Haematologic reference values in free-living red-backed shrike (Lanius collurio) nestlings in an agricultural habitat

Blood parameters are widely used to evaluate the condition of birds. Typically, only a few parameters are used, such as the haemoglobin concentration, haematocrit, or the erythrocyte and leukocyte counts. The aim of this study was to present reference values of the haematological parameters of a free-living population of red-backed shrike ( Lanius collurio ) nestlings in east-central Poland. We assessed various blood parameters (haemoglobin concentration, erythrocyte and leukocyte, on blood smears for the erythrogram, leukogram and thrombocyte count), and examined their relationships with the brood size, biometrics conditions and hatching date. To reduce dimensionality and address the potential collinearity among variables, we conducted a Principal Component Analysis (PCA) and used the resulting components in Generalised Linear Mixed Models. None of the models yielded statistically significant effects. These results suggest a relatively stable physiological status among these nestlings, likely supported by a food-rich agricultural landscape. The haematological profiles presented here offer valuable baseline data for assessing the health of red-backed shrike populations and may support future comparative studies in wild birds.