In an interesting paper published online recently in Proceedings of the Royal Society B (Boyle et al 2010: see link below), Dr. Alice Boyle and co-workers present research into the effect of storms upon migration in tropical birds. Many animals, including birds, bats and insects, migrate up and down elevational gradients, from the lowlands to higher ground in mountainous regions of tropical forest. This is known as altitudinal migration, and the factors that drive animals to migrate in this fashion remain obscure. One idea, proposed forty years ago by Alexander Skutch, is that tropical storms could force tropical birds to migrate downhill. In this paper, Dr. Boyle and her co-workers test this idea, and also suggest a mechanism. They propose that one explanation for these patterns may be that storms reduce the time a bird can spend finding food at higher altitudes more than at lower altitudes, and that this reduction in foraging time may pose a very real risk of starvation.
Dr. Boyle tested these hypotheses with a species of partially migratory tropical bird, the White-ruffed manakin. White-ruffed manakins are small fruit-eating birds that breed in the wet, mountainous forests of Central America. After the breeding season some birds migrate downhill to lower altitudes whilst some remain at higher altitudes. Dr. Boyle found that capture rates at higher elevation decrease during storms and increase at lower altitudes, which suggests that storms do play a role in altitudinal migration in this species. Interestingly, at high altitudes before storms there is a bias towards males, and after storms a bias towards females, which may be linked to differences in body size between sexes (males are around 15% smaller). By collecting blood samples from birds and analyzing physiological measures of body condition, Dr. Boyle also found support for her ‘limited foraging opportunities’ hypothesis: rainfall was associated with plasma coricosterone levels, fat stores, plasma metabolites and haematocrit. These results suggest that weather-related risks for species requiring high food intakes can explain the altitudinal migrations of tropical animals. This is an important advance in our understanding of animal migration in the tropics, which may be important in conservation efforts. Furthermore, this work suggests that climate change may have significant impacts upon migratory birds in the tropics, as global warming is predicted to alter the severity and timing of rainfall events in tropical regions of the world.
Here is the link to the full paper: