on the
Canary Islands. We thank two anonymous reviewers for their helpful
comments to improve and clarify this manuscript.
Author contributions All the authors were involved in conducting field-
work. TvO conceptualized research questions. TvO collected behavioural
data on agonistic and affiliative interactions. TvO carried out the statisti-
cal analyses with input from LG. TvO wrote the manuscript with editorial
input from all co-authors (LG, MG-A, WB, MdlR, JD). All the authors
gave final approval for publication.
Funding information TvO received funding from the European Unions
Horizon 2020 research and innovation programme under the Marie
Sklodowska-Curie grant agreement no. SocForVul 659008.During
writing, LG was supported by a Marie Sklodowska-Curie Fellowship of
the European Commission (grant number: 747729 EcoEvoClim). MG-
A was supported by a contract from Programa de FPU del Ministerio de
Educación, Cultura y Deporte(FPU13/05429). The long-term monitor-
ing of the vulture population has been funded by the projects REN 2000
1556 GLO, CGL2004-00270/BOS, CGL2009-12753-C02-02,
CGL2012-40013-C02-01 and CGL2015-66966-C2-1-2-R (Spanish
Ministry of Economy and Competitiveness and EU/FEDER). Further
support was provided by the Cabildo Insular de Fuerteventura and the
Dirección General de Protección de la Naturaleza (Viceconsejería de
Medio Ambiente, Canarian Government).
Compliance with ethical standards
Conflict of interest The authors declare no competing interests.
Ethical approval All applicable international, national and/or institu-
tional guidelines for the care and use of animals were followed. All
procedures performed in studies involving animals were in accordance
with the ethical standards of the institution or practice at which the studies
were conducted (Ethic Committee of CSIC (CEBA-EBD-12-56).
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... The Egyptian vulture is an obligate scavenger that frequently consumes both carrion from livestock and wild ungulates (Donázar, 1993). The species exhibit complex social behaviour forming large individual congregations outside the breeding season at highly preferred feeding stations (e.g., farms) and nearby temporary roosting sites (García-Alfonso, 2020;van Overveld et al., 2020a). Altogether, these traits make it an ideal study species to assess how different migratory strategies coexist. ...
... Moreover, the above-mentioned favourable conditions could also foster the grouping of the species that could attract more birds to such areas during both unfavourable and favourable seasons (see also García-Alfonso et al., 2020). Such congregations, which usually occur at roosting sites surrounding the primary trophic sources like farms, are vital for information exchange and population dynamics on the species (Donázar, 1993;García-Alfonso et al., 2020;van Overveld et al., 2020a). Furthermore, in a broader context, changes in livestock numbers in human-dominated landscapes could also modulate the dynamics at roosting sites (Chapter 5-6). ...
... Interestingly, our results suggest that such change in environmental conditions could be amplified by species-specific behavioural traits such as attraction to conspecifics(van Overveld et al., 2020a;. We also observed that each migratory form may maximise a certain fitness component (i.e., survival or reproduction). ...
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Human activities transformed virtually all landscapes worldwide to fulfil their basic needs (e.g., resource extraction, agriculture or leisure activities). By doing so, they also affect species inhabiting these human-dominated landscapes. Due to their historical link to human activities, apex predators, especially vultures, are especially vulnerable to human-induced behavioural alterations and have undergone population declines worldwide. Therefore, finding a solution that reconciles vulture conservation and human activities in such landscapes is necessary. By using a set of behavioural indicators (e.g., breeding, occupancy/detectability and space use) from long-term monitoring and movement ecology, this thesis aims to build links between behaviour and conservation of Egyptian vulture Neophron percnopterus in human-dominated landscapes. The current dissertation shows that the species invests similar effort in parental care and that incubation and hatching are important tipping points during the breeding season (Chapter 1). This information could be, in turn, used to design cost-effective monitoring while accounting for imperfect detection and breeding phenology and other environmental variables that could help to adapt monitoring programs to different available budgets (Chapter 2). Similarly, the knowledge of breeding behaviour of the species could be used to infer the impact of habitat alterations on species nest occupancy and reproduction patterns and to improve conservation programs (Chapter 3), and test whether management programs and collaboration networks resulted effective in reducing the synergistic effect of various human disturbances (Chapter 4). Finally, it poses an advance in the understanding of how certain human activities that provide continuous and predictable food pulses, such as farming, could alter species space use and favour residency in partial migratory species (Chapter 5), and that human-driven changes in migratory behaviour could even have consequences on fitness and energy use of different migratory phenotypes (Chapter 6). Overall, this work demonstrates the utility of increasing vulture behaviour knowledge to ascertain the effects of human activities on the species and find coherent conservation solutions that favour its persistence and promote vulture-human coexistence in anthropogenic landscapes.
