BIPHASIC MASS GAIN IN MIGRANT HUMMINGBIRDS - BODY-COMPOSITION CHANGES, TORPOR, AND ECOLOGICAL SIGNIFICANCE

Body mass of migrant Rufous Hummingbirds (Selasphorus rufus) on refueling stopovers increased on average from 3.2 to 4.6 g over a period ranging from several days to 3 wk. In birds arriving with body masses below almost-equal-to 3.5 g, the initial period of mass gain was very slow. This slow gain was not explained by energy costs associated with territory establishment or learning to secure food, since it occurred even in years when nectar resources were superabundant and territoriality was nearly nonexistent. Data on body composition indicate that mass gain up to almost-equal-to 3.5 g was due to deposition of nonlipid body components, which we hypothesize to be proteins involved in rebuilding muscle catabolized during the last stage of the recent migratory flight. Following the initial phase of slow mass gain, an accelerating rise in body mass consisted entirely of lipid gain. On average, overnight mass loss decreased prior to migration, suggesting that nocturnal torpor facilitated lipid deposition. The slow phase of mass gain is a potentially important constraint on migrating hummingbirds, because if they deplete their fat stores and allow their body mass to fall below 3.5 g, they incur a substantial cost in terms of greatly increased time spent on the subsequent stopover.