Does food abundance explain altitudinal migration in a tropical frugivorous bird?

Abstract

Many animals undergo annual migrations. These movements are well studied at proximate levels, but their fundamental causes are poorly understood. Among tropical frugivorous birds, annual migration is thought to have evolved in the context of exploiting reciprocal peaks in fruit abundance among locations and seasons, yet previous tests of this hypothesis have yielded equivocal results. In this paper, I tested whether protein and (or) fruit limitation explain both uphill and downhill migratory movements in a tropical frugivorous bird, the White-ruffed Manakin ( Corapipo altera Hellmayer, 1906). While White-ruffed Manakins likely migrate uphill to exploit peaks in fruit abundance, I found no evidence that elevational differences in fruit abundance explain the downhill portion of the migratory cycle. This result challenges long-standing ideas regarding the causes of altitudinal migration because it implies that birds seeking to maximize fruit intake should remain sedentary at higher elevations. Data are also inconsistent with the hypothesis that White-ruffed Manakins migrate (either uphill or downhill) to exploit arthropod prey. Future studies should consider how variation in weather, predators, or parasites could help explain altitudinal migrations of birds from breeding areas to nonbreeding areas.