... Applying such robust metrics especially to critically endangered species such as African white-backed vultures may provide useful ecological knowledge regarding the spatial and social arrangement of such species in ecosystems. For example, quantifying home range overlap zones may be a useful proxy for inferring potential species interactions and encounter rates (Robert et al., 2012;Sanchez & Hudgens, 2015), (but see Vander Wal et al., 2014), which is critical for expanding our understanding of infectious disease ecology (Gangoso et al., 2009;Hoover et al., 2020), mating behaviors, territorial behavior (van Overveld, Gangoso, et al., 2020), breeding behavior (Anderson, 2004;Leepile et al., 2020;Xirouchakkis & Mylonas, 2007), sociality, and information sharing as well as the location and pulling effect of high-quality habitats and resources (Cortés-Avizanda et al., 2014;Jackson et al., 2008;Kane et al., 2015;Murn & Anderson, 2008). High-quality habitats here refer to beneficial communal roosting and breeding colony zones where social and ecological questions can be explored further to expand our understanding on shared space use mechanisms among African vulture species. ...
... We also estimated dry breeding season and the wet non-breeding season pairwise home range overlaps to test whether home range overlap trends for the two time blocks vary significantly. We used seasonality as a proxy for cyclic variation in food abundance and predictability as well as reproduction status (Reading et al., 2019;van Overveld, Gangoso, et al., 2020) and thus a major driver for dynamics in space use sharing patterns (Gil et al., 2014). Finally, we quantified home range overlap for each individual vulture between the dry breeding and the wet non-breeding seasons to test for consistency in space use patterns which may be used as a strategy to optimize foraging efficiency. ...
... First, African white-backed vultures range extensively and tend to establish social networks to enhance their foraging success (Monadjem et al., 2018;Phipps et al., 2013). Second, several studies have identified vultures as highly gregarious species which breed and roost in loose colonies Harel et al., 2017;Mundy, 1992;van Overveld, Gangoso, et al., 2020). Such behavior may support intensive spatial overlaps in home ranges. ...
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Understanding key overlap zones and habitats which are intensively shared by species in space and time is crucial as it provides vital information to inform spatial conservation with maximum benefits. The advent of high‐resolution GPS technologies associated with new analytical algorithms is revolutionizing studies underpinning species spatial and social interaction patterns within ecosystems. Here, using a robust home range estimation algorithm, the autocorrelated kernel density estimator (AKDE) equipped with an equally powerful home range overlap metric, the Bhattacharyya's coefficient (BC), we provide one of the first attempts to estimate and delineate spatial home range overlap zones for critically endangered African white‐backed vultures to inform conservation planning. Six vultures were captured in Hwange National Park using a modified cannon net system after which they were tagged and tracked with high‐resolution GPS backpacks. Overall, results suggested weaker average home range overlaps based on both the pooled data (0.38 ± 0.26), wet non‐breeding seasonal data (0.32 ± 0.23), and dry breeding season data (0.34 ± 0.28). Vultures 4, 5, and 6 consistently revealed higher home range overlaps across all the scales with values ranging between 0.60 and 0.99. Individual vultures showed consistence in space use patterns as suggested by high between‐season home range overlaps, an indication that they may be largely resident within the Hwange ecosystem. Importantly, we also demonstrate that home range overlapping geographic zones are all concentrated within the protected area of Hwange National Park. Our study provides some of the first results on African vulture home range overlaps and segregation patterns in the savanna ecosystem based on unbiased telemetry data and rigorous analytical algorithms. Such knowledge may provide vital insights for prioritizing conservation efforts of key geographic overlap zones to derive maximum conservation benefits especially when targeting wide‐ranging and critically endangered African white‐backed vultures. To this end, spatial overlap zones estimated here, although based on a small sample size, could provide a strong foundation upon which other downstream social and ecological questions can be explored further to expand our understanding on shared space use mechanisms among African vulture species. Understanding home range spatial overlaps is critical for the conservation of critically endangered species. Application of spatially explicit remote‐tracking technologies is providing important tools for this purpose. Home range overlaps for African white‐backed vultures are complex and need further investigation.
