Heterothermy in small, migrating passerine birds during stopover: Use of hypothermia at rest accelerates fuel accumulation

For small endothermic animals, heterothermy serves as an energy-saving mechanism for survival in challenging environments, but it may also accelerate fat accumulation in individuals preparing for fuel-demanding activities. This is the first study to demonstrate adaptive hypothermic responses in migrating passerines. While monitoring body temperature (T_b) of eight blackcaps (Sylvia atricapilla) by radiotelemetry, we found that during daytime T _b=42.5±0.4°C (mean ± s.d.); at night T_b decreased to a minimum between 33 and 40°C. We determined the lower limit for normothermy at 37.4°C and found that on 12 out of 34 birdnights of observations under semi-natural conditions blackcaps reduced their Tb below normothermic resting levels with minimum values of 33 and 34.5°C compared with rest-phase normothermic T_b of 38.8±0.8°C. In birds of body mass (m_b) <16.3g, minimum T_b at night correlated with the individual's m_b (r=0.67, P<0.01, N=17), but this was not the case in birds with m_b>16.3g. Minimum nocturnal T_b did not correlate with night-time air temperature (T_a). Measurements of metabolic rate in birds subjected to a Ta of 15°C showed that hypothermia of this magnitude can lead to a reduction of some 30% in energy expenditure compared with birds remaining normothermic. Our data suggest that by reducing the T_b-T_a gradient, blackcaps accelerate their rate of fuel accumulation at a stopover. When body energy reserves are low blackcaps may achieve this reduction by entering hypothermia. Since hypothermia, as seen in blackcaps, may lead to significant energy savings and facilitate body mass gain, we predict that it is common among small migrating passerines.