Hypothermic abilities of migratory songbirds at a stopover site

Abstract

Migratory birds use stopover sites for resting and feeding. Breaks after prolonged flights may allow cooling down the muscles and save energy through hypothermia. We measured hypothermic abilities in two long-distance migratory songbirds (garden warbler, Sylvia borin, and icterine warbler, Hippolais polyglotta) at a stopover site during spring migration selecting (1) birds just arrived after a prolonged flight (morning) (2) birds that had been resting in the vegetation for several hours (afternoon) and (3) birds caught in the late afternoon and kept overnight in standard conditions (night). In the first two samples, body temperature was measured immediately, 30 and 180 min after capture. In the night sample, it was measured at 1 a.m. in birds awake and asleep, and at 7 a.m., when all birds were awake. Morning birds showed the highest values (39.7°C), dropping of about 3°C after 180 min. In the afternoon birds (2), the time course was similar, but values (38.5°C) were lower than that of morning birds of about 1°C. At night, asleep birds had lower temperature than awake birds, reaching up to 10°C less than daytime values, but recovering in the early morning. Such remarkable plasticity in thermogenic abilities during migration could allow small migrants to economize energy in emergency situations.