Torpor-assisted migration: Regular use of shallow torpor reduces rest-phase energy expenditure for songbirds during migration.

Endothermy evolved independently in birds and mammals providing many fitness benefits but requiring substantial energy to maintain a stable body temperature. Many endotherms have evolved heterothermic strategies to cope with daily and seasonally cold weather and reduced food availability. Heterothermy has been studied extensively in mammals, but comparatively few studies have considered heterothermy in birds. Heterothermy may be beneficial to migrating endotherms to reduce thermoregulatory energy expenditure and enhance refuelling rate at stopover sites, known as torpor-assisted migration. We hypothesized that migratory birds use shallow torpor to reduce rest-phase energy expenditure. We tested four species for shallow torpor use during migration: Brown Creeper Certhia americana, Golden-crowned Kinglet Regulus satrapa, Ruby-crowned Kinglet Corthylio calendula and Yellow-rumped Warbler (Myrtle subspecies) Setophaga coronata coronata. We measured body temperature using temperature-sensitive PIT tags and metabolic rate using open-flow respirometry at night. All four species from three different passerine families (Regulidae, Certhiidae and Parulidae) used shallow torpor frequently (69%-98% of birds per species) over a range of ambient temperatures (-1 to 30°C) during migration. Body condition was the main driver of torpor use. Birds with smaller fat stores used torpor more deeply and frequently than fatter birds, achieving up to 42% lower metabolic rates and saving more energy. Contrary to studies on deep torpor, ambient temperature had no effect on the frequency or depth of torpor use. Shallow torpor has wide-ranging implications for the energetics of migration with broader impacts on survival and reproduction and provides an opportunity to compare heterothermic strategies between birds and mammals.