Introduced predators and nest competitors shape distribution and breeding performance of seabirds: feral pigeons as a new threat

Abstract

Petrels are particularly sensitive to predation by introduced species. Many populations have reduced their breeding ranges, currently mainly occupying predator-free sites. Breeding range reduction leads to interspecific competition for nesting sites, which can be detrimental to petrels. Here, we evaluate how the presence of introduced mammals (cats Felis catus and rats Rattus spp.) and potential competitors for nest sites (Cory’s shearwaters Calonectris borealis and feral rock pigeons Columba livia ) shape the distribution, breeding density, and breeding performance of Bulwer’s petrel Bulweria bulwerii on Tenerife, the largest and most densely human populated of the Canary Islands. We estimated nest density, assessed the role of nest location and physical characteristics of nests on breeding success, and determined causes of breeding failure by introduced predators and competitors. Nest density was higher in predator-free colonies on marine rocks. Cat presence was the best predictor of nest density, but it was not correlated with either presence or abundance of competitors. Breeding success varied between years and colonies but was not related to nest characteristics. Pigeon competition for nests was the most frequent cause of breeding failure (7.3%), followed by rat predation (6.3%). We also compared petrel and pigeon nest cavities and found considerable overlap in the physical size of nest sites. Our study provides insights into an overlooked impact of the invasive rock pigeon: nest competition with small seabirds. We encourage more research on the effects of pigeons on nest density, as well as disease and pathogen transmission, and vegetation changes within seabird colonies.

Full text

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Biol Invasions (2022) 24:1561–1573
https://doi.org/10.1007/s10530-022-02746-1
INVASION NOTE
Introduced predators andnest competitors shape
distribution andbreeding performance ofseabirds: feral
pigeons asanew threat
BeneharoRodríguez · AiramRodríguez ·
FelipeSiverio · JuanM.Martínez·
EnriqueSacramento· YarciAcosta
Received: 21 May 2021 / Accepted: 27 January 2022 / Published online: 23 February 2022
© The Author(s) 2022
causes of breeding failure by introduced predators
and competitors. Nest density was higher in predator-
free colonies on marine rocks. Cat presence was the
best predictor of nest density, but it was not correlated
with either presence or abundance of competitors.
Breeding success varied between years and colonies
but was not related to nest characteristics. Pigeon
competition for nests was the most frequent cause
of breeding failure (7.3%), followed by rat predation
(6.3%). We also compared petrel and pigeon nest cav-
ities and found considerable overlap in the physical
size of nest sites. Our study provides insights into an
overlooked impact of the invasive rock pigeon: nest
competition with small seabirds. We encourage more
research on the effects of pigeons on nest density, as
well as disease and pathogen transmission, and veg-
etation changes within seabird colonies.
Keywords Breeding colony· Conservation·
Competition· Predation· Procellariiformes· Rock
dove· Seabird
Introduction
Seabirds and, particularly, petrels (order Procellari-
iformes) occur in vast ocean areas, but their breeding
colonies are usually restricted to islands that are mark-
edly limited in area in comparison with their large
pelagic feeding grounds (Brooke 2018). The reduced
breeding range is thought to be a consequence of
Abstract Petrels are particularly sensitive to preda-
tion by introduced species. Many populations have
reduced their breeding ranges, currently mainly occu-
pying predator-free sites. Breeding range reduction
leads to interspecific competition for nesting sites,
which can be detrimental to petrels. Here, we evaluate
how the presence of introduced mammals (cats Felis
catus and rats Rattus spp.) and potential competitors
for nest sites (Cory’s shearwaters Calonectris borea-
lis and feral rock pigeons Columba livia) shape the
distribution, breeding density, and breeding perfor-
mance of Bulwer’s petrel Bulweria bulwerii on Ten-
erife, the largest and most densely humanpopulated
of the Canary Islands. We estimated nest density,
assessed the role of nest location and physical charac-
teristics of nests on breeding success, and determined
Supplementary Information The online version
contains supplementary material available at https:// doi.
org/ 10. 1007/ s10530- 022- 02746-1.
B.Rodríguez· F.Siverio· E.Sacramento
Canary Islands’ Ornithology andNatural History Group
(GOHNIC), BuenavistadelNorte, CanaryIslands, Spain
B.Rodríguez· J.M.Martínez· Y.Acosta
Sociedad Española de Ornitología (SEO/BirdLife),
Delegación de Canarias, LaLaguna, CanaryIslands, Spain
A.Rodríguez(*)
Terrestrial Ecology Group (TEG-UAM), Department
ofEcology, Universidad Autónoma de Madrid, Madrid,
Spain
e-mail: airam.rodriguez@uam.es
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predation. Predation pressure is generally lower on
islands than on the mainland, although humans have
introduced many predators worldwide (Mulder etal.
