Adaptive management of body mass by Siberian jays

A profusion of theoretical and empirical studies has successfully explored the issue of adaptive management of body mass by small birds threatened with both starvation and predation. In addition to diurnal body mass cycles, body mass tends to increase with either poorer mean conditions or more variable conditions, such as those often experienced during winter. In many species, individual dominance status will affect access to food and predation risk, and is therefore predicted to influence patterns of optimal body mass further. We investigated body mass regulation via repeated measurements without capture in groups of Siberian jays, Perisoreus infaustus, during autumn. As predicted, body mass of individuals increased throughout the day and towards the winter and was higher during colder periods. Jay body mass was also correlated with wind chill conditions 23 h before. This makes sense since conditions 24 h earlier were a better predictor of current conditions than conditions either 12 or 48 h earlier. Dominant birds (i.e. breeders) that were large carried relatively lower fat reserves than large subordinate individuals. The opposite was true for morphologically small birds. Together this suggests tighter requirements for individual variation in mass for dominant breeders in these groups. Our findings in this food-hoarding corvid are in general agreement with models and results concerning much smaller birds.