... Secondly, there was an important effect of the spatial location of highly predictable feeding places on the number of vultures visiting the farms. Overall, we observed that these food-rich sites attract birds from very distant areas [13,18], which can congregate in large numbers, especially outside the breeding season (up to 147 birds in a single day, [65]). This suggests that the attractiveness of these HPFP is not only due to the spatiotemporal predictability of food but also the opportunities they offer for socialization [65]. ...
... Overall, we observed that these food-rich sites attract birds from very distant areas [13,18], which can congregate in large numbers, especially outside the breeding season (up to 147 birds in a single day, [65]). This suggests that the attractiveness of these HPFP is not only due to the spatiotemporal predictability of food but also the opportunities they offer for socialization [65]. Consequently, those farms located near HPFP would also have high probabilities of being visited by more birds simply because of the large number of vultures that congregate [18]. ...
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Recent changes in European legislation have legalized the abandonment of carcasses around livestock farms, but our understanding of how vultures exploit these semi-predictable food sources is still very limited. For filling this gap, we determine the individual and ecological drivers influencing vulture visits to farms. We assessed the effects of individual characteristics of both birds and farms on the frequency of vultures’ visits to livestock facilities using data collected from 45 GPS-tagged Egyptian Vultures (Neophron percnopterus) and 318 farms (>94% of livestock) on Fuerteventura Island, Spain. Farms were more visited during the vultures’ breeding season. Farms located closer to highly predictable feeding places (i.e., vulture restaurants and garbage dumps) or with more available feeding resources were visited by more vultures, whereas those located close to roads and vultures’ breeding territories received fewer visits. Younger territorial birds visited a farm more frequently than older territorial ones, whereas older non-territorial individuals concentrated those visits on farms closer to their activity core areas compared with younger ones. Our findings indicate that visits to farms were determined by their spatial distribution in relation to the age-specific birds’ activity centers, the availability of carcasses, seasonality, and individual characteristics of vultures. These interacting factors should be considered in vulture conservation, avoiding very general solutions that ignore population structure
... While migratory individuals regularly travel >4,000 km between their northern breeding and southern wintering grounds by using several distinct migratory yways (Phipps et (Donázar 1993). The species exhibit complex social behaviour forming large individual congregations outside the breeding season at highly preferred feeding stations (e.g., farms) and nearby temporary roosting sites (García-Alfonso et al. 2020;van Overveld et al. 2020a). Altogether, these traits make it an ideal study species to assess how different migratory strategies coexist. ...
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Partial migration is one of the most widespread migratory strategies among taxa. Investigating the trade-off between environmental/social factors-fitness and energetic consequences is essential to understanding the coexistence of migratory and resident behaviours. Here, we compiled eld monitoring data of wintering population size and telemetry data of 25 migrant and 14 resident Egyptian Vultures Neophron percnopterus to analyse how environmental and social factors modulate resident population size, compare tness components (i.e., survival and reproduction), and energetic consequences between migratory and resident individuals across wintering and non-wintering seasons. We observed that food availability positively correlated with the wintering population size and that subadult birds increased linearly with censused adult birds. Residents exhibited higher survival probabilities, but lower breeding activity and higher energy expenditure, less FLight efficiency, and lower activity due to shorter winter-day lengths. On the contrary, migratory birds showed lower survival but more breeding attempts but spent less energy and flew more efficiently at longer distances, bene tting from longer days in African wintering quarters. These results suggest that anthropogenic food mediated social attraction could modulate population dynamics and promote residency. Food availability may bene t resident individuals enhancing their survival which may offset higher energy expenditure. Migrant birds, on the contrary, may compensate for the higher costs in terms of survival by a reduction in the energy cost, which may bene t future breeding. Our results offer new insights to understand how species bene t from one strategy or another and that the coexistence of both migratory forms is context-dependent.
... This is the oldest feeding station and most used by the vultures, so it was considered as the main SFS (mSFS). Particularly during the non-breeding season, vultures gather in large numbers at this feeding station (up to 150 different individuals during a day), congregate at communal roosts on power lines but in lower numbers (up to 35 simultaneous individuals in a pylon), and conform small aggregations at the garbage dump (up to 15 simultaneous individuals) and large livestock farms (up to 10 simultaneous individuals) based on field observations (see also Donázar et al., 2002b;van Overveld et al., 2018van Overveld et al., , 2020b. ...
