Heat dissipation rate constrains reproductive investment in a wild bird

Abstract

The “heat dissipation limit” theory (HDL) posits that animals with higher capacity to dissipate metabolic heat can increase reproductive investment. This theory remains untested in the wild. We recently showed that increased workload in a small bird causally relates to maximum body temperature. Here, we have expanded this approach by experimentally facilitating sensible heat transfer rate in nestling‐feeding blue tits—a small bird with high resting‐ and work‐induced body temperatures—through removal of ventral plumage. Feather‐clipped parents did not increase work rate but sired larger, and sometimes heavier, nestlings while maintaining lower body temperature and losing less body mass than controls. Thus, when relieved of the demands to dissipate metabolic heat, parents could invest more into both current (nestling condition) and future (self‐maintenance) reproduction. In accordance with the HDL theory, we conclude that constraints on heat dissipation rate could be a potent mediator of life‐history trade‐offs in wild animals. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13243/suppinfo is available for this article.

Evidence that stress-induced changes in surface temperature serve a thermoregulatory function

“…To test this prediction, we sought to quantify and compare total heat transfer (q tot ) from the eye region of captive Chickadee, in stress-induced and control treatments. Here, heat transfer was calculated from thermographic images using methods described by Ward et al (1999), McCafferty et al (2011), and Nord and Nilsson (2019).…”

“…Here, the values for kinematic viscosity of air (m 2 /S; at an assumed atmospheric pressure of 101.325 kPa), thermal conductivity of air (W/m °C -1 ), and thermal expansion coefficient of air (1/K) used in the calculation of qRad and q Conv were calculated according to Ta at the time of image capture. Eye region surfaces were treated as planar structures, similar to the ventral surfaces of Blue Tits ( Parus caeruleus ) described in Nord and Nilsson (2019), and the surface area of the imaged eye region was estimated to be an oval of 1.0 cm vertical diameter and 1.1 cm horizontal diameter. Final q Total values were multiplied by two to represent total heat transfer across both eye regions of an individual.…”

Evidence that stress-induced changes in surface temperature serve a thermoregulatory function

“…To test this prediction, we sought to quantify and compare total heat transfer (q total ) from the eye region of captive chickadee, in stress-induced and control treatments. Here, heat transfer was calculated using maximum eye temperature measurements that were extracted from thermographic images, and by following methodology described by Ward et al (1999), McCafferty et al (2011 and Nord and Nilsson (2019), with slight modifications (see Appendix). Because individuals were sheltered from wind in flight enclosures, and were unlikely to transfer heat by direct contact between the eye region and a medium other than air, q total was assumed to be the sum of radiative heat transfer (q rad. )…”

Endocrinology of thermoregulation in birds in a changing climate

“…The challenge may be especially severe for species adapted to relatively narrow temperature ranges (Boyles et al, 2011a;Boyles et al, 2011b), such as birds in the tropic (Huey et al, 2012;Khaliq et al, 2014), and those already operating close to their upper thermal limit such as desert birds (Gerson et al, 2019). However, even birds inhabiting far less extreme, thermally variable, and generally colder habitats, may suffer the negative consequences of acute heat stress (blue tits and great tits (Andreasson et al, 2018;Nord and Nilsson, 2019;Rodriguez et al, 2016).…”

“…Behavioral changes are likely to be immediate and may, at least partly, mitigate effects of acute thermal stress. For example, several studies show that the amount of physical work in birds is reduced at high ambient temperature (du Plessis et al, 2012;Powers et al, 2017), presumably to avoid somatic costs of overheating (Nilsson and Nord, 2018), though at the potential cost of reduced reproductive success (Nord and Nilsson, 2019). Beyond endocrine regulation, a major constraint on plasticity is likely to be the energetic (heat production) or water constraint (of by evaporative cooling), see §3.…”