Abstract

At temperate latitudes, altricial birds and their nestlings need to handle night temperatures well below thermoneutrality during the breeding season. Thus, energy costs of thermoregulation might constrain nestling growth, and low nocturnal temperatures might require resources that parents could otherwise have invested into nestlings during the day. To manipulate parental work rate, we performed brood size manipulations in breeding marsh tits (Poecile palustris). Nest box temperatures were always well above ambient temperature and increased with increasing brood size. In line with predictions, a large majority of females (but no males) made use of this benign environment for roosting. Furthermore, females tending enlarged broods, thereby having to work harder during the day, reduced their body temperature at night. This might have reduced nocturnal energy expenditure. Our finding that a higher proportion of enlarged, as compared to control, females continued to use the nest box as roosting sites even after a simulated predation event despite increased vulnerability to predation, further highlighting the need for energy conservation in this group. High nest box attendance and reduced body temperature in brood-reduced females may indicate that these females prioritised self-maintenance by initiating other costly physiological adjustments, e.g. moult, when relieved from parental work. We suggest that the energy demand for defending homeothermy is an element of the general trade-off between current and future reproduction, i.e. between daytime investment in food provisioning and the potential short- and long-term costs of a reduction in body temperature and increased predation risk.Significance statementEven during summer at temperate latitudes, breeding birds need to use energy to maintain stable body temperature. Parents, thus, need to enter the night with sufficient body reserves to cover energy requirements for thermoregulation. As these resources could be used for feeding nestling during the day, adaptations to reduce the cost of thermoregulation would be selected for. We performed brood size manipulations, thereby increasing the need for nestling provisioning in marsh tits (Parus palustris). We found that females typically spent the night in the thermally benign environment of the nest box together with their brood. Females working hard during the day continued to roost in the nest box during the night despite an increase in the perceived risk of nest predation. Furthermore, these females reduced their body temperature at night, thereby reducing the gradient between ambient and body temperature, further reducing the cost of thermoregulation.

Highlights

  • In temperate areas, ambient night temperature is commonly below the thermoneutral zone (TNZ) of most birds throughout the year

  • Experimental category could explain a significant part of the variation in nocturnal nest box temperature (F2,69.1 = 23.8; P < 0.0001), our initial model containing brood size provided a significantly better fit to data (ΔAIC = 7.0)

  • Parents tending reduced broods did not seem to increase investment into feeding effort as nestling body mass did not differ between control and reduced broods

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Summary

Introduction

Ambient night temperature is commonly below the thermoneutral zone (TNZ) of most birds throughout the year. Behav Ecol Sociobiol (2017) 71: 171 environments have been extensively studied during winter (Nilsson and Svensson 1996; McKechnie and Lovegrove 2002; Nord et al 2009), almost no studies have investigated how birds handle the same problem during breeding. This is unfortunate, because nestling provisioning is among the most energetically costly activities of adult birds with altricial young (Drent and Daan 1980). The extent to which adults and nestlings need to devote energy to thermoregulation should affect their breeding and fledging success, respectively

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