Predictability of food supply modulates nocturnal hypothermia in a small passerine.

Johan F Nilsson,Juli Broggi,Hannah Watson,Jan-Åke Nilsson

doi:10.1098/rsbl.2020.0133

Abstract

The combination of short days and long cold winter nights, in temperate regions, presents a major challenge for small diurnal birds. Small birds regularly employ heterothermy and enter rest-phase hypothermia during winter nights to conserve energy. However, we know little about how environmental conditions, such as food availability, shape these strategies. We experimentally manipulated food availability in winter to free-living great tits Parus major. A ‘predictable' and constant food supply was provided to birds in one area of a forest, while birds in another area did not have access to a reliable supplementary food source. We found that predictability of food affected the extent of nocturnal hypothermia, but the response differed between the sexes. Whereas male nocturnal body temperature was similar regardless of food availability, females exposed to a naturally ‘unpredictable' food supply entered deeper hypothermia at night, compared with females that had access to predictable food and compared with males in both treatment groups. We suggest that this response is likely a consequence of dominance, and subdominant females subject to unpredictable food resources cannot maintain sufficient energy intake, resulting in a higher demand for energy conservation at night.

Highlights

Energy is the currency of life, and animals must obtain sufficient resources to meet their metabolic demands
The average distance between a nest-box and a feeder in the predictable area was 0.12 ± 0.07 km. Another area within the same forest received no food supplementation, and birds wintering in this area were reliant on natural food resources, which are typically unpredictable in winter and certainly less predictable than permanent feeding stations
While food deprivation has been shown to induce reductions in nocturnal Tb in birds, studies have been performed in captivity and imposed a more extreme scenario of reduced food availability [13,14]

Summary

Introduction

Energy is the currency of life, and animals must obtain sufficient resources to meet their metabolic demands. Small birds demand high energy intake to fuel a high metabolic rate, in part resulting from a high rate of heat loss owing to a large surface-area-to-volume-ratio [2,3,4]. This becomes especially critical during the nocturnal roosting period, when individuals require sufficient energy resources to survive the long winter night, but a combination of low food availability, low ambient temperatures (Ta), inclement weather and short days can impose energetic constraints. The use of nocturnal hypothermia by small diurnal birds could reduce metabolic demands by as much as 50% [7] and increase winter survival by up to 58% [8]. Birds do not consistently use a heterothermic strategy, suggesting that a regulated reduction in Tb carries costs, such as increased predation risk [9,10], altered sleep patterns [11] and reduced efficiency of cellular body temperature (°C)

Methods

Results

Conclusion