Storms drive altitudinal migration in a tropical bird

Abstract

Although migration is a widespread and taxonomically diverse behaviour, the ecological factors shaping migratory behaviour are poorly understood. Like other montane taxa, many birds migrate along elevational gradients in the tropics. Forty years ago, Alexander Skutch postulated that severe storms could drive birds to migrate downhill. Here, we articulate a novel mechanism that could link storms to mortality risks via reductions in foraging time and provide, to our knowledge, the first tests of this hypothesis in the White-ruffed Manakin (Corapipo altera), a small partially migratory frugivore breeding on the Atlantic slope of Costa Rica. As predicted, variation in rainfall was associated with plasma corticosterone levels, fat stores, plasma metabolites and haematocrit. By collecting data at high and low elevation sites simultaneously, we also found that high-elevation residents were more adversely affected by storms than low elevation migrants. These results, together with striking temporal capture patterns of altitudinal migrants relative to storms, provide, to our knowledge, the first evidence that weather-related risks incurred by species requiring high food intake rates can explain altitudinal migrations of tropical animals. These findings resolve conflicting evidence for and against food limitation being important in the evolution of this behaviour, and highlight how endogenous and exogenous processes influence life-history trade-offs made by individuals in the wild. Because seasonal storms are a defining characteristic of most tropical ecosystems and rainfall patterns will probably change in ensuing decades, these results have important implications for understanding the ecology, evolution and conservation of tropical animals.

Lekking birds in a tropical forest forego sex for migration

“…Differences between years in dD values are consistent with the costs of residency being weather-related [10]. In 2008, when the preceding season was rainier, more individuals abandoned breeding areas, regardless of social status.…”

“…This frugivorous species breeds in montane forests and some, but not all, individuals migrate to lower elevations after breeding [6] (electronic supplementary material). After eliminating alternative hypotheses regarding the causes of altitudinal migration in this community and population [7][8][9], we showed that residents incur physiological costs of high-elevation storms that limit foraging opportunities, compromising condition [10]. Here, we test the hypothesis that partial migration results from these costs being countered by reproductive benefits of residency, with migratory decisions dependent upon mating prospects the following year (electronic supplementary material, figure S1).…”

Experimental effects of early‐life corticosterone on the hypothalamic–pituitary–adrenal axis and pre‐migratory behaviour in a wild songbird

“…Further, it has been shown in adult birds that adverse weather activates the HPA axis and alters behaviour to facilitate survival (Wingfield et al 1983;Silverin 1986;Rogers et al 1993;Breuner & Hahn 2003;Boyle, Norris & Guglielmo 2010).…”

Phenology of tropical understory trees: patterns and correlates

“…If attack rates vary in seasonally predictable ways, pathogens could also contribute to shaping temporal patterns of fruiting. Finally, because rainfall is a major axis of seasonality in these forests, influencing plant growth (Brearley et al 2007), seed disperser behavior (Boyle et al 2010), and likely pathogen populations, we related precipitation to community-level phenological patterns.…”