Patterns and dynamics of rest-phase hypothermia in wild and captive blue tits during winter

Abstract

We evaluated biotic and abiotic predictors of rest-phase hypothermia in wintering blue tits (Cyanistes caeruleus) and also assessed how food availability influences nightly thermoregulation. On any given night, captive blue tits (with unrestricted access to food) remained largely homeothermic, whereas free-ranging birds decreased their body temperature (T(b)) by about 5 degrees C. This was not an effect of increased stress in the aviary as we found no difference in circulating corticosterone between groups. Nocturnal T(b) in free-ranging birds varied with ambient temperature, date and time. Conversely, T(b) in captive birds could not be explained by climatic or temporal factors, but differed slightly between the sexes. We argue that the degree of hypothermia is controlled predominantly by birds' ability to obtain sufficient energy reserves during the day. However, environmental factors became increasingly important for thermoregulation when resources were limited. Moreover, as birds did not enter hypothermia in captivity when food was abundant, we suggest that this strategy has associated costs and hence is avoided whenever resource levels permit.

Challenges of measuring body temperatures of free-ranging birds and mammals

“…This fast, affordable, and minimally invasive method relies on inserting a thin temperaturesensitive probe into the rectum via the anus or cloaca [20][21][22]. It is commonly undertaken using thermocouples or thermistors either custom adapted for use or using standard medical or veterinary thermometers.…”

“…Thermometry has wide applicability in ecology, and has been used for example to record functional thermoregulation in relation to climatic variables, immune function, natural occurrence of pathogen infection and when studying the ontogeny of thermoregulation [20,[24][25][26][27]. While highly accurate and repeatable [20], thermometry generally only provides a cross-sectional temperature sample, because subjects must be captured and handled during sampling.…”

“…Thermometry has wide applicability in ecology, and has been used for example to record functional thermoregulation in relation to climatic variables, immune function, natural occurrence of pathogen infection and when studying the ontogeny of thermoregulation [20,[24][25][26][27]. While highly accurate and repeatable [20], thermometry generally only provides a cross-sectional temperature sample, because subjects must be captured and handled during sampling. The stress of capture and indeed the temperature measurement itself can bring about stress-induced hyperthermia (SIH) mediated by the sympathetic adrenal and the hypothalamic-pituitary-adrenal (HPA) axes, as well as through increased metabolic heat production associated with the escape response [28,29].…”

Nocturnal body temperature in wintering blue tits is affected by roost-site temperature and body reserves

“…However, hypothermia is rarely used at its maximum energy-conserving potential. Rather, it appears to be closely regulated in relation to prevailing ambient conditions (Merola-Zwartjes and Ligon 2000; Lane et al 2004) and energy reserve levels (Reinertsen and Haftorn 1984;Nord et al 2009), and some species refrain from using hypothermia unless they have been starved (McKechnie and Lovegrove 2003;Laurila and Hohtola 2005). We therefore predicted that an experimental alleviation of the energetic requirements of T b regulation would be paralleled by a reduction in the extent of hypothermia.…”

“…Rest-phase hypothermia (hereafter referred to as hypothermia), during which T b is mildly reduced (·10°C below normothermic levels) and responsiveness to external stimuli is maintained, is the most widespread heterothermic strategy in birds (McKechnie and Lovegrove 2002). It is typically employed to counter increased metabolic demands during periods of energetic stress (Nord et al 2009) and can provide important energetic beneWts even at a comparatively mild degree of T b reduction (Reinertsen and Haftorn 1984;Maddocks and Geiser 1997;Cooper and Gessaman 2005). However, hypothermia is rarely used at its maximum energy-conserving potential.…”