Partial migration of tropical birds was long believed to be driven by variation in food abundance. Recent evidence from a partially-migratory species suggests that in contrast, limited foraging opportunities at high elevations during severe wet season storms drives the most metabolically-challenged individuals down to elevations where rainfall is lighter. Here, I test community-level predictions of this hypothesis by examining the relationship between high-elevation rainfall in the second half of the year and counts of migrant birds in lowland forest during late December each year from 19902009. I contrast results derived from analysis of all migrant species with both analyses of only the frugivorous migrants, and analyses of resident species. Counts of migrant species were on average positively associated with montane rainfall with differences of up to 72% in the numbers of birds counted in drier or wetter years. Frugivores and smaller birds responded more strongly to variation in rainfall compared to the broader migrant species pool. Interestingly, counts of resident species were also higher following wetter montane wet seasons. Results of analyses exploring the cause of resident responses were not consistent with climatic effects on breeding productivity or short-term weather effects on detectability. Results were, however, consistent with cryptic down-slope migration of individuals breeding at higher elevations augmenting lowland resident populations in wet years. These results suggest that changes in rainfall amount, storm intensity, and timing of severe weather events would lead to large increases in or losses of an important behaviour.
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