Abstract
Stress in birds has been widely studied through the measurement of heterophil-to-lymphocyte ratio (H/L ratio). In this study we aimed to assess for the first time the potential variation of stress, measured as H/L ratio, associated to geography (between-country variation) and seasonality (between seasons and within the breeding season), as well as the leukocyte profiles, in the threatened Dupont’s Lark (Chersophilus duponti), using samples from Spain and Morocco. Furthermore, we tested whether variation in H/L ratio was associated with variables such as population density, presence of blood parasites and individual body condition. We found that H/L ratio did not vary between countries, but individuals captured during the breeding season showed higher values of H/L compared to non-breeding ones. Neither male density, nor date within the breeding season had an effect on the H/L ratio. In Spain, individuals with higher body condition showed lower H/L ratio regardless of whether they were malaria-infected. In Morocco, malaria-infected individuals showed higher values of H/L ratio than the non-infected birds. Moreover, we found that our average values of H/L ratio in Morocco were within the ranges of other passerines, but not in Spain. Individuals with higher H/L ratios may be more stressed or present higher capability to face stressful situations. Although H/L ratio is a useful and relatively easy way to obtain measure of stress, the impact that the environment might have on stress and the way it is explained by H/L ratio must be addressed carefully. This study provides new insight for this species’ biology and provides useful reference information to test the status and survival of other populations.
Highlights
In many species, the physiological stress of individuals is often the first compensatory response to intrinsic or extrinsic stressors—including those mediated by human activities— aimed to preserve homeostasis and improve the chances of survival and reproduction (Dantzer et al 2014)
H/L ratios detected in Morocco roughly coincide with the average H/L values calculated over different passerine species in a recent review (0.53 ± 0.02; Minias et al 2019), while the values from Spain (1.11 ± 1.18) are clearly above such average and more similar to the mean reported for ancestral avian lineages (1.13 ± 0.08, Minias et al 2019) or passerines experiencing stress levels (Cornelius et al 2014)
The limitations in the knowledge about the relationship between H/L ratio and health status and the unbalanced nature of our dataset implies some caution about the conclusions, but we believe that our results may inform about the health status of populations of this threatened lark
Summary
The physiological stress of individuals is often the first compensatory response to intrinsic or extrinsic stressors—including those mediated by human activities— aimed to preserve homeostasis and improve the chances of survival and reproduction (Dantzer et al 2014). Hormones (e.g. corticosterone) have been widely used to assess physiological stress (Davis et al 2008; Cockrem 2013; Skwarska 2018) Their concentrations in blood might change rapidly (1–3 min) following disturbance (short-term stress indicator), which makes its use challenging (Scanes et al 2020) when working with individuals in the wild, whose control and manipulation is not immediate. The use of haematological parameters, such as the heterophil-to-lymphocyte ratio (hereafter H/L ratio) quantified in blood smears, is considered a reliable measure of long-term physiological stress on birds (Davis and Maney 2018; Gormally and Romero 2020), as they vary after some time (1–4 h depending on the species and the stressor’s nature) since exposure to the stressor and they tend to endure longer than corticosterone levels in blood The use of haematological parameters, such as the heterophil-to-lymphocyte ratio (hereafter H/L ratio) quantified in blood smears, is considered a reliable measure of long-term physiological stress on birds (Davis and Maney 2018; Gormally and Romero 2020), as they vary after some time (1–4 h depending on the species and the stressor’s nature) since exposure to the stressor and they tend to endure longer than corticosterone levels in blood (Vleck et al 2000; Davis et al 2008; Müller et al 2011; Lebigre et al 2012; Goessling et al 2015; Davis and Maney. 2018; Skwarska 2018; Minias 2019; Latimer et al 2020)
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