Authors: Bradley J. Bergstrom, Matthew D. Johnson, Jared C. Harris, and Thomas W. Sherry
Publication: Journal of Field Ornithology
Publication Link: https://onlinelibrary.wiley.com/doi/abs/10.1111/jofo.12294
Keywords: energy stores, fasting, Neotropical migrant, winter ecology, total-body electrical conductivity (TOBEC)
Abstract: Most small birds wintering in the tropics should show little subcutaneous fat deposition (SFD), except in habitats where food availability may decline in late winter or, for some resident species, to prepare for incubation or brooding fasts. However, these predictions need re-examination in light of a new, precise, cross-validated method to compare SFD among habitats and species. We sampled 170 Nearctic-Neotropical migrant and 279 resident birds during early and late winter in 1993 and 1994 in Jamaica, West Indies. Habitats, from greatest to least expected availability of insect prey, were (1) mangrove forest, (2) montane/foothills forest and cultivation, (3) dry limestone forest, and (4) acacia scrub. Percent lipid, estimated from multiple-regression models using visual fat scoring (0–8 scale), total-body electrical conductivity, and a variety of morphometrics, was categorized by percentile ranks to determine if SFD varied by habitat, season, or age for all species, resident species, migrant species, and several individual species. SFD averaged ~ 13% total mass for all birds, ranging from 8–24% for well-sampled species. The few bird species in acacia scrub, primarily two facultative long-distance migrants, averaged ~ 26% lipid content, significantly more than birds in other habitats. Most birds did not vary in SFD in the other three habitats, although Common Yellowthroats (Geothlypis trichas) had greater SFD in dry limestone habitat than in montane habitat. Bananaquits (Coereba flaveola) and Jamaican Euphonias (Euphonia jamaica) in montane habitat, especially in early winter, had higher SFD than other resident species. Contrary to our prediction, adults and juveniles had
similar SFD, with the exception of juveniles having more SFD than adults in acacia scrub habitat. Winter fat deposition (or, in some cases, muscle-protein catabolism) in the tropics may be an overlooked strategy, potentially important as a hedge against fasting for floaters, facultative migrants, some territorial migrants in habitats with seasonal declines in food resources, and some resident species prior to breeding.