2011). Petrels are in decline mainly due to preda-
tion by introduced specieson their breeding grounds,
being one of the most endangered avian groups (Rod-
ríguez etal. 2019; Dias etal. 2019). So, invasive spe-
cies drive petrel local extinctions and restrict breed-
ing petrel distribution to predator-free sites, such as
islets or sea cliffs (Spatz etal. 2014, 2017).
The breeding density of birds is usually limited by
the availability of nesting sites and, for cavity-nesting
bird species, cavities are a limited resource (Newton
1994). Many seabirds, especially petrels, nest under-
ground, and share nesting grounds with other spe-
cies. Under these circumstances, sympatric species
fiercely compete for favourable nesting sites, with
the smaller species usually defeated (Cory’s shearwa-
ter Calonectris borealis vs. Bulwer’s petrel Bulweria
bulwerii, Macaronesian shearwater Puffinus baroli,
and band-rumped storm-petrel Hydrobates castro;
Ramos etal. 1997; Wedge-tailed shearwater Ardenna
pacifica vs. Tahiti petrel Pseudobulweria rostrata;
Villard etal. 2006; Streaked shearwater Calonectris
leucomelas vs. H. castro and Swinhoe’s storm-petrel
Hydrobatesmonorhis; Sato etal. 2010; Bonin petrel
Pterodroma hypoleuca and A. pacifica vs. Tristam’s
storm-petrel Hydrobates tristami; McClelland et al.
2008), although differences in microhabitat or nest
site characteristics can partially alleviate interspecific
competition (Sullivan and Wilson 2001; Bourgeois
and Vidal 2007; Troy etal. 2016).
Our knowledge on interspecific interactions
between petrels and other non-petrel native spe-
cies is limited. Evidence suggests that competition
for nesting holes has little effect on petrels under
natural conditions, i.e. without cumulative effects
of other anthropogenic threats (Rodríguez et al.
2019). However, when native species abundance
increases as a result of human actions, such as pro-
vision of supplementary food or nesting habitat,
interspecific competition can be detrimental for
threatened petrel populations, e.g. the nest compe-
tition of white-tailed tropicbird Phaethon lepturus
hampered the recovery of the endangered Bermuda
petrel Pterodroma cahow (Madeiros et al. 2012).
Future research on interspecific interactions must be
further intensified and focused on identifying prob-
lematic species for endangered petrels, because this
basic information is still unavailable for many of
them (Rodríguez etal. 2019).
In the Canary Islands, seven procellariiform spe-
cies breed currently (Bulwer’s petrel B. bulwerii,
Cory’s shearwater C. borealis, Manx shearwa-
ter Puffinus puffinus, Macaronesian shearwater P.
baroli, European storm-petrel Hydrobates pelagicus,
band-rumped storm-petrel H. castro and white-faced
storm-petrel Pelagrodroma marina). All of them are
threatened with declining small populations, except
Cory’s shearwater (Lorenzo 2007). Small-sized spe-
cies show small and patchily distributed breeding
colonies, mainly located on predator-free islets and
marine rocks which are difficult to access (Lorenzo
2007). Due to the limited surface of these breeding
areas, competition between birds for nest sites may
be intense as in other nearby locations (Ramos etal.
1997; Bried and Bourgeois 2005).
The rock pigeon Columba livia is an invasive spe-
cies introduced worldwide (CABI 2008), which has
been eradicated from the Galápagos Islands to protect
its unique biodiversity applying the “precautionary
principle” (Phillips etal. 2012). This species, native
to Europe and the Canary Islands (geographically
closer to Africa than mainland Europe), has suffered
a genetic introgression with domestic pigeons dur-
ing the last few centuries (Johnston and Janiga 1995;
Lorenzo 2007; Giunchi et al. 2020). Thanks to its
high reproductive rates and behavioural plasticity,
feral rock pigeons can become pests (Johnston and
Janiga 1995). The negative ecological impacts of
feral rock pigeon populations include transmission
of infectious diseases and pathogens (Foronda etal.
2004; Burt etal. 2018; Mori etal. 2019), changes in
chemical soil properties by means of the acidity of its
excrement (Spennemann and Watson 2017), and com-
petition with native species for resources such as food
or nesting sites (Forero etal. 1996; Hernández-Brito
etal. 2014). Despite its impacts, field studies focusing
on their role as nest competitors are few (e.g. Forero
etal. 1996; Hernández-Brito etal. 2014; this study).
Here, we assess the role of feral rockpigeons on
the nesting density and breeding performance of a
small seabird, Bulwer’s petrel, on a densely human-
populated island of the Canary Islands. To compare
the impact of pigeons with that of introduced preda-
tors and other nest competitors, we also evaluated
the presence of cats Felis catus and rats Rattus spp.,
and the abundance of native Cory’s shearwaters.
