Periorbital temperature responses to natural air temperature variation in wild birds.

With global temperatures rapidly increasing, biologists require tools to assess how wild animals are responding to heat. Thermal imaging of the eye region offers a potential non-invasive alternative to traditional techniques to study thermoregulation and stress responses in wild animals. However, we currently have a poor understanding of how the temperature of the eye region is regulated under increasing temperature and whether this regulation differs among individuals. Here, we use thermal imaging to repeatedly measure the maximum temperature of the eye region (periorbital temperature) in 42 wild pied babblers (Turdoides bicolor) under natural air temperatures ranging from 14.3 to 42.5 °C. Our aim was to determine the relationship between periorbital temperature and air temperature, whether this relationship is repeatable, and whether it differs according to individual attributes. Periorbital temperature showed a non-linear increase with air temperature, becoming independent of air temperature above 38 °C. Above 38 °C, periorbital temperature was not explained by any individual attributes. Below 38 °C, periorbital temperature increased more steeply in individuals with low body mass and it was lower in older compared to younger females. However, the effect of these individual attributes was small compared to the effect of wind speed, air temperature and head tilt. Additionally, the repeatability of individual periorbital temperature was low (R < 0.25) and non-significant both below and above 38 °C. Our findings warrant caution in the use of periorbital temperature to infer individual thermoregulatory responses to increasing temperatures, especially in the wild, where control over confounding non-physiological factors is limited.