Authors: Lisa C. Horn, Tarmo K. Remmel, and Bridget J. M. Stutchbury
Year: 2021
Publication: Ornithological Applications
Publication Link: https://academic.oup.com/condor/article-abstract/123/2/duab012/6246117
Keywords: body condition, carry-over effects, climate change, El Niño–Southern Oscillation, Normalized Difference Vegetation Index, spring migration timing, winter habitat productivity
Abstract:
Reduced rainfall in tropical regions decreases the availability of food resources for overwintering songbirds, with negative
impacts on their body condition. Increasingly dry conditions at tropical wintering sites as a result of climate change may
impact the ability of temperate-breeding songbirds to prepare for and execute their spring migration. We studied the carryover
effects of natural climatic fluctuations created by the El Niño–Southern Oscillation (ENSO) in tropical
overwintering areas on 7 long-distance migratory songbirds at a Canadian stopover site. We used the Normalized Difference Vegetation
Index as a proxy for tropical habitat productivity and resource availability and a 34-year bird banding dataset from Long
Point, Ontario, Canada to assess migration timing and stopover body condition. To increase our ability to detect potential
carry-over effects, we employed a novel approach of using recent migratory connectivity studies to restrict the wintering
ranges to areas most likely used by individuals passing through southern Ontario. Using linear models, we found that
overwinter habitat productivity was significantly negatively influenced by dry ENSO events in the overwintering ranges in
3 of 7 species, with a fourth near-significant. Subregional differences in the effect of ENSO on precipitation patterns may
explain why we did not detect a consistent effect of ENSO on overwinter habitat productivity for all species. Despite restricting
the wintering range and using a robust dataset for species with diverse life histories, we detected only weak and
often conflicting evidence of population-level carry-over effects from dry ENSO events and overwinter habitat productivity.
Negative carry-over effects may be strongest and most evident during the earlier stages of migration because birds
may be able to compensate to some extent for poor departure condition and late departure while en route.