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During 2017–2020, we (1) estimated Bulwer’s pet-
rel nest density in colonies, (2) assessed the impact
of nest location and physical characteristics of nests
on breeding success, and (3) quantified causes of
breeding failure as a result of introduced predators
and native competitors. We hypothesized that at sites
where invasive species occur, i.e. cats and rats, nest
density and breeding success would be lower than
at sites free of introduced predators. Given that the
potential nest competitors studied here, i.e. pigeons
and shearwaters, are benefited by the absence of
introduced predators, i.e. cats and rats, we hypoth-
esized that nest competition could lead to breeding
failure at predator-free sites. We also compared physi-
cal characteristics of nest cavities used by petrels
and pigeons to assess nesting niche segregation. We
predict that some overlap occurs between the nesting
niches of these two species.
Materials andmethods
Study area and species
The study was conducted on Tenerife, the largest
and the highest (2034 km2 and 3718 m above sea
level—a.s.l.) of the Canary Islands (Fig. S1). The
coastline (342km) is predominantly rocky with boul-
der shores, cliffs up to 400m high, and small marine
rocks on the north coast. The climate is subtropical
and oceanic. Oceanographic conditions in this archi-
pelago are influenced by northeast trade winds and
marine upwelling that occurs off the north-west Afri-
can coast. In 2019, the local human population was
approximately 949,000 inhabitants, but as the island’s
economy is highly dependent upon the tourism, each
year several millions of visitors are received (Instituto
Canario de Estadística, www. gobie rnode canar ias. org/
istac/).
Bulwer’s petrel is a small pelagic procellariiform
(75–130g), with a disjointed pan-oceanic distribu-
tion in tropical and subtropical waters of the Pacific,
Indian, and Atlantic Oceans. Canarian Bulwer’s pet-
rels visit breeding grounds from April to September
and nest mainly in small crevices, caves, and holes
under rocks, usually close to the sea, where they lay
a single egg per breeding attempt in late May-early
June (Hernández et al. 1990; Martín and Lorenzo
2001). Chicks fledge mainly during the second fort-
night of September (Martín and Lorenzo 2001).
Cory’s shearwater is the largest (700–800g) and
most abundant procellariiform species in the Canary
Islands (Lorenzo 2007). It is distributed extensively
on the island under 1000m a.s.l. and is well known
to compete for nests with Bulwer’s petrels on other
nearby colonies (Ramos etal. 1997; Bried and Bour-
geois 2005). Cory’s shearwaters visit their breed-
ing colonies from March to November. Single-egg
clutches are laid in early June and chicks fledge from
late October to early November (Martín and Lorenzo
2001).
In the Canaries, the feral rock pigeon is abun-
dant and distributed widely in rural and urban envi-
ronments, but especially in coastal sectors (Lor-
enzo 2007). This sedentary and medium-sized bird
(230–370g) uses holes and crevices on sea cliffs and
marine rocks as roosting and breeding sites (Lorenzo
2007). Rock pigeon clutch size is 1–2 eggs and pairs
can make several breeding attempts per breeding sea-
son. Its breeding period is concentrated in spring and
summer, but pigeons can breed throughout the year
(Martín and Lorenzo 2001). Nowadays in Tenerife,
most rock pigeons seen in the wild resemble birds of
domestic origin (authors’ pers. obs.). Studies con-
ducted on Tenerife indicate that feral rock pigeons
could be vectors of pathogens for the endemic
pigeons, Bolle’s pigeon Columba bollii and the white-
tailed laurel pigeon Columba junoniae (Foronda etal.
2004; Abreu-Acosta etal. 2009).
Two species of rats (black rat Rattus rattus and
brown rat R. norvegicus) and feral domestic cats
Felis catus have been introduced in Tenerife sincefif-
teenth century (Nogales et al. 2006). Rats and cats
are widely distributed on the main island, but they
are absent on the marine rocks (Nogales etal. 2006;
authors’ pers. obs.). Introduced mammals intensively
depredate eggs, chicks, and adults, which lead to
seabird population declines worldwide (Spatz etal.
2017).
Nesting density, nest characteristics and breeding
performance
During the 2017–2020 breeding seasons (June-early
September), we visited all known Bulwer’s pet-
rel nesting sites (Hernández et al. 1990; Lorenzo
2007). Other sites with the potential to accommodate
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colonies (apparently free of introduced predators and
with suitable holes) were also visited. We visited 43
sites, spending a total of 218h/observer of effective
prospection (Fig. S1; Table S1). During daylight
hours, we counted the number of Bulwer’s petrel
and Cory’s shearwater nests in places accessible on
foot using a hand-held flashlight to inspect holes and
crevices. We also recorded the presence or absence
of feral rock pigeons, rats, and feral cats according to
field signs (individuals, feathers, nests, prey remains,
or droppings). Studied site edges, including those
without petrels, were delineated and plotted using
landscape features in aerial photographs, maps, and
a geographic information system (Qgis v3.14.16).
