Social stability and daily body mass gain in great tits

This paper presents the results of an experiment on the influence of changes in flock composition and social rank on the pattern of daily mass gain in captive wintering great tits. We created flocks of three male great tits and scored dominance when ranks had stabilized after a few days. We then moved the dominant bird from each flock into a neighboring flock, thus creating a certain amount of social instability and unpredictable resource access. Both before and after moving the dominants, we collected individual body mass data three times a day. When the hierarchies had stabilized in the second flocks, individual ranks were scored. After the dominant birds were moved, the rate of daily mass gain increased for the average individual. There was also a decrease in average morning body mass, so the birds started the day at a lower mass and ended at a slightly higher mass than previously. Dominants were on average heavier than subordinates, but we found no statistically significant effect of rank change on body mass. The pattern of daily mass gain was linear over time both before and after treatment. We conclude that an impression of unpredictable resource access can be created by disturbing the social structure in a flock, with similar effect as from restricting individual access to feeders. Within a flock, instability of the social hierarchy thus seems to have an influence on body mass management in addition to that of environmental stochasticity. Copyright 2004.