Energetic determinants of abundance in winter landbird communities

There is increasing evidence that individual energetics constrain macroecological patterns. Here we model total abundance within winter landbird communities as a function of (1) energy supply, as measured by ecosystem net primary productivity, and (2) energy use of individuals, as influenced by body mass and ambient temperature. Using data from the North American Christmas Bird Count, we find that total abundance increases with productivity to the 0.61 power, and decreases with body mass and environmental temperature as predicted by metabolic theory when individuals are below their thermoneutral zone. We note a negative relationship between ambient temperature and average body mass, and suggest that this community-level pattern, reminiscent of Bergmann's Rule, is related to a tendency for small species to be less-abundant or absent from cold locations. Results from this study emphasize the importance of individual-level metabolism for understanding large-scale ecological patterns.