Four inaccessible sites were inspected using a ther-
mal monocular Helion XP50 (Pulsar, Lithuania) from
vantage points (80–130 m) to estimate the number
of nests according to attendance behaviour of adults.
Bulwer’s petrel and feral rockpigeon nests from five
petrel colonies were described with nine variables
(Table1).
We assessed breeding success of 209 breeding
attempts (2018, n = 71; 2019, n = 70; 2020, n = 68)
in five colonies, four of them located on marine rocks
(without introduced predators) and one on a coastal
cliff (Fig. S1). Each nest was visited at least twice: to
confirm incubation in late June or early July and to
record the presence or absence of a full-grown chick
in late August or early September. We assumed that
nestlings observed during the second visit would
fledge successfully.
To identify causes of breeding failure, the nest and
its surroundings were inspected, paying special atten-
tion to the presence of unhatched eggs, eggshells,
dead chicks, droppings, and signs of introduced pred-
ators. Pigeon nests at sites (e.g. crevices, caves, holes,
or burrows) previously used by petrelsfor nesting, i.e.
in the previous breeding season or early in the same
breeding season, were considered breeding failures.
Statistical analysis
To assess the role of the colony location (main island
vs. marine rock), presence of predators (cats and
rats), and nest competitors (pigeons and Cory’s shear-
waters) on the density of Bulwer’s petrel nests, we ran
Linear Models (LMs). We did not include year as a
response variable as nest density is relatively consist-
ent among years and we had just a density estimate
to many colonies. Given our limited sample size (43
sites), we compared LMs including just a single pre-
dictor. Then, we ranked the models according to its
AICc value (the lower AICc, the better the model),
including a null model containing just the intercept.
To test for potential differences in breeding success
(0 = failure; 1 = success) among colonies and years,
we ran Generalized Linear Mixed Models (GLMMs)
with family binomial (link = logit). We included nest
identity as a random factor, given that the same nests
were studied each breeding season, and year and
colony as predictors. We ranked competitive models
according to AICc.
To assess potential relationships between breeding
success (0 = failure; 1 = success) and nest features,
we ran GLMMs with family binomial (link = logit).
We included nest identity as a random factor and
used seven variables describing nests (height, width,
depth, chamber height, vertical difference height,
Table 1 Variables used to
describe nests of Bulwer’s
petrels Bulweria bulwerii
and feral rockpigeons
Columba livia on Tenerife,
Canary Islands
Variable Description
Nest type Proportion of nest cavity sides bounded by rock: all rock, rock crev-
ice; soil bed, soil crevice; and all soil, burrow
Entrance height Minimum height of the nest entrance (cm)
Entrance width Minimum width of the nest entrance (cm)
Depth Maximum length from entrance to the nesting chamber (cm)
Chamber height Height of the nesting chamber (cm)
Difference in heights Difference in height from the entrance and the nesting chamber (cm)
Entrance curvature In degrees (0–90°), curve between the entrance and the chamber
Slope In degrees (0–180°), angle measured in a 1 m radii from the nest
entrance considering vertical up as 0°
Nest material Absent (0), Present (1)
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curved, and slope; see Table1 for further details) as
predictors in univariate models. We ran all competing
GLMMs including just a single factor or variable, as
well as the null model including just the intercept, to
rank them according to the AICc value.
To test for potential differences between the nest
sites of petrels and pigeons, we ran permutations tests
and plotted the data on the seven continuous vari-
ables of Table1 describing physical characteristics of
the nests. In addition, we ran a Principal Component
Analysis (PCA) to assess the nesting niche. Variables
were zero centered and scaled to have unit variance
before the PCA, as they were taken in different scales
and units and showed different variance.
Statistical analyses were run in R (v4.0.4). Models
were run using the glm.nb and glmer functions of the
MASS and lme4 packages. Models were compared to
null models using the AICc function (MuMIn pack-
age) and assumptions were checked using diagnostic
plots. We used the R package coin to conduct the per-
mutation tests and an alpha threshold of 0.05. PCA
was conducted by using the prcomp function in the
stats package.
Results
Distribution and nest density
Most nests (86%) were on marine rocks off the north
coast of the island where they were at the highest den-
sity (TableS1; Fig.1). Nest density was higher in col-
onies without predators like cats or rats, but it was not
related to nest competitors such as feral rockpigeons
or Cory’s shearwaters (Fig. 2). The linear models
indicated that the presence of cats and rats, and col-
ony location (mainland island vs. marine rocks) were
the best predictors of nest density (Table2; Estimates
± S.E. and 95% CIs: Presence of cats = − 6.93 ±
2.91 [−12.81, −1.04]; Presence of rats = −6.83 ±
2.99 [−12.88, −0.79]; Marine rocks = 6.36 ± 2.99
[0.33, 12.40].
