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.