Environmental determinants of seasonal changes in bird diversity of Mediterranean oakwoods

The strong season-to-season variation (seasonality) in abiotic factors and productivity shape the changing patterns of species distribution and diversity throughout the year in temperate ecosystems. However, the determinants of seasonal changes within animal communities have rarely been explored, and the prognosis of community variation typically relies on identifying simple factors (e.g., mean temperature) that are assumed to have a constant effect throughout the year. Here we analyze the competing and changing roles of biotic (vegetation structure and phenology) and abiotic (temperature and elevation) factors in determining the richness and nestedness of montane Mediterranean oakwoods (central Spain) bird species in winter and spring. In winter, the most energy-demanding period, birds prefer mature forests with higher nocturnal temperatures where they can minimize thermoregulation costs during the long winter nights. In spring, which is the breeding season, spatial variation of species richness and nestedness is more deterministic than in winter. Breeding birds prefer lower forests with cooler temperatures at midday (presumably to avoid summer overheating stress), less unpredictable weather, and where trees develop leaves earlier (suggesting that birds, particularly those that prey on folivorous insects, would be able to breed early in the season). Thus, although both biotic and abiotic factors take part in the assemblage of local communities, the intervening specific components vary between seasons. For example, temperature—the factor most widely used to forecast future community changes—had opposite effects in winter than in spring. These results highlight the importance of fine-grained scale studies in accounting for temporal variation to understand both current and future regional biodiversity patterns.