Breeding success and causes of breeding failure
Breeding success varied among colonies and years
(Table2 and TableS2). According to the AICc val-
ues of the competitive models, the model containing
colony and year as predictors obtained the lowest
AICc value (Table2; see estimates ± S.E. and 95%
CIs for simple terms in TableS3). The colony located
on the main island had the lowest breeding success,
where no chicks fledged in 2019 and 2020 breed-
ing seasons (Table S2). According to the GLMMs
explaining the breeding success in relation to physical
characteristics of nests, any of our studied nest vari-
ables were related to breeding success (Table2). The
null model obtained the lowest AICc value (Table2).
Breeding failure was hard to determine on the
ground. Thus, the cause of breeding failure in most
breeding attempts was unknown (75.0%). At least
seven breeding attempts (7.3%) failed due to Bulwer’s
petrels being out-competed by feral rockpigeons for
nest cavities. Such breeding failure was confirmed
in three colonies on marine rocks (Garachico and
both of San Juan de La Rambla; #22, 28, and 29 in
Table S1). At least 6.3% of the breeding attempts
failed due to rat predation, all of them located on
Los Pedrones on the main island (#14 in TableS1),
although most of failed breeding attempts may have
been caused by predation (Table3).
Fig. 1 Box-and-whisker plots of the nesting density of Bul-
wer’s petrels Bulweria bulwerii on the main island and on
marine rocks of Tenerife, Canary Islands. Dots have been jit-
tered for a better visualization. The line within boxes indicates
the median, the bottom and top of the box represent the first
and third quartiles, and the whiskers extend 1.5 times the inter-
quartile range
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Bulwer’s petrel and feral rockpigeon nest
characteristics
A considerable overlap in features describing Bul-
wer’s petrel and pigeon nests was observed accord-
ing to the Principal Component Analysis, indicat-
ing both species share part of their nesting niches
(Fig.3; TableS4). PC1 retained 33.5% of the varia-
tion and was highly correlated with entrance height
(− 0.89), chamber height (− 0.74), depth (− 0.65),
and entrance width (− 0.54). PC2 retained 16% of the
variation and was highly correlated with difference
in heights (− 0.71), entrance curvature (− 0.52), and
entrance width (− 0.51) (TableS4). Five out of the
seven variables describing the petrel and pigeon nests
reached significant differences (Table S5, Fig. S2).
Petrel nests had smaller entrances (i.e. lower height
and width) and smallernesting chambers (i.e. height).
Petrel nest entrances were more curved than those
of pigeon nests, and slopes were steeper in pigeon
nests. Depth and vertical difference in height between
entrance and nest chambers were similar in the two
species’ nests (TableS5, Fig. S2).
Discussion
Our results support our hypotheses and indicate that
the main factor shaping breeding distribution of small
procellariiforms, such as Bulwer’s petrel, is the pres-
ence of invasive mammals (Spatz etal. 2014, 2017).
In addition, nest competition with feral rockpigeons
was the main identified cause of breeding failure of
Bulwer’s petrels, highlighting an overlooked impact
of this species (which can become a pest or invasive
species depending on its native or introduced range)
on small seabirds. This was supported by the over-
lap in the nesting niches of both species. Contrary to
our predictions, our data do not provide any evidence
for impacts of Cory’s shearwaters on Bulwer’s petrel
nesting density or breeding success.
Rats and cats have been present in the Canary
Islands for several centuries (Nogales et al. 2006).
So, these predators have shaped the current breed-
ing distribution of Bulwer’s petrels, mainly relegat-
ing them to isolated predator-free marine rocks on
which dense colonies form (Hernández etal. 1990).
At predator-free sites, such as the Desertas Islands,
Fig. 2 Box-and-whisker
plots of the nesting density
of Bulwer’s petrels Bulwe-
ria bulwerii according to
the presence of introduced
predators (cats Felis catus
and rats Rattus sp.) and
competitors (feralrock
pigeons Columba livia
and Cory’s shearwaters
Calonectris borealis) on
Tenerife, Canary Islands.
Dots have been jittered for
a better visualization. The
line within boxes indicates
the median, the bottom and
top of the box represent
the first and third quartiles,
and the whiskers extend 1.5
times the interquartile range
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Madeira, breeding success ranges between 60 and
75% (Nunes and Vicente 1998), but in rat-invaded
colonies on the Bonin Islands, Japan, marked die-off
of breeding adults by predation can occur (Kawakami
etal. 2010). As a whole, breeding success in Tenerife
is low (54.1% of laid eggs produced a fledgling), but
we observed strong spatiotemporal variation. No
fledglings were produced during two consecutive sea-
sons on a coastal locality on the main island (#14 in
TableS1) where invasive rats were present. By con-
trast, in the predator-free sites more than 50% of the
breeding attempts produced a fledgling (TableS2).
