Thermal Imaging to Study Stress Non-invasively in Unrestrained Birds

Abstract

Stress, a central concept in biology, describes a suite of emergency responses to challenges. Among other responses, stress leads to a change in blood flow that results in a net influx of blood to key organs and an increase in core temperature. This stress-induced hyperthermia is used to assess stress. However, measuring core temperature is invasive. As blood flow is redirected to the core, the periphery of the body can cool. This paper describes a protocol where peripheral body temperature is measured non-invasively in wild blue tits (Cyanistes caeruleus) using infrared thermography. In the field we created a set-up bringing the birds to an ideal position in front of the camera by using a baited box. The camera takes a short thermal video recording of the undisturbed bird before applying a mild stressor (closing the box and therefore capturing the bird), and the bird's response to being trapped is recorded. The bare skin of the eye-region is the warmest area in the image. This allows an automated extraction of the maximum eye-region temperature from each image frame, followed by further steps of manual data filtering removing the most common sources of errors (motion blur, blinking). This protocol provides a time series of eye-region temperature with a fine temporal resolution that allows us to study the dynamics of the stress response non-invasively. Further work needs to demonstrate the usefulness of the method to assess stress, for instance to investigate whether eye-region temperature response is proportional to the strength of the stressor. If this can be confirmed, it will provide a valuable alternative method of stress assessment in animals and will be useful to a wide range of researchers from ecologists, conservation biologists, physiologists to animal welfare researchers.

Using field photography to study avian moult

“…The photographic method can also be carried out in association with other imaging techniques such as thermal imaging cameras to monitor stress levels under challenging conditions (Jerem et al . ).…”

“…Moreover, photographs can benefit from associations with the citizen science movement to cover wider geographical areas. The photographic method can also be carried out in association with other imaging techniques such as thermal imaging cameras to monitor stress levels under challenging conditions (Jerem et al 2015).…”

A temperate endotherm trades thermoregulation for self-preservation: stress-induced changes in surface temperature as thermoregulatory trade-offs

“…leg scales or feathers; discussed by Jerem et al, 2018), and it has previously been shown to be thermally responsive to stress exposure (e.g. Jerem et al, 2015;Robertson et al, 2020). Additionally, we chose to monitor the maximum of the eye region temperature rather than the mean of the eye region temperature because it is thought to be less vulnerable to measurement error, and more consistent across image angles .…”

“…Stress-induced changes in surface temperature have previously been viewed as evolutionary "spandrels" that emerge as a by-product of experimental design, or vasomotor responses that serve alternative functional purposes (e.g. Andreasson et al, 2019;Jerem et al, 2015Jerem et al, , 2018.…”

Welfare of zebra finches used in research

“…This phenomenon known as stress-induced hyperthermia could be utilized to measure stress. Indeed, infrared thermography has been used to determine the effect of stressful procedures in birds (Edgar et al, 2013;Herborn et al, 2015;Jerem et al, 2015) . Contrary to the stress snapshot of corticosterone, this non-invasive imaging method might provide longitudinal measurements of stress.…”