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Observations on alloparental care of fledglings
in Osprey Pandion haliaetus (Aves, Pandionidae)
MANUEL SIVERIO1, FELIPE SIVERIO2& BENEHARO RODRÍGUEZ3
1C/ Constitución 17-3, E-38410 Los Realejos, Tenerife, Canary Islands, Spain
(mansiverio@telefonica.net)
2Los Barros 21, E-38410 Los Realejos, Tenerife, Canary Islands, Spain
(felipe.siverio@telefonica.net)
3La Malecita s/n, E-38480 Buenavista del Norte, Tenerife, Canary Islands, Spain
(benerguez@terra.es)
SIVERIO, M., F. SIVERIO & B. RODRÍGUEZ (2011). Observaciones de atención aloparental de pollos
volanderos en el águila pescadora Pandion haliaetus (Aves, Pandionidae). VIERAEA 39: 105-110.
RESUMEN: Describimos la adopción y la atención aloparental de pollos
volanderos por una pareja de águila pescadora, durante dos temporadas de
cría consecutivas (2008-09), en la pequeña población semicolonial y rupícola
de Tenerife (islas Canarias). Las polladas intrusas (dos pollos volanderos
cada año) cambiaron de nido pocos días después de volar (≤ 10 días en 2008
y ≤ 6 en 2009). Usaron el nido receptor y los posaderos junto con las polladas
genéticas, y fueron alimentadas por la pareja adoptiva. La corta distancia
entre nidos vecinos y unas características orográficas ventajosas del territorio
receptor parecen ser las causas principales que motivaron estos comporta-
mientos.
Palabras clave: águila pescadora, Pandion haliaetus, adopción, atención alo-
parental, nidificación semicolonial, Tenerife, islas Canarias.
ABSTRACT: We describe the adoption and alloparental care of fledglings
by an Osprey pair during two consecutive breeding seasons (2008-09) in the
small and semi-colonial cliff nesting population of Tenerife (Canary Islands).
The intruding broods (two fledglings each year) switched nest a few days
after fledging (≤ 10 days in 2008 and ≤ 6 in 2009). They used the recipient
nest and perches together with the genetic broods, and received alloparental
feeding. The short distance between neighbouring nests and advantageous
terrain characteristics of the recipient territory seem to be the main reasons
that led to these behaviours.
Key words: Osprey, Pandion haliaetus, adoption, alloparental care, semi-
colonial nesting, Tenerife, Canary Islands.
VIERAEA Vol. 39 105-110 Santa Cruz de Tenerife, octubre 2011 ISSN 0210-945X
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106 SIVERIO ET AL.
INTRODUCTION
Adoption and alloparental care is a phenomenon present in many animal species
(Riedman, 1982). In birds, several hypotheses have been raised to explain facultative nest
switching in fledglings, among others these include: distance between nests (Bustamante
& Hiraldo, 1990; Ferrer, 1993), inter-generational conflict (Pierotti & Murphy, 1987; Re-
dondo et al., 1995; Berggren, 2006), kin selection (Poole, 1982), reciprocal altruism
(Pierotti, 1980), and reducing the ectoparasite exposure through redistribution among adop-
tive family members (Bize et al., 2003).
Records of natural adoption and alloparental care in raptors include both diurnal
(Donázar & Ceballos, 1990; Arroyo & García, 2002) and nocturnal species (Roulin, 1999;
Penteriani & Delgado, 2008). This behaviour occurs not only in colonial or semi-colonial
birds of prey (Donázar et al., 1991; Arroyo & García, 2002), but also in species whose nests
are usually widely isolated (Donázar & Ceballos, 1990; Ferrer, 1993). In general, the altricial
state of most raptor nestlings restricts this behaviour, although in dense colonies, such as
those of the Lesser Kestrel (Falco naumanni; nesting on tiled roofs), nestlings are able to
walk to neighbouring nests before fledging (Tella et al., 1997).
The few studies that have addressed this behaviour in Osprey (Pandion haliaetus)
generally suggest that the younger fledgling (the subordinates) of large broods are the ones
that move to a neighbouring nest (with a younger brood) where they can become dominant
(Poole, 1982; Gilson & Marzluff, 2000). In this paper we describe the natural adoption of
intruding fledglings by an Osprey pair in a neighbouring territory during two consecutive
years. We also discussed the possible scenarios involved, and warn of the errors that could
result if population monitoring failed to detect these events.
