Migratory strategies, fat loads, and torpor use in migrating hummingbirds

Abstract North American bee-clade hummingbirds (Mellisugini) frequently enter torpor during autumn migrations to save energy. The frequency with which they use torpor is mediated by lipid (fat) loads. To estimate frequencies and temporal patterns of torpor use, we measured fat loads and feather hydrogen stable isotope ratios (δ2H) in 942 individuals of 4 species of migrating bee hummingbirds: Selasphorus rufus (Rufous Hummingbird), S. calliope (Calliope Hummingbird), S. platycercus (Broad-tailed Hummingbird), and Archilochus alexandri (Black-chinned Hummingbird), at a stopover site in Southwestern New Mexico. In a subsample of 63 juveniles across the 4 species, we constructed predictive functions relating fat loads and feather δ2H to torpor incidence, using direct measurements of torpor use via respirometry. We used these functions, along with fat and δ2H data from the remaining 563 captured juvenile birds, to estimate their temporal patterns of torpor use. Fat loads 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, fat loads decreased with capture date, whereas in A. alexandri they increased. We found no effect of capture date on fat loads in S. platycercus and S. calliope. In all species, torpor incidence decreased with increasing fat 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. platycercus individuals entering torpor increased with capture date, whereas that of A. alexandri decreased. Our results revealed similarities and unexpected interspecific differences in fat loads and torpor incidence in migratory bee-clade hummingbirds. Some of these differences are likely the result of dissimilar migration strategies. Others remain unexplained but represent an opportunity to better understand the causes and consequences of torpor-mediated migration.