Do avian and mammalian nest predators select for different nest dispersion patterns of Fieldfares Turdus pilaris? A 15‐year study

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Abstract
The Fieldfare Turdus pilaris is an open‐nesting passerine that breeds either solitarily or in colonies. The nest‐spacing pattern of this species in a subalpine homogeneous Birch Betula pubescens forest in central Norway (c. 63°N) was studied during the period 1980–1994. Nest‐spacing patterns varied annually, and the number of Fieldfare colonies present (0–6) within a 3.5‐km2 study area was positively correlated with the annual density of small rodents and the birds' fat index and mean clutch size. After the rodent population crashes, prey switching seems to occur among mustelids. The nest‐spacing pattern of Fieldfares in the Birch forest therefore provides indirect support for the alternative prey hypothesis. Since the nest predation rate in Fieldfares is density dependent, and since their use of ejected faeces in communal defence may be detrimental, even lethal, to avian but not mammalian predators, it is suggested that a high density of mustelids results in selection for solitary breeding, whilst a predominance of avian predation risk selects for colonial breeding by the Fieldfare. The physical condition (fat index) of the Fieldfares is apparently a contributing factor in determining their nest dispersion pattern. The proportion of colonial nesting pairs was negatively correlated with the Fieldfare density and indicated a distributional pattern in a patchy habitat in agreement with the ideal free distribution theory.

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