Migratory strategies, fat loads and torpor use in migrating hummingbirds

North American bee hummingbirds frequently enter torpor during autumn
migrations to save energy. The frequency with which they use torpor is
mediated by fat loads. To estimate frequencies and temporal patterns of
torpor use, we measured lipid loads and feather hydrogen stable isotope
ratios (δ2H) in 942 individuals of four species of migrating bee
hummingbirds: Selasphorus rufus (Rufous Hummingbird), Selasphorus calliope
(Calliope Hummingbird), Selasphorus platycercus  (Broad-tailed
Hummingbird), and Archilocus alexandri (Black-chinned Hummingbird), at a
stopover site in Southwestern New Mexico. In a subsample of juvenile
individuals of each species, we constructed functions relating lipid loads
and feather δ2H values to torpor incidence, using respirometry. We used
these functions, along with lipid load and δ2H data from the remaining
captured juvenile birds to estimate their temporal patterns of torpor use.
Lipids were significantly higher in adult S. rufus and S. platycercus than
in juveniles but did not differ with age in S. calliope and A. alexandri.
In S. rufus, lipid loads decreased with capture date, while in A.
alexandri they increased. There was no effect of capture date on lipid
loads in S. platycercus and S. calliope. In all species torpor incidence
decreased with increasing fat loads in a predictable fashion. A large
fraction (70%) of S. calliope and a smaller fraction (30%) of S. rufus
individuals entered torpor, but these fractions remained constant over
time. In contrast, the fraction of S. playtcercus individuals that entered
torpor increased with capture date, whereas that of A. alexandri
decreased. Our results revealed similarities, but also unexpected
interspecific differences in fat loads and torpor incidence in migratory
bee hummingbirds. Some of the correlates of these differences remain
unexplained but represent an opportunity to better understand the causes
and consequences of torpor-mediated migration.