The Influence of Biological Factors on Haematological Values in Wild Marsh Harrier (Circus aeruginosus) Nestlings

SIMPLE SUMMARY: Haematological examination can be used to address the health state of birds. The present study aimed to investigate how hatching order, brood size, and nest initiation date influence the haemoglobin level, red blood cell count, white blood cell count, and glucose level in the blood of raptor nestlings in which brood reduction occurs during nestling rearing. Blood samples were collected to assess the health status of wild marsh harrier nestlings in the agricultural landscape of eastern Poland. Statistical analyses revealed that hatching order, but not brood size or nest initiation date, had an impact on the haematological data of nestlings. Hatching order affected these data in that haemoglobin levels and red blood cell counts gradually decreased, whereas glucose levels and white blood cell counts increased, from the first- to the last-hatched nestlings. The poor condition of the youngest nestlings reflected in these indices may increase the likelihood of their perishing, and consequently, of brood reduction. ABSTRACT: Marsh harrier (Circus aeruginosus) is a species with obligatory cainism, in which hatching asynchrony creates a pronounced size hierarchy in nestlings. The size-related competitive advantage of older nestlings means that they tend to dominate the younger ones, and brood reduction occurs in most nests. The aim of the study was to reference values and carry out a haematological examination in order to evaluate the physiological status and health of nestlings with respect to hatching order, brood size, and nest initiation date. To do so, we examined 19 nests with a total of 58 nestlings from a free-living population of this species located in fishpond complexes in the agricultural landscape of eastern Poland. Repeated blood samples (118 in all) were collected from nestlings. The following parameters were measured using fresh full blood: red blood cell count (RBC), haemoglobin level (Hb), white blood cell count (WBC), and plasma glucose level (Glu). The data were analysed using generalized linear mixed models and linear mixed models ((G)LMM). The study revealed that hatching order, but not brood size or nest initiation date, affected the physical condition of marsh harrier nestlings. Hb levels and RBC counts gradually decreased, whereas Glu levels and WBC counts increased from the first- to the last-hatched nestlings. This result points to the generally poorer condition of the youngest nestlings compared with their older siblings. The poor physiological condition of the youngest nestlings may consequently increase the likelihood of their perishing, and hence, of brood reduction.