Nest competition with feralrock pigeons was the
main identified cause of breeding failure of Bulwer’s
petrels. A minimum of 7.3% of Bulwer’s petrel breed-
ing attempts failed due to interactions with pigeons.
In those cases, we confirmed that pigeons occupied
petrel nests causing breeding failure (signified by the
presence of unattended eggs or dead chicks) or dis-
ruption to the petrel’s use of the nest sites. Similar
to small petrels, feralrock pigeons also benefit from
nesting and roosting on predator-free marine rocks to
avoid predation (authors’ pers. obs.). Other agonistic
interactions may occur between nesting or roosting
pigeons and breeding petrels. Aggressive interac-
tions might disrupt petrel nest attendance and trans-
mit pathogens and diseases, leading finally to petrel
breeding failure (Forero etal. 1996; Foronda et al.
2004; Hernández-Brito etal. 2014).
Although we failed to detect a clear relationship
between petrel nest density and pigeon competition,
Table 2 Results of the LMs and GLMMs explaining nest den-
sity and breeding success of Bulwer’s petrel Bulweria bulwerii
on Tenerife, Canary Islands, during 2018–2020 breeding sea-
sons. See main text for details
Predictor df AICc ∆AICc
Nest density
Presence of Felis catus 3 320.66 0.00
Presence of Rattus sp. 3 321.06 0.40
Colony location (main island or marine
rock)
3 321.70 1.04
Null model 2 323.89 3.24
Presence of Columba livia 3 326.13 5.47
Abundance of Calonectris borealis 3 326.13 5.47
Breeding success vs. colony and year
Colony * Year 16 279.33 0
Colony + Year 8 280.98 1.65
Colony 6 281.81 2.48
Null model 2 285.46 6.13
Year 4 285.74 6.41
Breeding success vs. nest traits
Null model 2 285.46 0
Difference in heights 3 286.21 0.75
Chamber height 3 286.87 1.41
Slope 3 287.07 1.61
Entrance curvature 3 287.19 1.73
Entrance height 3 287.43 1.97
Entrance width 3 287.51 2.05
Depth 3 287.51 2.05
Table 3 Causes of breeding failure of Bulwer’s petrel Bulwe-
ria bulwerii breeding attempts on Tenerife, Canary Islands,
during 2018–2020 breeding seasons
Causes of breeding failure (n = 96) n (%)
Unknown 72 (75.0)
Nest competition with pigeons 7 (7.3)
Predation by rats 6 (6.3)
Abandoned egg 4 (4.2)
Dead chick 4 (4.2)
Broken egg 2 (2.1)
Chick falling from nest site 1 (1.0)
Fig. 3 Scores on the second principal component axis (PC2)
plotted against those on the first principal component axis
(PC1) from a PCA carried out on seven continuous variables
describing Bulwer’s petrel Bulweria bulwerii (red) and feral
rock pigeon Columba livia (blue) nests on Tenerife, Canary
Islands, during 2018–2020 breeding seasons
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systematic breeding failure mediated by pigeon com-
petition for nest sites might deflate petrel nesting den-
sity to a point of local extinction. Field observations
support this reasoning. First, the highest nesting den-
sity of petrels was recorded on the small rock La Cor-
onela (#25 in TableS1), where no feral rockpigeons
were present. Second, on Roque La Playa (#33 in
TableS1), we found only six petrel nests in the same
area where a minimum of 18 nests were counted in
August–September 1983 (E. Hernández, com. pers.).
Furthermore, we confirmed the presence of seven
pigeon nests, three of them at sites previously used by
petrels (E. Hernández, com. pers.). Third, we could
not detect petrels on two predator-free rocks repre-
senting a potentially large extension to apparent suit-
able nesting habitat (#30 and #31 in TableS1). These
rocks held at least four Cory’s shearwater breeding
pairs and a large number of breeding and roosting
pigeons (>110 pigeons counted at dusk). Unfortu-
nately, no previous estimates of petrel abundance
were available for these rocks. A plausible explana-
tion for the absence of petrels is the interspecific com-
petition with increasing number of pigeons. In sup-
port, qualitative comparisons between old and current
photographs show marked loss of vegetation in recent
decades. The Canary spurge Euphorbia canariensis
population has declined from nine large plants to only
one small plant (Fig. S3), which is probably related
to the acidity and accumulation of pigeon excrement
(Spennemann and Watson 2017; Fig. S4).
Pigeon population size increases primarily around
human-modified areas (e.g. livestock farms, cereal
silos, garbage dumps, dunghills, and urban centres)
(Giunchi et al. 2012). Therefore, petrel breeding
colonies closer to these sites are more susceptible to
pigeon nest competition. At a global scale, the breed-
ing distribution of Bulwer’s petrels, but also other
endangered small petrel species, greatly overlaps with
the range of native and introduced pigeons. Therefore,
nest competition between them may occur, particu-
larly in proximity to human-transformed landscapes.
