Age differences in night-time metabolic rate and body temperature in a small passerine
Spending the winter in northern climes with short days and cold ambient temperatures ( T a ) can be energetically challenging for small birds that have high metabolic and heat loss rates. Hence, maintaining body temperature ( T b ) in T a below thermoneutrality can be energetically costly for a small bird. We still know little about how increased heat production below thermoneutrality affects the level at which T b is maintained, and if these patterns are age specific. To test this, we measured subcutaneous body temperature ( T s ) and resting metabolic rate (RMR) simultaneously in blue tits ( Cyanistes caeruleus ) during winter nights in T a ’s ranging from 25 to − 15 °C. RMR increased below the lower critical temperature (LCT, estimated at 14 °C) and was 6% higher in young (birds in their first winter) compared to old birds (birds in their second winter or older). The higher RMR was also mirrored in higher T s and thermal conductance ( C ) in young birds, which we suggest could be caused by age differences in plumage quality, likely driven by time constraints during moult. Reduction in nightly predicted T b was modest and increased again at the coldest ambient temperatures, suggesting that either heat retention or heat production (or both) improved when T a reached levels which are cold by the standards of birds in our population. Our results show that levels of heat production and T b can be age specific. Further studies should address age-specific differences on quality, structure, and thermal conductivity of plumage more explicitly, to investigate the role of variation in insulation in age-linked metabolic phenotypes.