STUDY AREA AND METHODS
We collected data at the sea cliffs of the Teno massif, NW of Tenerife Island (Canarian
archipelago, 27°37’-29°25’ N and 13°20’-18°19’ W), during the post-fledging dependence
period in the 2008 and 2009 breeding seasons. We recorded observations as part of a long-
term research project monitoring density, laying phenology, hatching success and nesting
success of the Osprey population in this area (Siverio, 2006; Siverio, 2008). Over two years
of study the population has ranged between four and five pairs, and the average distance
between neighbouring nests was 1700 m (range 687-3385, n= 4 pairs) and 1098 m (range
388-3000, n = 5 pairs), respectively. As in much of the Osprey’s range, these nearest-neigh-
bour distances also reveal the trend of this rupicolous population towards semi-colonialism
(Poole, 1989).
We conducted three observation sessions during the post-fledging dependence period,
one in the first year (16 June 2008; 09:10-13:35 h GMT) and two in the second (18 and 27
June 2009; 10:00-16:00 and 08:30-15:30, respectively), over a total of 17.25 hr. Observa-
tions were made using binoculars and telescopes (20x-60x) from the top of the cliff where
there is an optimal visibility of the nest (about 200 m distant) and of the commonly used
perches of the recipient territory, as well as a large segment of the emitting territory. The
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ATENCIÓN ALOPARENTAL EN EL ÁGUILA PESCADORA 107
genetic broods of the recipient pair were marked with metal and colour rings with alpha
numeric code (intruding broods were not marked), and the recipient territory pair was indi-
vidually identified by the plumage pattern on the pileus (Bretagnolle et al., 1994). Each ob-
servation session was taken as a sampling unit, and the attack rate (AR) was calculated by
dividing the number of attacks (aggressive intraspecific interactions) between the hours of
observation (see Margalida & Bertran, 2005).
RESULTS
The recipient nest and the intruders’ natal nest (687 m apart) were in the same location
in 2008 and 2009, and were the only successful nests in the two nesting seasons. In 2008
and in the first observation session of 2009, the intruding broods (two fledglings each year)
and the genetic broods of the recipient pair (one in 2008 and two in 2009) were about 70
and 65 days (d) old, i.e. about 10 and 6 d after fledging, respectively. During these two ob-
servation periods the intruding fledglings were recorded in the nest of the recipient pair for
an average of 50 min (± 59 SD; range 3-180, n= 9; 43% of the whole observation time).
During the rest of the time the intruders remained in the recipient territory, either in flight
or on well defined perches. No agonistic behaviours were recorded between intruder and
genetic fledglings, neither when they were together in the recipient nest (both broods), nor
when they used communal perches.
In general, the recipient pair tolerated the presence of the intruders, with just two in-
cidents recorded (one by the male and another by the female) of agonistic behaviour toward
the intruder fledglings (AR = 0.13); this persecution lasted 3.20 min, 0.3% of the total ob-
servation time. Alloparental feeding was recorded in both years. In 2008, the two intruders
were fed at the same time as the genetic fledgling, and in 2009 (second observation session)
an intruder was observed to be fed at the same time as the genetic fledgling. During the
three observation periods the recipient pair defended the territory when the genetic parents
of the intruders flew into it (AR = 0.4 ± 0.2 SD), investing in this 1.34 min on average (±
2.53 SD; range 0.5-8; n = 7).
DISCUSSION
Given that the Osprey is generally a semi-colonial bird (Poole, 1989), fledgling nest-
switching behaviour observed in Tenerife appears to be related to the proximity of the emit-
ter and recipient nests (687 m apart). Distance between nests was also considered as one of
the main motivations for nest-switching by fledglings in North America (Poole, 1982;
Gilson & Marzluff, 2000) and on the island of Menorca, Mediterranean Sea (R. Triay, pers.
comm.). In studies of other raptors that normally nest in isolation, e.g. Red Kite (Milvus
milvus), Imperial Eagle (Aquila adalberti), Egyptian Vulture (Neophron percnopterus), and
Eagle Owl (Bubo bubo), nest-switching also occurs when population densities are high and
distances between neighbouring nests are reduced (Bustamante & Hiraldo, 1990; Donázar
& Ceballos, 1990; Ferrer, 1993; Penteriani & Delgado, 2008).
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108 SIVERIO ET AL.
