Abstract
Spending the winter in northern climes with short days and cold ambient temperatures (Ta) can be energetically challenging for small birds that have high metabolic and heat loss rates. Hence, maintaining body temperature (Tb) in Ta 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 Tb is maintained, and if these patterns are age specific. To test this, we measured subcutaneous body temperature (Ts) and resting metabolic rate (RMR) simultaneously in blue tits (Cyanistes caeruleus) during winter nights in Ta’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 Ts 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 Tb was modest and increased again at the coldest ambient temperatures, suggesting that either heat retention or heat production (or both) improved when Ta reached levels which are cold by the standards of birds in our population. Our results show that levels of heat production and Tb 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.
Highlights
Animals that overwinter at northern latitudes face several energetic challenges
Young birds had a 6% higher resting metabolic rate (RMR) compared to old birds throughout the Ta-range below lower critical temperature (LCT) (P = 0.013, Table 1, Fig. 2a), but the difference between the age groups did not change with Ta
RMR above LCT was higher in young birds and LCT did not differ between the age groups
Summary
Animals that overwinter at northern latitudes face several energetic challenges. It is often cold, days are short, and food supply can be scarce. Days are short, and food supply can be scarce Such conditions are especially challenging for small birds that have high surface area to volume ratio (which increases heat loss rate), high massspecific metabolic rate, and limited capacity to store energy as fat (e.g., Haftorn 1992). A small bird in winter needs to minimize energy expenditure while simultaneously.
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