Volume 19 Issue 2
Pages 62 - 110 (October 2002)
Citation: Ruiz-Olmo, J. and Marsol, R. (2002) New
Information on the Predation of Fish Eating Birds by the Eurasian Otter (Lutra
lutra). IUCN Otter Spec. Group Bull. 19(2): 103 - 106
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New Information on the Predation of Fish Eating Birds by
the Eurasian Otter (Lutra lutra)
Jordi Ruiz-Olmo and Rosa Marsol
Servei de Protecció i Gestió de la Fauna,
Direcció general del Patrimoni Natural i del Medi Fisic, Dr. Roux 80, 08017
Barcelona, Spain, e-mail: ajruiol@gencat.net
(received 24th January 2002, accepted 12rth
June 2002)
Abstract: The Eurasian Otter (Lutra lutra)
mainly eats aquatic and semiaquatic prey (such as fish, crabs, amphibians
and snakes), although in some places, and at some times, mammals, birds
and insects can represent a significant part of the diet. A meticulous
review of the literature shows a few cases of otters preying on large
birds. Three cases are examined: a heron killed and eaten by a wild otter
in the Pyrenees, gulls killed and eaten by an escaped otter near
Barcelona, and a red kite caught by a captive otter, but rescued by staff,
at the Pont de Suert otter centre in the Pyrenees. Further research is
needed to establish whether otters are superpredators of birds and play a
role in population control.
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The Eurasian otter (Lutra lutra) feeds mainly on fish in the wild, but
amphibians and other types of prey, including birds, can be important in certain
areas or times of the year. In most regions of Europe, including the
Mediterranean area, ducks (Anas, especially A. plathyrhynchos),
Moorhen (Gallinula chloropus) and small songbirds are the types of bird
most frequently consumed, although usually to a very small degree in overall
proportionate terms (see summary in RUIZ-OLMO and PALAZÓN,
1997). A detailed study of available literature (e.g. see summaries in MASON
and MACDONALD, 1986; KRUUK, 1995) shows that the
Eurasian Otter does not regularly consume predatory birds. GREEN
(2000) provides some interesting data showing that pheasants (Phasianus
colchicus), wrens (Trogoldytes troglodytes), song thrushes (Turdus
philomelos), geese (Anser sp.) and gulls (Larus sp.) were all
caught and eaten by otters (Lutra lutra) in captivity, and that Crows (Corvus
corax) and Grey Heron (Ardea cinerea) were chased off whilst
attempting to steal their fish. These data show L. lutra is capable of
catching relatively large birds (on occasions weighing up to several kg),
although these birds are generally non-predatory. At most, according to general
opinion, the otter just scares such predatory birds off because they compete for
food, although in one case a Yellow-legged Gull (L. argentatus) was
caught. This gull is an aggressive species, capable of catching large prey,
stealing prey (kleptoparasitism), and strongly defending itself. For example, POLECHLA
et al. (1993) reported cases of a Mew Gull (Larus canus) attacking Lontra
canadensis, but it was also pointed out that this was probably related to
kleptoparasitic behaviour. According to these authors, three other species: the
Common Crow, the Bald Eagle (Haliaetus albicilla) and the Osprey (Pandion
haliaetus), act in the same way towards this species of otter.
In this report, we highlight some more interesting data on otters preying on
ichthyophagous predators:
- The first case refers to a Grey Heron, apparently caught and eaten near
the Noguera Ribagorçana River (Pyrenees, Lleida, Spain) in February 1990.
In mid-winter, an adult sized heron corpse was found five metres from the
riverbank next to a large rock, where it had apparently either been caught
or dragged by the otter. Wounds on the heron were compatible with an Otter
attack. Otter tracks (probably of a male) found in the mud led away from the
river and surrounded the grey heron (which had been pulled from the water).
Part of the heron's pectoral muscle mass had been eaten and the area was
strewn with feathers that showed the typical feather bite of a carnivore.
This, together with one dropping on top of the heron carcass, certainly
suggests an otter was responsible. The harsh mid-winter climate, combined
with the difficulty of catching food, may have weakened the heron, making it
easier to catch. No other occurrence such as this has been reported during
18 years of L. lutra studies in Spain and no heron remains have been
found in thousands of analysed droppings.
- One otter, from a reintroduction project in NE Spain, escaped from an
enclosure in the town of Barcelona. This animal was living completely free
for some weeks (January and February 1996) in the Ciutadella Park (Deli
Saavedra & Jordi Ruiz-Olmo, unpublished). The otter intensively used two
artificial lakes where several species of fish provided plentiful food. Many
gulls and a population of more than 300 ducks also inhabited these waters
(with daily population fluctuations). During a survey conducted on a more
isolated island (used by ducks and other birds for resting and sleeping), we
found the remains of more than 10 birds, some of them uneaten. It was clear
that a carnivore killed the birds as both tooth wounds were found in necks
and other parts and the feathers were bitten and broken in a typical manner.
The otter was the only animal that could have killed the birds as no dogs or
cats, etc, could reach the island (otter spraints were also found). Of these
birds, two were Common Black-headed Gulls (L. ridibundus) and one a
Yellow-legged Gull (L. cachinnans).
- Another interesting piece of data refers to a Red Kite (Milvus milvus)
that was attacked by a captive otter at the otter centre in Pont de Suert
(Pyrenees, Lleida) on 11 August 1999. Kites often enter the otter enclosures
to steal fish remains. In this particular case, the otter was fast enough to
catch the kite by its wing, which broke. The bird barely managed to escape
when centre personnel rescued it and took it to a recovery centre. In this
otter centre, otters fairly frequently attack different species of winged
predators (herons, kites, and crows) that come to eat their fish. Although
this could be a captivity-induced behaviour pattern, the otter captured the
kite relatively easily, suggesting that such birds could certainly be caught
in the wild.
It has been noted that otters tend to catch and eat ichthyophagous and
crayfish-eating birds from similar families (TILER
et al., 1994). Predatory birds can often be found eating together when they
take advantage of foraging otters, as has been shown in the case of L.
perspicillata (KRUUK et al., 1993). Further, cases
where predatory birds (especially small and medium-sized water birds) have been
attacked by otters have tended to occur in places where they live close to large
bird colonies, as when L. canadensis was reported catching gulls, terns
and petrels (FOOTTIT and BUTLER, 1977; POLECHLA
et al., 1993; CAMERON, 1995). Attacks on gulls are
frequent in these areas, although they are normally limited to an attack
followed by the rapid escape of the gull (personal observation in Alaska; see
photograph). In certain circumstances (surprise, weakness of the bird being
attacked, etc.), otters may, however, catch larger and more aggressive birds (as
in the cases of the heron and kite reported earlier). The sea otter (Enhydra
lutris) occasionally feeds on predatory birds, such as cormorants, gulls,
grebes, teals and fulmars, resting on the water surface (RIEDMAN
and ESTES, 1988), possibly as the otter's larger size and more pelagic
habits make such birds easier to catch.
If we discount the possible effects of captivity, there are a number of
hypotheses as to why such birds should be caught by the otter in the wild,
including the chasing, and occasional catching, of competitors, or actual
hunting for food. However, in many cases, this would entail a high outlay of
energy and entail some risk to the otter, probably accounting for the fact that
they are only rarely taken. Perhaps all these data represent only single
incidents, without great relevance; indeed, very few records are found in the
literature. However, the data above are interesting, bearing in mind that they
may be situations involving cases of super-predation (one predator killing
competing predators, which may contribute to their regulation and thereby to the
conservation or increase in the stock of available prey). This mechanism has
been reported in some other species of predators. For example, PALOMARES
et al. (1995) showed that the Iberian lynx (Lynx pardellus) kills
other carnivores (red fox, Vulpes vulpes, mongoose, Herpestes
ichneumon, and genet, Genetta genetta) that prey on rabbit (the usual
prey of the lynx), without actually eating them, resulting in a simultaneous
increase in the abundance of rabbits. This could also be the case with otters,
where its role as a super-predator, feeding on fish eaters or fish egg eaters
(e.g. crayfish, amphibians, water snakes (Natrix ssp.) and predatory
fish) has been highlighted in previous studies (RUIZ-OLMO and
CASADESUS, 1998; RUIZ-OLMO, 2001). The isolated
data presented above (in addition to similar reports made by GREEN,
2000), showing that otters can attack and kill medium and large-sized
ichthyophagous birds, may also provide evidence of this type of behaviour. Given
that this type of behaviour is infrequently observed and difficult to establish,
it is possible that it has been overlooked in the past. There is evidently a
lack of information on this matter and more data will be needed in the future if
super-predation on ichthyophagous birds is to be confirmed.
REFERENCES
Cameron, A. 1995. Apparent River Otter Predation
at an Aleutian Tern Colony. Colonial Waterbirds 18, 91-92.
Foottit, R.G., Butler, R.W. 1977. Predation on
nesting Glaucous-winged Gulls by river otter. Can. Field-Nat. 91,
189-190.
Green, R. 2000. Avian predation by captive otters. IUCN
Otter Spec. Group Bull. 17,
83.
