Testing the heat dissipation limit theory in a breeding passerine

Area of Science:

• Physiology
• Ecology
• Animal Behavior

Background:

• The maximum work rate in animals may be constrained by the capacity to dissipate metabolic heat (heat dissipation limitation (HDL) theory).
• This theory has primarily been explored in the context of animals in hot environments, with limited testing in wild populations from temperate regions.

Purpose of the Study:

• To experimentally test the heat dissipation limitation (HDL) theory in a wild bird species, the marsh tit (Poecile palustris).
• To investigate the relationship between reproductive effort, work rate, and body temperature regulation in breeding birds.

Main Methods:

• Experimental manipulation of brood size in breeding marsh tits to alter parental work rate.
• Monitoring of body temperatures in male and female marsh tits under varying ambient temperatures and manipulated work rates.
• Comparison of body temperatures with known thermal tolerance limits for birds.

Main Results:

• Marsh tit parents exhibited elevated body temperatures during nestling feeding, often exceeding 45°C, particularly with increased work rate and ambient temperature.
• These high body temperatures, previously observed mainly in birds from arid regions, suggest potential heat stress in a temperate species.
• Females displayed lower body temperatures than males, potentially due to heat loss via the brood patch.

Conclusions:

• Reproductive effort in marsh tits may be limited by heat dissipation capacity, supporting the HDL theory in wild, non-arid environments.
• Elevated body temperatures indicate potential somatic costs, suggesting HDL theory mediates the trade-off between current and future reproduction.
• Increasing global temperatures may pose future challenges to animal work capacity and sustained physiological function.