Abstract
AbstractAvailable energy stores determine stopover length, migration speed and likely survival in migrating birds. We measured energy stores by estimating fuel load in 11 species of Neotropical migrant songbirds in the Darién of Colombia over five years. We evaluated 1) whether individuals flying further from breeding origin arrived with smaller fuel loads, 2) if the ENSO (El Niño-Southern Oscillation) cycle affected fuel load and 3) if species known to migrate mostly overwater arrived with less fuel relative to those migrating overland. Breeding origin, inferred from feather δ2H values, only had a significant positive effect on fuel load in Swainson’s Thrush (Catharus ustulatus). Veery (Catharus fuscescens) and Swainson’s thrush had higher and lower fuel loads, respectively, in El Niño years. Multi-species mixed-effects models revealed support for larger fuel loads in larger-bodied species and in species taking overwater routes, contrary to our prediction. Across species, we found no support for common effects of breeding origin or ENSO on fuel loads, in contrast to community-wide effects of migration route and body-size. In general, the variables considered here explained little of the variance in fuel loads, suggesting that inter-individual differences likely have a greater impact than broad-scale factors in our study system.
Highlights
As a group and throughout the World, long-distance migrant birds are declining at faster rates than short-distance migrants and resident species [1, 2]
To determine which factors most strongly influence fuel loads and vulnerability during migration in Neotropical migratory landbirds, we evaluated the impact of several factors on arrival fuel loads in a group of 11 Neotropical migrants moving through a major stopover region [3, 14, 21], the Darién, in northwestern Colombia on fall migration
We aimed to determine: 1) whether individuals from northern origins that travel further to reach the Darién, arrive with smaller fuel loads than those from southern origins, 2) if migrants are affected by the El Niño Southern Oscillation (ENSO) cycle, whereby El Niño years result in smaller fuel loads on arrival relative to la Niña years, and 3) if species known to migrate mostly over water arrived with lower fuel loads relative to those that migrate mostly overland
Summary
As a group and throughout the World, long-distance migrant birds are declining at faster rates than short-distance migrants and resident species [1, 2]. Why this group appears to be more vulnerable than other groups of birds is still open to speculation. Breeding origin is one factor that can influence survivorship [3, 4], with longer migrations involving greater risks [5]. For Neotropical migratory birds that cross the Gulf of Mexico and the Caribbean Sea, such factors are linked to the El Niño Southern Oscillation (ENSO)[7, 8]. Migratory success in songbirds has been linked to experience, with younger, more inexperienced birds expected to have lower survival during migratory episodes [9]
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