Kruuk, H. 1995. Wild Otters. Predation and
Populations. Oxford University Press. Oxford. 304 pp.
Kruuk, H., Kanchanasaka, B., O'Sullivan, S.,
Wanghongsa, C. 1993. Kingfishers Halcyon capensis and Alcedo athis
and Pond-Heron Ardelola bacchus associating with otters Lutra
perspicillata. Nat. Hist.Bull. Siam Soc. 41, 67-68.
Mason, C.F., Macdonald, S.M. 1986. Otters. Ecology
and Conservation. Cambridge Univ. Press. 236 pp.
Palomares, F., Gaona, P., Ferreras, P., Delibes, M.
1995. Positive effects on game species of top predators by controlling
smaller predator populations: An example with lynx, moongooses, and rabbits.Cons.
Biol. 9, 295-305.
Polechla, P., Aiaginar, Y.-J., Whiteman, S. 1993. Agonistic behaviour
of a mew gull (Larus canus) towards a nearctic river otter (Lontra
canadensis) in Western Alaska. The River Otter Journal, 3,
1-2.
Riedman, M.L., Estes, J.A. 1988. Predation on
seabirds by sea otters. Can. J. Zool. 66, 1396-1402. Ruiz-Olmo,
J. 2001. Pla de Conservació de la Llúdriga: Biologia i Conservació. Documents
dels Quaderns del Medi Ambient 6, 1-87.
Ruiz-Olmo, J., Casadesús, F. 1998. Effects of the
otter on sport fishing in the Pyrenean Mountains (North-East Spain). BOKU-Rep. Wildl.
Res. & Game Manag. 14, 35-36.
Ruiz-Olmo, J., Palazón, S. 1997. The diet
of the European otter (Lutra lutra L.) in Mediterranean freshwaters
habitats. J. Wildl. Res. 2, 171-181.
Tiler, C., Evans, M., Heardman, C., Houghton, C. 1994.
Diet of the smooth-coated indian otter (Lutra perspicillata) and of fish
eating birds: a field survey. J. Bombay Nat. Hist. Soc. 86,65-70.
Résumé : Information Nouvelle sur l'Ornithophagie
de la Loutre (Lutra Lutra): la Consommation d'Oiseaux Predateurs
La loutre eurasienne (Lutra lutra) se nourrit principalement de
poissons et d'écrevisses, bien qu'amphibiens ou autres catégories de proies
puissent représenter, localement ou saisonnièrement, une part importante du
régime de l'espèce. L. lutra est apte, à certaines périodes, de
capturer des oiseaux de grande taille (pesant jusqu'à plusieurs kg), mais
généralement non-prédateurs. Une revue minutieuse de la littérature montre
que la loutre eurasienne ne consomme pas régulièrement les oiseaux prédateurs
(MASON and MACDONALD, 1986; KRUUK, 1995). C'est également vrai sur le pourtour
méditerranéen, ou canards (g. Anas, notamment A. plathyrhynchos),
poules d'eau (Gallinula chloropus) et autres sont les catégories
d'oiseaux les plus fréquemment consommées, bien que leur proportion soit
toujours globalement très faible dans le régime (c.f. résumé de RUIZ-OLMO et
PALAZON, 1997). Le présent article rapporte ici quelques cas intéressants de
prédation d'oiseaux ichtyophages chez la loutre.
Revenez au dessus
Resumen: Nuevas Informaciones sobre la
Depredación de Aves por la Nutria Euroasiática (Lutra lutra): Cosumo de
Depredadores
Las nutrias consumen usualmente especies acuaticas y semiacuaticas (especialmente
peces, cangrejos, anfibios, culebras de agua), aunque a veces también pueden
consumir mamíferos, insectos y aves. El trabajo recoje la bibligrafía
existente de consumo de grandes aves (a veces depredadoras) y de interacciones
con estas por las dierentes especies de nutrias del Mundo. Se revisan diferentes
tipos de comportamiento (desde comensalismo o cleptoparasitismo, hasta la propia
depredación). Se presentan tres casos de este tipo: una garza real (Ardea
cinerea) muerta por una nutria en un río Pirenaico, un milano real (Milvus
milvus) salvaje atacadao por una nutria captiva en el Centro de nutrias del
Pont de Suert (que resultó herido y con un ala rota y sólo pudo ser salvado
por el personal del centra), y varias gaviotas (Larus cachinnans, Larus
ridibundus) muertas por una nutria salvaje en un parque urbano de Barcelona.
Se discute el posible efecto beneficioso que la superdeprdación puede tener
sobre las presas de la nutria.
Vuelva a la tapa
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