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Data on the effect of predation by an Eagle Owl (Bubo bubo) of captive origin on fauna of an island environment (Aves, Strigidae)

  • Grupo de Ornitología e Historia Natural de las islas Canarias
Invasive species are today one of the main causes of decline in the biota of a
region. This is particularly severe in island environments, where native biodi-
versity is much more vulnerable, above all due to the small size of their popu-
lations and unique evolutionary features brought about by isolation (see Russell
et al., 2017 [Environ. Conserv. 44: 359-370] & refs. cited therein). Among exotic
birds and specifically birds of prey, the escape of species held in captivity (e.g.
for falconry or other recreational purposes) is seriously detrimental to their na-
tive relatives. Besides the risk of genetic introgression (see Rodríguez et al.,
2019 [J. Raptor Res. 53: 189-197]), this is because they may compete for breeding
sites and food, transmit diseases or prey on them directly (Krone et al., 2004
[Vet. Rec. 154: 110-113]; Naldo & Samour, 2004 [J. Avian Med. Surg. 18: 229-241];
Cugnasse et al., 2017 [Ornithos 11: 1-11]). Furthermore, their introduction for
biological control purposes has had highly adverse consequences for local non-
target taxa (Simberloff & Stiling, 1996 [Biol. Conserv. 78: 185-192]), such as
seabirds in the Hawaiian archipelago following release of the American Barn
Owl (Tyto furcata) for control of rats (Rattus spp.) in the 1950s (see Raine et al.,
2019 [Mar. Ornithol. 47: 33-38]).
The consequences for native oceanic island fauna can be expected to be much
more tragic when a top predator such as the Eagle Owl (Bubo bubo) is introduced,
since it is even known to catch the largest numbers of other nocturnal raptors
(Mikkola, 1983 [Owls of Europe]). The purpose of this note is therefore to provide
data on the diet of an adult of this species, escaped from captivity on the island of
Fuerteventura (eastern Canary Islands), and thereby contribute to evaluating its
potential effect on the local fauna. The first record of its presence in the natural
environment of the island dates back to November 2016 (L. M. Gutiérrez, pers.
Fecha de recepción: 21/02/2021 / Fecha de aceptación: 06/04/2021 173
Data on the effect of predation by an Eagle Owl
(Bubo bubo) of captive origin on fauna of an island
environment (Aves, Strigidae)
SIVERIO, F., L. M. GUTIÉRREZ & M. CABRERA (2021). Datos sobre el efecto de la depre-
dación de un búho real (Bubo bubo) escapado de cautividad sobre la fauna de un ambiente
insular (Aves, Strigidae). Vieraea, 47: 173-177.
VIERAEA Vol. 47 pp. 173177 Santa Cruz de Tenerife, diciembre 2021 ISSN 0210-945X
174 VIERAEA | 2021 | vol. 47 | pp. 173-177 | ISSN: 0210-945X
obs.). In January 2017, its calls were recorded (Pieterse [Xeno-canto, XC356279])
and in March 2019 it was finally captured and its origin elucidated (REDEXOS - Go-
bierno de Canarias).
The Eagle Owl was established during this period on Risco de Las Peñas (approx.
1 km2), a rocky outcrop of plutonic origin which, as an interfluve, separates two
ravines in the central-western part of the island (Betancuria Rural Park). It is a
considerably steep place with little vegetation, dominated by scarps and gullies,
as well as crevices, ledges and shallow caves of varying sizes. Together, these form
a highly optimal environment for birds with a marked predilection for such rocky
enclaves. Somewhat less rugged, the SE sector of the area is crossed by a road
that includes a wide viewpoint, which is usually quite busy. In the surrounding areas
there are more similar hills, a succession of small ravines where xeric-thermophile
scrub predominates, a dam silted up with sediment covered with Tamarix canarien-
sis, and some rural farmhouses with many cultivated plots.
