Winter feeding influences the cost of living in boreal passerines

Abstract

The plastic regulation of internal energy reserves is acknowledged as the main adaptive response to winter conditions of resident small birds in northern latitudes, a strategy that may be altered whenever human-supplemented food is available. We investigated the effects of supplementary feeding on the energy management strategy of two wild passerine species, the Willow Tit Poecile montanus and Blue Tit Cyanistes caeruleus, wintering in boreal conditions by measuring body mass and the energy cost of living, i.e. basal metabolic rate. Individuals of both species were heavier, larger and exhibited a higher energy cost of living when captured at the feeders than were individuals captured away from feeders. Fed Willow Tits expended more energy in maintenance, although this difference disappeared once mass was accounted for. Conversely, Blue Tits at feeders had higher mass-adjusted energy cost of living, but only at low ambient temperatures. The results indicate that winter feeding has species-specific effects on overall energy management strategy and modifies the response to environmental conditions of wintering passerines.