Intrinsic factors influence a physiological measure across a forest bird community

Physiological stress parameters have the potential to serve as valuable
early warning indicators for the conservation of animal populations.
However, measuring stress in wildlife is often challenging, due to the
lack of knowledge about baseline levels, and intrinsic differences between
individuals across species. This study is aimed at filling this gap by
investigating the influence of intrinsic factors, including sex, age, body
condition, and reproductive status on the physiological stress of a forest
bird community. For measuring stress levels, we used the heterophil to
lymphocyte (H/L) ratio of the bird community, which was assessed using a
novel deep learning approach based on Convolutional Neural Networks
applied to whole blood smear scans. Using phylogenetically controlled
analyses across the bird species, we found higher H/L ratios in adult
birds than in juveniles and observed higher stress levels in females than
in males. While body condition had no effect on the H/L ratio,
reproductive birds tended to have higher H/L ratios than non-reproductive
birds, regardless of their sex. Furthermore, we found a robust
phylogenetic signal of the H/L ratio in the studied bird community. Our
results emphasize the importance of considering intrinsic factors when
using stress physiology for assessing the condition of bird populations
and communities.