Accidents on power lines are the leading cause of mortality for many raptor species. In order to prioritise corrective measures, much effort has been focused on identifying the factors associated with collision and electrocution risk. However, most studies lack of precise data about the use of pylons and its underlying driving factors, often relying on biased information based on recorded fatalities. Here, we used multiple years of high-resolution data from 49-GPS tagged Canarian Egyptian Vultures (Neophron percnopterus majorensis) to overcome these typical biases. Birds of our target population use electric pylons extensively for perching (diurnal) and roosting (nocturnal), so accidents with these infrastructures are nowadays the main cause of mortality. Predictive models of pylon intensity of use were fitted for diurnal and nocturnal behaviour, accounting for power line, environmental, and individual vulture's features. Using these measures as a proxy for mortality risk, our model predictions were validated with out-of-sample data of actual mortality recorded during 17 years. Vultures used more pylons during daytime, but those chosen at night were used more intensively. In both time periods, the intensity of use of pylons was determined by similar drivers: vultures avoided pylons close to roads and territories of conspecifics, preferentially used pylons located in areas with higher abundance of food resources, and spread their use during the breeding season. Individuals used pylons unevenly according to their sex, age, and territorial status, indicating that site-specific mitigation measures may affect different fractions of the population. Our modelling procedures predicted actual mortality reasonably well, showing that prioritising mitigation measures on relatively few pylons (6%) could drastically reduce accidents (50%). Our findings demonstrate that combining knowledge on fine-scale individual behaviour and pylon type and distribution is key to target cost-effective conservation actions aimed at effectively reducing avian mortality on power lines.
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Despite important recent advances in cognitive ecology, our current understanding of avian cognition still largely rests on research conducted on a few model taxa. Vultures are an ecologically distinctive group of species by being the only obligate carrion consumers across terrestrial vertebrates. Their unique scavenging lifestyle suggests they have been subject to particular selective pressures to locate scarce, unpredictable, ephemeral, and nutritionally challenging food. However, substantial variation exists among species in diet, foraging techniques and social structure of populations. Here, we provide an overview of the current knowledge on vulture cognition through a comprehensive literature review and a compilation of our own observations. We find evidence for a variety of innovative foraging behaviors, scrounging tactics, collective problem-solving abilities and tool-use, skills that are considered indicative of enhanced cognition and that bear clear connections with the eco-social lifestyles of species. However, we also find that the cognitive basis of these skills remain insufficiently studied, and identify new research areas that require further attention in the future. Despite these knowledge gaps and the challenges of working with such large animals, we conclude that vultures may provide fresh insight into our knowledge of the ecology and evolution of cognition.
When social animals forage together, they may engage in frequency-dependent strategies either as producers, those who acquire food with their own energy, or as scroungers, those who feed on what the producers discover. An individual's choice of strategy may depend on the frequency of strategies within the group, or it may depend on the individual's own traits. Most of our understanding of producer–scrounger relationships comes from highly social species, but we may gain different insights by studying less social species. We used novel puzzle box experiments to study social foraging relationships and identify the traits associated with strategy choice in facultatively social yellow-bellied marmots, Marmota flaviventer. We found that marmots had consistent strategies as either producers or scroungers. Furthermore, several individuals specialized as scroungers, following the producers to the puzzle box before other scroungers. However, their roles were only associated with sex (males were more likely to produce), and not with age, boldness or dominance rank. Marmots' lack of trait-regulated foraging roles may be associated with their relatively abundant and not-entirely defensible food, or may emerge from their lack of obligate social structure. We discuss how multiple factors may explain variation in social foraging behaviour.
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Fission-fusion dynamics have evolved in a broad range of animal taxa and are thought to allow individuals to mitigate feeding competition. While this is the principal benefit of fission-fusion, few studies have evaluated its costs. We compared gregariousness, foraging budgets, and social budgets between lactating bonobos and chimpanzees from wild populations to evaluate potential costs. Both species exhibit fission-fusion dynamics, but chimpanzees, particularly in East African populations, appear to experience higher feeding competition than bonobos. We expected lactating chimpanzees to be less gregarious than lactating bonobos; reduced gregarious-ness should allow lactating chimpanzees to mitigate the costs of higher feeding competition without requiring more foraging effort. However, we expected the reduced gregariousness of lactating chimpanzees to limit their time available for affiliative interactions. Using long-term data from LuiKotale bonobos and Gombe chimpanzees, we found that lactating chimpanzees were indeed less gregarious than lactating bonobos, while feeding and travel time did not differ between species. Contrary to our predictions, lactating females did not differ in social interaction time, and lactating chimpanzees spent proportionately more time interacting with individuals other than their immature offspring. Our results indicate that lactating chimpanzees can maintain social budgets comparable to lactating bonobos despite reduced gregariousness and without incurring additional foraging costs. We discuss potential explanations for why lactating bonobos are more gregarious.