Despite the information gap on nest competition nest
between feral rockpigeons and seabirds in scientific
journals, our results are not the only evidence. We
have compiled evidence of pigeon nest competition
with at least 15 species across the world (Table4).
More studies are required to understand in greater
detail the role of pigeons on seabird nest density,
transmission of diseases and pathogens to seabirds,
and vegetation changes at seabirdcolonies.
Conservation implications
The Canarian Bulwer’s petrel faces several human-
related threats on land, i.e. predation by introduced
mammal predators (Hernández et al. 1990), colli-
sions with electricity transmission wires (Gómez-
Catasús et al. 2021), road casualties (Tejera et al.
2018), habitat destruction and attraction to artificial
night lights (Rodríguez et al. 2012). These threats
mainly occur on the main human-inhabited islands.
Owing to this fact, the bulk of the breeding pairs are
currently restricted to geographically small secure
breeding sites (mostly marine rocks). We highlight an
overlooked threat to these petrel sanctuaries: pigeon
competition for nesting sites. To minimize such inter-
specific competition as a matter of urgency, artificial
nest boxes should be installed in the main petrel sanc-
tuaries (marine rocks) with high densities of pigeons.
Artificial nests must be designed to exclude pigeons
and larger Procellariiformes, i.e. Cory’s shearwater,
by designing them with long and narrow entrances.
This action would facilitate long-term monitoring of
population and breeding success, and would help to
improve the conservation status of Bulwer’s petrel
by providing secure nest sites. This action has suc-
cessfully increased the annual productivity of other
endangered petrel species (Bolton etal. 2004). Given
that rock pigeons can easily become a pest (CABI
2008), we also advocate their culling in some petrel
colonies to avoid unnatural high densities (Giunchi
et al. 2012). Of course, other small petrels (e.g.
Macaronesian shearwater Puffinus baroli and band-
rumped storm-petrel Hydrobates castro) and endemic
species (i.e. land snails, lizards and plants) living
in those marine rocks might also benefit from feral
rockpigeon removal or control. On the main island
colonies, control of invasive mammals is the most
urgently required intervention, but additional actions
to reduce artificial light attraction, and wire and vehi-
cle collisions should be also considered.
Acknowledgements We thank the collaboration and essential
help in the field work of Manuel Siverio, José Juan Hernán-
dez, David Rodal Santiago “Vilayta”, Ángeles Taoro, M.
Nazaret Carrasco, Aurelio J. Acevedo, and David P. Padilla.
We accessed some dangerous and steep marine rocks thanks
to Javier Martín Carbajal and Raúl Martínez. Thanks to the
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Table 4 Evidence of seabird competition for nest sites with rock pigeon Columba livia
Seabird species Location Rock pigeon range Observations Source
Bulwer’s petrel Bulweria bulw-
erii, Monteiro’s storm-petrel
Hydrobates monteiroi
Vila Islet, Santa Maria,
and Praia Islet, Gra-
ciosa, Azores
Native One instance of a Bulwer’s petrel nest taken over by feral
rockpigeons on Vila Islet. The former owner had settled
in a neighbouring cavity and the nest was re-occupied by
a prospecting adult/sub-adult the night following pigeon
nest removal. Also a case in a Monteiro’s storm-petrel nest
on Praia Islet. The nest contained a half-grown Monteiro’s
storm-petrel chick in early August and pigeon feathers inside
the nest in early September. Chick may have fledged or been
killed or ousted by the pigeons
Joël Bried pers. comm.
Macaronesian shearwater
Puffinus baroli, band-rumped
storm-petrel Hydrobates
castro
Azores Native We have the feeling that rock pigeons may interfere with the
breeding of small Procellariiformes in rock cliffs. However,
the areas are impossible to access, so there are no data avail-
able
Jaime Ramos pers.
comm.
European storm-petrel Hydro-
bates pelagicus Goulien and Biarritz,
France
Native Interactions between pigeons and storm-petrels explain popula-
tion reductions in small colonies
Cadiou etal. (2004)
European storm-petrel Hydro-
bates pelagicus Terreros Islet, Almería,
Spain
Native Given that there are few available caves and crevices for
storm-petrel nests on the islet, the recent islet colonization
by pigeons (2012) could be a threat to the small storm-petrel
breeding population (around 10 pairs). The pigeon population
is nine breeding pairs and camera traps have demonstrated
that pigeons visit storm-petrels nest sites
Sergio Eguía pers.
comm. Eguía and Pérez
Morales (2020)
Yelkouan shearwater Puffinus
yelkouan Malta Native Pigeons and shearwaters share several nest crevices/caves.