In studies that have addressed adoptions in Osprey, most nests involved (emitters and
recipients) were on artificial platforms (Gilson & Marzluff, 2000), usually less than 10 m
hight and in open habitats where neighbouring nests are in line-of-sight (Poole, 1982). Such
situations of proximity would facilitate nest-switching, perhaps due to the stimulus that pro-
duces on fledglings food deliveries seeing in neighbouring nests (Poole, 1982). The nests
under study on Tenerife are not within sight of each other, but peculiarities of the recipient
territory terrain may be more favorable for intrusions to occur. The two territories are in the
same bay, facing SW, with a high insolation level after midday. However, the intruders’
natal nest is 100 m above sea level (a.s.l.) at one end of the bay on a vertical cliff (with no
beach at the base) and more than 300 m in height. In contrast, the recipient nest is situated
in the centre of the bay, at 50 m a.s.l., on a cliff (with a beach on the base) which is less ver-
tical and lower in height, and it is dissected by the perpendicular mouth of a ravine. This
territory has many more shaded perching areas which appear to have a strong attraction for
the broods (intruder and resident) and the foster adults, where they spend long periods of
time. In this regard, before the subject nests of this study had been built, in 2006, ringed
fledglings from nearby territories used these same shaded perches, including the natural
ledge where the recipient nest is now placed. One fledgling came from a nest at about 1200
m away, whereas the other two were from a nest 300 m away (M. Siverio, unpubl. data).
The growth phase of the intruder nestlings before fledging was apparently adequate,
so it is unlikely that our examples of nest-switching were related to intergenerational con-
flict (poor nest provisioning), i.e. when an inadequate amount of food is offered to a
nestling by its parents this may induce it to beg for food at a foreign nest (Pierotti & Mur-
phy, 1987). Moreover, the adoptions occurred when intruders had been flying ≤ 10 d in
2008 and ≤ 6 in 2009, suggesting that their parents were still feeding them frequently. The
intruders, on the other hand, could not behave as dominants, because fledglings from the
recipient nest were of the same age and because dominance behaviour is more likely to
occur when they are older than the recipient nest fledglings (Poole, 1982; Bustamante &
Hiraldo, 1990; Redondo et al., 1995; Gilson & Marzluff, 2000). The adoption, in our study,
of a whole brood by a pair with young of the same age would supposedly diminish the
food intake of all fledglings (intruders and genetic). However, this was not reflected in the
external appearance or behaviour of the four fledgings during the third observation session
in 2009 (nine days after the second), leading to suspicion that the genetic parents of the
intruders also may feed both their own and other local fledglings as they were seen carrying
prey near the recipient nest.
Generally, Osprey pairs seem to recognize their own descendants, tolerating fledg-
lings that are not their own, but defend their territory from conspecific adults (Poole, 1982).
The foster pair of our study showed no abnormal behaviour towards their genetic fledg-
lings, but on two occasions attacked the intruders (e.g. the female chased the two intruders
at the same time). However, alloparental feeding, AR and the limited time spent in these
chases suggest that a high level of tolerance prevails. Moreover, on one occasion the foster
female spent an hour at the nest near an intruder while it was eating, and on another occa-
sion the foster male delivered prey to an intruder fledgling in response to begging when it
was perched in the recipient nest. We do not know if this tolerance is associated with the
degree of relatedness between the intruders and the recipient pair (kin selection hypothesis),
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ATENCIÓN ALOPARENTAL EN EL ÁGUILA PESCADORA 109
but it seems conceivable, given the high level of philopatry in this species (Poole, 1989;
Dennis, 2007). However, this is potentially a maladaptive reproductive error for the foster
pair (Gilson & Marzluff, 2000), and it is more likely that adoption is the result of a “con-
formity” or instinctive response behaviour by these adults, since the energy cost invested
in frequent chases towards the intruders would not be compensated (see Penteriani & Del-
gado, 2008).
Although Bustamante (1995) found no adoptions or any type of alloparental care over
35 years monitoring an Osprey nest, these behaviours may occur more frequently than
thought (Poole, 1982; Gilson & Marzluff, 2000). Therefore, in populations that are poorly
monitored, undetected cases of alloparental care could lead to errors in the calculation of
nest success. Especially if fledglings are not ringed or nests are not regularly checked before
fledging, i.e. casual observation of the number of fledglings at a nest does not necessarily
infer that all are at their natal nest.
ACKNOWLEDGMENTS
We would like to thank Terry E. Dennis, Airam Rodríguez and Karen Steenhof for
their suggestions which contributed to improve the manuscript. Thanks also to Rubén
Barone and Guillermo Delgado for their comments to the final text.
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Fecha de recepción: 23 junio 2011 Fecha de aceptación: 13 julio 2011
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