P1.184 Wednesday, Jan. 4 Altitudinal differences in songbird fattening rates during fall migration WILLIAMS, Tony D*; EVANS OGDEN, Lesley J; MARTIN, Kathy; Simon Fraser Univ., Burnaby, Canada; Univ. of British Columbia, Vancouver, Canada; Univ. of British Columbia, Vancouver, Canada email@example.com
It is clear that stopover habitats can be limiting for some migratory songbirds and this can lead to population limitation. Many songbird species have been observed using high altitude ‘stopover’ habitat throughout North America. Thus, it is important to determine if these ‘alternative’ habitats represent high quality stopover sites, supporting high fattening rates, such that birds can refuel without deviating far from their ‘optimal’ migration schedules. In this study we used plasma metabolite analysis to estimate fattening rates of four species of migratory songbirds during fall migration over three years at two low altitude (< 25 m ASL) and two high altitude (1200 m ASL) sites in southwestern British Columbia, Canada. We predicted that fattening rates would be greater at high altitude sites during fall migration, especially in frugivorous species, due to the delayed phenology of food availability at higher altitude. Our analyses supported this prediction for three of four species: fox sparrow, golden-crowned sparrow and hermit thrush all had higher estimated fattening rates at higher altitude sites. These three species have the most frugivorous diets during fall migration. In contrast, the orange-crowned warbler, which is largely insectivorous during fall migration showed the opposite pattern, with higher estimated fattening rates at low altitude sites. There were no differences in estimated fattening rates among the three more frugivorous species at high altitude sites, but all species had higher fattening rates than the orange-crowned warbler. These data support the hypothesis that altitudinal differences in estimated fattening rate are related to species differences in diet and seasonal phenology of food availability.