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Vultures are one of the most threatened bird groups globally. Although many of the threats faced by vultures have been identified, the impact of human activities on the social life of vultures has received little attention. In this paper, we emphasize the need to integrate vulture sociality into conservation practice. First, we summarize current knowledge on vulture social behavior, and the evolutionary and ecological roots of their breeding systems. We describe the existence of contrasting gradients in social foraging strategies and hierarchical social structures among colonial and territorial breeders associated with species (and population) reliance on carrion differing in size and predictability. We also highlight the potential role of vulture gatherings in maintaining population-level social structures and for mate-finding given high mate-selectivity. Next, based on this social framework, we discuss the impact of human activities on social foraging, territory structures, resource partitioning processes, and mating dynamics. However, little is known about how disruptions of social habits may have contributed to vulture population declines and/or may impede their recovery. Lastly, we provide directions for future research on vulture socio-ecology that may improve current conservation efforts. We encourage researchers and wildlife managers to pay more attention to natural carrion diversity underlying vulture social system diversity, especially when implementing supplementary feeding programs, and to consider the complex mating and settlement dynamics in reintroduction programs. Overall, we stress that understanding the complex social life of vultures is critical to harmonize their conservation with anthropogenic activities.
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Many waterbirds have fully (totipalmate) or partially webbed (palmate) feet that are used for locomotion in aquatic environments. If webbed feet and wings both contribute to efficient diving, we predicted a positive association between the area of webbed feet and the size of the frontal locomotor apparatus (wing area, heart mass, and breast muscle, after adjusting for any partial effects of body size). We predicted that individuals able to acquire more and better quality food due to larger webbed feet should have larger livers with higher concentrations of fat‐soluble antioxidants such as vitamin E, and invest more in immune function as reflected by the relative size of the uropygial gland than individuals with small webbed feet. Here, we examine if the area of webbed feet is correlated with locomotion, diet, and body condition in a sea‐duck, the eider (Somateria mollissima). We analyzed an extensive database of 233 eiders shot in Danish waters and at Åland, Finland during winter and early spring. Eiders with larger webbed feet had a larger locomotor apparatus, but did not have larger body size, they had larger uropygial glands that waterproof the plumage, they had larger beak volume and larger gizzards, and they had higher body condition. These findings imply that eiders with large webbed feet benefitted in terms of locomotion, feeding, and reproduction. If webbed feet contribute to efficient diving, we predicted a positive association between the area of webbed feet and the size of the frontal locomotor apparatus (wing area, heart mass, and breast muscle). We predicted and found that individuals able to acquire more and better quality food should have larger livers, and invest more in immune function than individuals with small webbed feed. The findings were consistent with predictions, showing that the size of webbed feet in waterbirds plays a functional role in locomotion, foraging, and development of high body condition.
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Population size is generally limited by resource availability during and outside the breeding season. Therefore, maximum size of flocks may provide important information on population regulation and the influence of diet and trophic level on maximal degree of sociality. We hypothesized that (a) flock size should increase with nutrient availability; (b) flock size should decrease with latitude because productivity is higher at lower latitude; (c) aquatic habitats should have larger flocks than terrestrial habitats because the former are less accessible; (d) smaller species should have larger flocks because they require overall less food; (e) human‐impacted species that live in perturbed habitats should have smaller flocks than other species; (f) flock size should decrease with increasing trophic level because there is a reduction in biomass due to conversion at each trophic level; and (g) flocks of species depending on ancestral landscapes should have decreased in size in recent years due to human impact (e.g., land‐use). We obtained 1564 observations of flocks that exceeded 100,000 individuals in order to test the predictions listed above. Most effect sizes were small to medium accounting for 1%–9% of the variance, while large effects accounting for 25% or more were only found for total nitrogen used per km² and area used for agriculture. Changes in large bird flocks were caused by habitat degradation and persecution, and temporal decline in size of large flocks revealed changes in nutrient use, reductions in nutrient cycling, and changes in flock size linked to trophic level.
