Travel distance and mass gain in wintering blackbirds

Birds that range over a large area will have a greater mass-dependent risk of predation than more sedentary birds. Birds that travel more may then red uce winter mass gain to compensate for the increased predation risk that gr eater travelling entails. I tested whether European blackbirds, Turdus meru la, that travelled more in winter had a lower mass than more sedentary bird s, independently of any confounding effects of food supply on both ranging behaviour and mass gain. I measured change in winter mass and amount of foo d eaten in conjunction with the distance that blackbirds travelled to a ran domly sited mobile feeder. Blackbirds that travelled shorter distances (per trip and in total) and less often to the feeder had the highest mass midwi nter relative to their spring mass. Blackbirds with a higher mean mass midw inter also travelled, on average, shorter distances to the feeder. The dist ance an individual blackbird travelled to the feeder at any one time was pr obably independent of the state of its daily energy reserves (how much of i ts daily total mass gain it had achieved at that point). The relationship b etween distance travelled and mass was probably independent of food supply because distances actually increased at the end of the winter and the amoun t of food eaten per individual changed little. More mobile blackbirds were therefore likely to have compensated for any increases in predation risk as sociated with their greater ranges by decreasing winter mass gain, but will have had an increased risk of starvation because of their lower mass. (C) 1999 The Association for the Study of Animal Behaviour.