Pigeons displace shearwaters in a few nests monitored for
several years in two colonies. One out of 11 shearwater nest
sites was occupied by pigeons in a cave at Majjistral colony
in 2021. One out of 10 shearwater nest sites was occupied by
pigeons on St Paul’s Island in 2020. A shearwater re-occupied
the nest site in the following breeding season, but the attempt
failed (broken egg) and pigeons were nesting in its place
Martin Austad pers.
comm.
Wedge-tailed shearwater
Ardenna pacifica Mokulua Nui, Oahu,
Hawaii
Introduced Images of the pigeon chicks in the shearwater nest and observa-
tions of territorial and aggressive behaviours of pigeons chas-
ing away adult shearwaters from their nests
Brooke Friswold pers.
comm.
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Table 4 (continued)
Seabird species Location Rock pigeon range Observations Source
White-tailed tropicbird Pha-
ethon lepturus Bermuda Introduced The tropicbirds nest only over late spring to early autumn in
natural erosion cavities in the limestone cliffs. The pigeons
tend to move into nesting spaces when they depart soiling
the nests and infusing them with mites. The Department
of Environment and Natural Resources actively controls
the feral rockpigeons to prevent this as much as possible.
Major storm activity resulting in nesting habitat destruction
increases competition for nest sites which can be further com-
pounded by the pigeons’ year-long presence using surviving
nest sites
Patrick Talbot pers.
comm. Dowson and
Madeiros (2009)
Red-tailed Tropicbird Phaethon
rubricauda, white tern Gygis
alba
Oahu, Hawaii Introduced There are no quantitative data showing reduced nest success,
but both species avoid sites that are used for roosting or nest-
ing by pigeons. The tropicbirds nest in small caves and on
ledges on rocky cliffs along the coast. There are several suit-
able areas with caves that are used for nesting by pigeons, and
the tropicbirds have never used those areas even though they
nest close by. There are many caves so there may not be much
competition, but the tropicbirds avoid those areas. Terns nest
in trees in urban areas where pigeons are abundant, and they
have abandoned several large trees that have become roosting
sites for pigeons or where human garbage has attracted a large
number of pigeons
Eric VanderWerf pers.
comm.
Bridled tern Onychoprion
anaethetus Penguin Island, Australia Introduced At least 22 nests were occupied by pigeons in one study plot of
97 tern pairs, but pigeon occupation rate was higher on the
cliffs than under the Rhagodia sp. bushes and artificial nest
boxes
Aurélie Labbé pers.
comm.
Grey ternlet Anous albivitta Lord Howe Island,
Australia
Introduced Cliff nesting grey ternlets, attempting to find sites inaccessible
to black rats, were nesting in large sea caves but commonly
out-competed by feralrock pigeons. No quantitative data, but
Lord Howe Board staff ran a programme of shooting during
the ternlets non-breeding season up until 2015. While the
success of the rodent eradication has yet to be confirmed, the
ternlets returned to non-sea cave rock cavities in 2020, reduc-
ing the conflict with pigeons
Nicholas Carlile pers.
comm.
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skills of Iván Méndez, Juan Curbelo, Moisés de León, Héc-
tor Manuel Figueroa, Adrián Borges, José María Benítez, and
Eligio Benítez as skippers, we landed in all rocks and inac-
cessible coastal sectors. This study was coordinated by the
Canarian regional SEO/BirdLife’s office. Our special thanks
to its staff, Juan Antonio Lorenzo and Elena Ramos. We thank
Chloe Carothers-Liske for improving the English grammar.
We also thank Benjamin Metzger and James Crymble of Bird-
Life Malta and those colleagues included in Table4 for shar-
ing their observations on nesting competition between seabirds
and pigeons. Two anonymous reviewers improved an earlier
version.
Author contributions BR and AR designed the study. BR,
FS, JMM, ES, and AR collected field data. AR analyzed the
data. YA managed and coordinated the research project. BR
and AR drafted the manuscript and all authors commented and
approved the final version of the manuscript.
Funding Open Access funding provided thanks to the
CRUE-CSIC agreement with Springer Nature. This study was
funded by the project INTERREG MAC/4.6d/157 LuMinAves.
Data availability All data are included in the article and its
supplementary information files.
Declarations
Conflict of interest The authors declare that they have no
conflict of interest.
Ethical approval All procedures in this study were approved
by insular and regional governments. Permits to conduct field-
work were granted by Cabildo de Tenerife (Permit Numbers:
AFF82/17, AFF94/18, and AFF142/19) and Gobierno de
Canarias (permit numbers: 2017/10464 and 2018/6842).
Open Access This article is licensed under a Creative Com-
mons Attribution 4.0 International License, which permits
use, sharing, adaptation, distribution and reproduction in any
medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Crea-
tive Commons licence, and indicate if changes were made. The
images or other third party material in this article are included
in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not
included in the article’s Creative Commons licence and your
intended use is not permitted by statutory regulation or exceeds
the permitted use, you will need to obtain permission directly
from the copyright holder. To view a copy of this licence, visit
http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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