Tw. Sherry et Rt. Holmes, WINTER HABITAT QUALITY, POPULATION LIMITATION, AND CONSERVATION OF NEOTROPICAL NEARCTIC MIGRANT BIRDS, Ecology, 77(1), 1996, pp. 36-48
Recent declines in Neotropical-Nearctic migrant songbird populations a
re often attributed to events during the nonbreeding season, such as t
ropical habitat conversion and drought. Support for this hypothesis in
most species, however, is largely anecdotal or conjectural. There is
a dearth of demographic information about migrants on their Neotropica
l winter grounds. Such data are needed to identify specific ecological
factors influencing survival, dispersal, and, ultimately, population
abundances aggregated over multiple habitats at regional spatial scale
s. In this paper, we review several lines of evidence, emphasizing res
ults of our research on paruline warblers in Jamaica, which indicate t
hat migrant passerines often compete intraspecifically in winter for p
referred quality habitats and that their populations may be limited at
least in part by ecological conditions in winter. The demographic and
ecological evidence supporting this hypothesis for migrant passerines
includes: (1) differing densities among habitats, suggesting variatio
n in habitat suitability; (2) strong territoriality, site attachment,
and site fidelity; (3) experimental demonstrations of habitat saturati
on; (4) nonrandom distributions of sex and age classes among habitats;
(5) overwinter decline of body mass by individuals occupying the most
drought-stressed habitats; and (6) different residence times among ha
bitats, suggesting differences in survival or dispersal. We review eco
logical and behavioral explanations for these demographic patterns, an
d make conservation recommendations based on our understanding of how
local demographic circumstances affect broader scale population proces
ses.