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In the last decades, the assumption that complex social life is cognitively challenging, and thus can drive mental evolution, has received much support from empirical studies in nonhuman primates. While extending the scope to other mammals and birds, different views have been adopted on what constitutes social complexity and which specific cognitive skills are selected for. Notably, many avian species form Bopen^ groups as non-breeders (i.e., seasonally and before sexual maturity) that have been largely ignored as potential sources of social complexity. Reviewing 30 years of research on ravens, we illustrate the socio-ecological conditions faced by these birds as non-breeders and discuss how these relate to their socio-cognitive skills. We argue that the non-breeding period is key to understand raven social life and, to a larger extent, avian social life in general. We furthermore emphasize how the combination of the large-scale perspective (defining social system components: e.g., social organization, mating system) and the individual-scale perspective on social systems allows to better capture the complete set of social challenges experienced by individuals throughout their life, ultimately resulting on a more comprehensive understanding of species’ social complexity.
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Social complexity has been one of the recent emerging topics in the study of animal and human societies, but the concept remains both poorly defined and understood. In this paper, I critically review definitions and studies of social complexity in invertebrate and vertebrate societies, arguing that the concept is being used inconsistently in studies of vertebrate sociality. Group size and cohesion define one cornerstone of social complexity, but the nature and patterning of social interactions contribute more to interspecific variation in social complexity in species with individual recognition and repeated interactions. Humans provide the only example where many other unique criteria are used, and they are the only species for which intraspecific variation in social complexity has been studied in detail. While there is agreement that complex patterns emerge at the group level as a result of simple interactions and as a result of cognitive abilities, there is consensus neither on their relative importance nor on the role of specific cognitive abilities in different lineages. Moreover, aspects of reproduction and parental care have also been invoked to characterize levels of social complexity, so that no single comprehensive measure is readily available. Because even fundamental components of social complexity are difficult to compare across studies and species because of inconsistent definitions and operationalization of key social traits, I define and characterize social organization, social structure, mating system, and care system as distinct components of a social system. Based on this framework, I outline how different aspects of the evolution of social complexity are being studied and suggest questions for future research. Significance statement Animal and human societies differ in social complexity, i.e., the number and association patterns of group members as well as the nature and patterning of their social relationships, but the dimensions of social complexity, the processes that generate it, the selective forces that engender different levels of social complexity, and the evolutionary consequences of this variation remain to be comprehensively understood. Here, I offer a conceptual framework for the systematic and comparative studies of social complexity by defining its main components as well as their proximate and ultimate relationships.
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Social complexity arises from the formation of social relationships like social bonds and dominance hierarchies. In turn, these aspects may be affected by the degree of fission-fusion dynamics, i.e., changes in group size and composition over time. Whilst fission-fusion dynamics has been studied in mammals, birds have received comparably little attention, despite some species having equally complex social lives. Here, we investigated the influence of environmental factors on aspects of fission-fusion dynamics in a free-ranging population of carrion and hooded crows (Corvus corone ssp.) in the urban zoo of Vienna, Austria over a 1-year period. We investigated 1) the size and 2) spatio-temporal structure of the local flock, and 3) environmental influences on local flock and subgroup size. The local flock size varied considerably over the year, with fewest birds being present during the breeding season. The spatio-temporal structure of the local flock showed 4 distinct presence categories, of which the proportions changed significantly throughout the year. Environmental effects on both local flock and subgroup size were time of day, season, temperature, and weather, with additional pronounced effects of the structure of the surroundings and age class on subgroup size. Our findings show environmental influences on party size at the local flock and subgroup level, as well as indications of structured party composition in respect to the 4 presence categories. These results suggest that environmental factors have significant effects on fission-fusion dynamics in free-ranging crows, thereby influencing social complexity.
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Despite increasing work detailing the presence of foraging specializations across a range of taxa, limited attention so far has been given to the role of spatiotemporal variation in food predictability in shaping individual resource selection. Here, we studied the exploitation of human-provided carrion resources differing in predictability by Canarian Egyptian vultures (Neophron percnopterus majorensis). We focussed specifically on the role of individual characteristics and spatial constraints in shaping patterns of resource use. Using high-resolution GPS data obtained from 45 vultures tracked for 1 year, we show that individual vultures were repeatable in both their monthly use of predictable and semi-predicable resources (feeding station vs. farms) and monthly levels of mobility (home range size and flight activity). However, individual foraging activities were simultaneously characterized by a high degree of (temporal) plasticity in the use of the feeding station in specific months. Individual rank within dominance hierarchy revealed sex-dependent effects of social status on resource preference in breeding adults, illustrating the potential complex social mechanisms underpinning status-dependent resource use patterns. Our results show that predictable food at feeding stations may lead to broad-scale patterns of resource partitioning and affect both the foraging and social dynamics within local vulture populations.
