Nest defence strategies in the Fieldfare Turdus pilaris: the responses on an avian and a mammalian predator.

Fieldfare (Turdus pilaris) Science Article 1

abstract

The Fieldfare Turdus pilaris, a medium-sized passerine, is aggressive in nest defence and its use of f ces may be detrimental to avian but not to mammalian predators. The birds breed either solitarily or in colonies, the nesting pattern in subalpine forests is apparently dependent on the abundance of small rodents. In the crash year following a year with high density of small rodents, the staple food of several mammals, mustelids as Stoats Mustela erminea are forced to find alternative prey as eggs and nestlings. The year 2003 was such a small rodent crash year in subalpine birch forests in the middle of Norway. The Fieldfares then bred solitarily and behaved more inconspicuous towards a human observer compared to the previous year. The majority of the pairs studied, left the nest silently when a life-like Stoat was mounted 10 m from the nest tree, whilst most pairs attacked a Hooded Crow Corvus corone cornix mounted at the same place. When these dummies were placed 1 m from the nest tree, the Fieldfares attacked both dummy models intensely. The birds return to the nest after the dummy was removed, was later for the Stoat than for the Hooded Crow dummy. These results suggest that the Fieldfare responds specifically to nest predators: trying to avoid disclosing the nest containing eggs or nestlings for an olfactory oriented mammal, the Stoat that is scarcely deterred by the birds defecation, but attacking the Hooded Crow with use of ejected f ces that may be detrimental for the avian nest predator. When the predator is close to the nest, however, the best strategy may be to distract the predator away from the nest. The late return to the nest after the Stoat exposure, may be explained by the Fieldfares’ behaviour towards a predator that is a lethal threat to the breeding bird.

Hogstad O. 2004, Ardea 92(1): 79-84

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