Investigating Natural Variation in Physiological Indicators Across a Forest Bird Community

Submitted by Michi on 25 March 2025.

Photo above: The European robin (Erithacus rubecula) was the most frequently captured species in our forest community (Photo: Finja Strehmann)

Birds face a wide range of challenges throughout their lives—some imposed by the environment, others shaped by intrinsic factors such as age, sex, and breeding status. Their ability to physiologically adapt to these challenges determines not only their individual survival but also their reproductive success. Studying these physiological responses can provide valuable insights into early warning indicators of population health, ultimately contributing to conservation efforts. However, before we can assess the impact of environmental factors on bird communities, it is crucial to first understand natural variation in physiological indicators.

A community-wide perspective on physiological indicators 
In their recently published study, Finja Strehmann and colleagues examined the natural variation in the heterophil-to-lymphocyte (H/L) ratio—a well-established physiological indicator of immune function, overall health, and long-term physiological adaptation—across a bird community inhabiting a managed mixed forest in Germany. Building on previous research that has often focused on single species, their study examined multiple species within a community, incorporating their evolutionary relatedness through a phylogenetically controlled approach.

                                                               Team of researchers and student assistants during field work in the early breeding season (Photo: Sascha Rösner)

Over four years, from March until August, with a dedicated team of researchers and student assistants, they collected blood samples from 409 individuals spanning 22 species at ten mist-netting sites across the forest ecosystem. The study design included both standard mist nets and high nets to sample individuals at different canopy levels, enhancing the diversity of captured species. No baits or lures were used to reduce confounding influences on the variation of the H/L ratio.

                                                            Taking a blood sample of a blackbird (Turdus merula) by puncturing the wing vein (Photo Finja Strehmann)

Leveraging deep learning for automated blood cell analysis 
One of the significant advancements in the study by Strehmann et al. was the use of a deep learning approach to automate the identification and quantification of blood cells. By developing a convolutional neural network (CNN), they streamlined the process of analyzing blood smears, reducing human error and making large-scale physiological assessments more feasible. This novel method significantly enhanced the efficiency and accuracy of H/L ratio calculations, opening the door for broader applications in ecological and conservation physiology.

Key findings: natural variation in physiological indicators 
The study revealed that intrinsic factors such as age, sex, and breeding status influence H/L ratios across the bird community. Adult birds exhibited higher ratios than juveniles, while females showed slightly elevated values compared to males, particularly during the breeding season. Incubating birds also tended to have higher H/L ratios. Additionally, a significant phylogenetic signal was observed, highlighting the importance of considering evolutionary relationships when interpreting physiological indicators. These findings provide a foundation for further research exploring environmental influences such as habitat disturbance, climate change, and parasite infestations on bird health.

                                                       Early morning at one of the mist netting locations just before opening the nets for capture routine in the forest (Photo: Finja Strehmann)

Text by Finja Strehmann

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