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Groups of animals (including humans) may show flexible grouping patterns, in which temporary aggregations or subgroups come together and split, changing composition over short temporal scales, (i.e. fission and fusion). A high degree of fission-fusion dynamics may constrain the regulation of social relationships, introducing uncertainty in interactions between group members. Here we use Shannon's entropy to quantify the predictability of subgroup composition for three species known to differ in the way their subgroups come together and split over time: spider monkeys (Ateles geoffroyi), chimpanzees (Pan troglodytes) and geladas (Theropithecus gelada). We formulate a random expectation of entropy that considers subgroup size variation and sample size, against which the observed entropy in subgroup composition can be compared. Using the theory of set partitioning, we also develop a method to estimate the number of subgroups that the group is likely to be divided into, based on the composition and size of single focal subgroups. Our results indicate that Shannon's entropy and the estimated number of subgroups present at a given time provide quantitative metrics of uncertainty in the social environment (within which social relationships must be regulated) for groups with different degrees of fission-fusion dynamics. These metrics also represent an indirect quantification of the cognitive challenges posed by socially dynamic environments. Overall, our novel methodological approach provides new insight for understanding the evolution of social complexity and the mechanisms to cope with the uncertainty that results from fission-fusion dynamics.
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Long-lived avian scavengers are threatened worldwide and thus, are common targets of conservation plans. However, scientific evidence of both the factors limiting populations and effectiveness of management actions are greatly needed in order to develop more efficient and successful conservation strategies. We assessed the effectiveness of conservation actions applied within a LIFE-Nature project aimed at improving the long-term survival of the critically endangered Canarian Egyptian vulture: including education campaigns for public awareness and control of illegal poisoning and the modification of power lines to reduce the risk of accidents. We formulated a multievent capture–mark–recapture model to obtain estimates of survival for juvenile, subadult and adult birds accounting for probabilities of resight, recovery and losses of metal and colour rings. Models supported a substantial enhancement in survival for subadult and adult birds and a moderate improvement for juveniles after the implementation of LIFE actions. Ring loss probabilities became notably high in the middle to long term. Poisoning events became very rare after LIFE was implemented, suggesting a positive effect of environmental education and awareness campaigns. Entanglements and collisions in power lines were also efficiently mitigated. Instead, electrocutions became the most identified cause of death in the post-LIFE stage. Synthesis and applications. Our results highlight the improvement of survival in a threatened island vulture population after the implementation of a European LIFE conservation project. On small islands, with small human populations and few stakeholders, education and awareness campaigns can be especially effective for biodiversity conservation. We also demonstrate the need to complement conservation programmes with long-term monitoring, which is essential to evaluate the effectiveness of actions, especially for long-lived species. © 2019 The Authors. Journal of Applied Ecology
Animal and human societies exhibit extreme diversity in the size, composition and cohesion of their social units, in the patterning of sex-specific reproductive skew, in the nature of parental care, in the form and frequency of cooperation and in their competitive regime, creating a diversity of socially complex societies. However, there is an ongoing debate about whether social complexity is a real, emergent property of a society or whether it only provides a conceptual framework for studying the diversity and evolution of societies. In this introduction to our topical collection, we identify three areas of current research addressing relevant challenges in the study of social complexity. First, previous studies have suffered from a lack of a common conceptual framework, including shared definitions, and existing measures of social complexity do not acknowledge its multiple components and dimensions. Second, most previous studies have ignored intraspecific variation, and the proximate and ultimate determinants of variation in social complexity, as well as their interactions, remain poorly known. Third, comparative studies of social complexity offer opportunities to explore its biological causes and correlates and but it is frequently difficult to identify the causal relationships involved and the development of general insights has been hampered by conceptual and methodological difficulties. In this paper, we briefly characterize these three challenges and offer guidance to the other contributions to this topical collection on social complexity by placing their key results in the context of these three topics. Significance statement In this paper, we address the main current challenges in the study of social complexity and combine our brief overview with a synopsis of the other contributions to this topical collection on various aspects of social complexity.