Premigratory ruby-throated hummingbirds, Archilochus colubris, exhibit multiple strategies for fuelling migration

Many avian species fatten to fuel migratory flights. However, the amount of fat deposited prior to departure is variable depending on individual migration strategies. Despite their small size and high mass-specific metabolic rates, migratory hummingbirds at isolated meadows can fatten up to 44% in just 4 days prior to resuming migration, suggesting profound changes in energy acquisition. However, it remains to be seen whether hummingbirds fatten at the breeding grounds prior to initiating migration. Using feeder stations outfitted with radiofrequency identification readers and digital scales, we identified a subset of premigratory ruby-throated hummingbirds that exhibited significant mass gain in the 4 days leading up to migration (premigratory fattening) and identified others that did not (premigratory nonfattening). We further assessed foraging behaviour, monitored individual mass throughout the day and calculated rates of overnight mass loss to understand what behavioural variation allowed some premigratory birds to rapidly fatten. Premigratory fattening hummingbirds abandoned foraging restraint during the middle of the day, a behaviour thought to enhance aerial agility, and increased foraging effort during both the middle of the day and the evenings by increasing the duration but not the frequency of feeder visits. Groups did not differ in their morning foraging strategy. Premigratory fattening hummingbirds also lost mass overnight at reduced rates, implying that birds conserved energy to minimize the depletion of existing fat stores, possibly via increased nocturnal torpor use. Fattening hummingbirds used a two-pronged approach of increasing energy intake during specific daily periods and reducing overnight energy expenditure to achieve substantial premigratory mass gain over just 4 days. However, not all hummingbirds adopted this premigratory fuelling strategy; those that did were adults (>1 year old), suggesting that the use of a premigratory fuelling strategy may be age related