Abstract
Energy availability explains the spatial variation of species richness. However, in arctic to temperate regions species richness in birds shows also a seasonal pattern with high species numbers in summer and low numbers in winter. Is it possible to explain this pattern by high energy availability in summer and low availability in winter? The ‘energy availability hypothesis’ rests on a positive correlation between abundance and species richness. Using weekly counts along a transect in Northern Bavaria (S Germany), we found the expected seasonal annual as well as biannual cycles in species richness (108 species observed in total). In contrast, the number of individuals showed no clear seasonal cycle, but erratic fluctuations particularly in winter. Weather conditions had only small effects on the number of individuals and on species richness. Adding weather variables to analyses increased explained variance only by 1.5% to 10%. The energy availability hypothesis is not able to explain the seasonal variation of species richness in temperate regions. However, bird assemblages during winter consist of species able to feed on various resource types, in particular seeds produced during summer. These left-overs provide the resources for diet generalists to survive the winter.
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