Eagle Owl feeding data were obtained by analysing both complete and disaggre-
gated (c. 82) pellets collected in 2017 (December) and 2019 (January, June, Septem-
ber and October) at the foot of several rock perches or plucking sites (e.g. Donázar
& Ceballos, 1989 [Ornis Scand. 20: 117-122]) near its usual roosts. To determine
the prey species, we took into account the high degree of bone fragmentation, se-
lecting the most diagnostic parts and those belonging to mammals (e.g. dental
and parietal remains, pelvis, proximal epiphyses of cubitus and femur) and birds
(e.g. tarsus and sternum). Finally, in order to quantify the minimum number of in-
dividuals (MNI), as well as the trophic niche breadth, we proceeded to match these
bony parts (see Laudet et al., 2002 [Acta Zool. Cracov. 45: 341-355] & refs. cited
therein) and applied Levin’s index, B = 1/∑ pi2 (where pi is the proportion each taxon;
the values were standardised [Bsta] on the scale 0 [trophic specialisation] – 1 [wide
range]: Krebs, 1998 [Ecological methodology]), respectively.
Eight taxa were identified from the 89 prey items counted, four introduced mam-
mals and four birds (Table 1). The European Rabbit (Oryctolagus cuniculus), which
made up about 88% of the total prey species, was by far the most depredated, es-
pecially individuals within the sub-adult age class. Together with the low diversity
of prey species, this high rate of predation determined the level of dietary special-
isation (Bsta = 0.04) (Table 1). Despite decreased abundance due to the arrival of
variant 2 of viral haemorrhagic septicaemia (VHS), rabbits are a well distributed
species on Fuerteventura (López-Darias & Lobo, 2009 [Biodivers. Conserv. 18: 3687-
3704]) and still maintain acceptable densities in the game reserves. One of these
areas corresponds to the that where the Eagle Owl became established (P. García,
pers. comm.). In fact, the predominance of this introduced lagomorph as the main
SIVERIO, F. et al.
VIERAEA | 2021 | vol. 47 | pp. 173-177 | ISSN: 0210-945X 175
prey species on the island was to be expected, owing especially to its great impor-
tance in the owl’s trophic spectrum in other latitudes (e.g. Pérez Mellado, 1980
[Ardeola 25: 83-112]; Donázar et al., 1989 [Ornis Scand. 20: 298-306]).
Among the remaining mammals captured, all with a minimal contribution to the
diet, the Barbary Ground Squirrel (Atlantoxerus getulus) (Table 1) stands out be-
cause of its exclusively diurnal habits (van der Marel et al., 2019 [Anim. Behav. 151:
43-52] & refs. cited therein). Among the few options, we cannot rule out that this
occurred due to alteration of the individual’s activity pattern attributable to an-
thropic impact (M. López-Darias, pers. comm.) in the vicinity of the owl roost. In
fact, there is a lookout point at which visitors habitually feed squirrels where they
often remain until dusk (L. M. Gutiérrez, pers. obs.).
Within the small group of birds of prey, also with a low quantitative contribution
to the diet (Table 1), the Barn Owl individual (T. alba) stands out starkly. Its attribu-
tion to the endemic subspecies of the eastern islands and islets, the Eastern Ca-
nary Islands Barn Owl (T. a. gracilirostris), makes this predation a worrying loss to
the native island biota. This exclusive taxon appears as “Vulnerable” in the regional
and national catalogues of threatened species. It currently has a very small popu-
Prey taxa MNI (%)
Rattus rattusa 1 (1.1)
Rattus sp.a 2 (2.2)
Mus musculusb 1 (1.1)
Atlantoxerus getulus 1 (1.1)
Oryctolagus cuniculus 78 (87.6)
Alectoris barbara 2 (2.2)
Columba livia 2 (2.2)
Tyto alba 1 (1.1)
Aves indet. 1 (1.1)
Bsta 0.04
Table 1.- Diet composition and niche breadth, based on a sample of c. 82 pellets, for an
Eagle Owl (Bubo bubo) escaped from captivity on the island of Fuerteventura, Canary Islands.
a Taxa grouped generically to calculate Levin’s index. b Given its occurrence in the same pel-
let, this individual is likely associated with the capture of Tyto alba. Photo: owl at one of its
roosting sites at Risco de Las Peñas on February 23, 2019 (© L. M. Gutiérrez).
176 VIERAEA | 2021 | vol. 47 | pp. 173-177 | ISSN: 0210-945X
lation on Fuerteventura (F. Siverio, pers. obs.), which is not exempt from other se-
rious conservation problems (Siverio & Palacios, 2004 [in Madroño et al. (eds.),
Libro rojo de las aves de España]). If we consider that the foraging radius of the Eagle
Owl may even exceed 5 km (Mikkola, 1983; van Nieuland et al., 2018 [Ardea 106:
147-162]), it may have preyed on Eastern Canary Islands Barn Owls from at least
two of their breeding territories. One of these latter coincides precisely with the
enclave where it was settled (F. Siverio, pers. obs.). Moreover, considering the size
of our sample of pellets, their rate of formation in Bubo (approx. 1 per day; Marti,
1973 [Wilson Bull. 85: 178-181]) and the time the individual remained at liberty (min.
850 days), we have only analysed about 10% of its total predatory activity in this
study. This suggests that, apart from more owls, other birds with which it shares
habitat could also have been its prey, including the Egyptian Vulture (Neophron per-
cnopterus). This species, which is preyed upon by the Eagle Owl in other regions
(Tella & Mañosa, 1993 [J. Raptor Res. 27: 111-112]), is in danger of extinction and
constitutes a subspecies (N. p. majorensis) in the Canary Islands (Donázar et al.,
2002 [J. Raptor Res. 36: 27-33]).
Bubo species N Island Date Remarks Source
B. buboa 1 L 1980s Electrocuted 2
B. buboa 1 F Dec 1993 Seen at Barranco de Río Cabras 2
B. bubo 1 T Jun 2007 Recovered 3
B. bubo 1 T Feb 2009 Recovered with band and injury 3
B. bubo 1 T Feb 2009 Only wearing anklets 3
B. virginianus 1 F Jan 2013 Recovered wearing anklets and bands 4
B. virginianus 1 T Jul 2013 Not recovered 3
B. bubo 1 T Dec 2013 Recovered 3
B. bubo 1 T May 2014 Falconry 3
B. bubo 1 T Mar 2015 Falconry 3
B. ascalaphus 1 T Dec 2019 Electrocuted wearing anklets and bands 5
B. bubob 1 F Dec 2019 Settled at Risco de Las Peñas 6
Table 2.- Individuals of different species of the genus Bubo escaped from captivity in the Ca-
nary Islands (L = Lanzarote; F = Fuerteventura; T = Tenerife) that have been detected during
recent decades. aAlthough unconfirmed, very likely an escaped bird; balmost certainly again
the individual dealt with in this note. Sources: 2) Martín & Lorenzo, 2001 (Aves del archip-
iélago canario); 3) La Tahonilla Wildlife Rehabilitation Centre, Cabildo de Tenerife (in litt.); 4)
rangers of the Fuerteventura Natural Protected Areas, Cabildo de Fuerteventura (pers.
comm.); 5) rangers of the Tenerife Natural Protected Areas, Cabildo de Tenerife (pers.
comm.); 6) this study.
SIVERIO, F. et al.
VIERAEA | 2021 | vol. 47 | pp. 173-177 | ISSN: 0210-945X 177
There is clearly a need to modify the current law on falconry in the Canary Is-
lands, firstly in order to avoid genetic mixing between exotic and native birds of
prey (Rodríguez et al., 2019). More generally, it is now urgent to draw up and approve
regulations that prohibit the entry and/or possession on the islands –in both pri-
vate and zoological centres– of species considered to be major predators, such
as the genus Bubo. Its apparently strong ability to escape from captivity is proven
by several known examples in the archipelago, among them the apparent recidi-
vism of the individual described here (Table 2). Apart from damage to the local
fauna (this study), recapture of these birds of prey escaped from zoos and private
individuals –who use them for profit and/or recreation– involves considerable
costs to public funds.
We would like to thank Juan Carlos Rando (ULL) for his willingness to identify
the remains of bird prey found in the pellets, and also Marta López-Darias (IPNA-
CSIC) and Rubén Barone for helpful critical readings of a first draft of this note.
Beneharo Rodríguez (GOHNIC) compiled part of the information on owls escaped
from captivity in the Canaries.
1 Canary Islands’ Ornithology and Natural History Group (GOHNIC)
La Malecita s/n, E-38480 Buenavista del Norte
Tenerife, Canary Islands
2 Casillas del Ángel 114-A, E-35611 Puerto del Rosario
Fuerteventura, Canary Islands
3 Asociación Ajicán, Ctra. El Aceitún 2, E-35629 Tuineje
Fuerteventura, Canary Islands
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