Knowledge about interactions between habitat quality and individual condition is vital for understanding ecological processes. We examined variation in commonly used hematological indices and body mass in incubating and chick-rearing Red-necked Grebes (Podiceps grisegena) in Poland breeding under contrasting foraging conditions in ponds used for aquaculture. Grebes used either ponds with low prey availability in the early breeding period and abundant food in the brood-rearing stage (high-quality habitat), or ponds stocked with fish attractive as prey to spring-arriving adults yet later unavailable to chicks due to size constraints (low-quality habitat acting as an ecological trap). Parental effort, measured as the proportion of time allocated to food acquisition and provisioning of chicks, was greater in the low-quality habitat, although adult grebes spent less time foraging there than in the high-quality habitat early in the season. Scaled body mass was higher in incubating grebes than in brood-rearing grebes, regardless of habitat quality. Breeding stage by habitat interactions were significant for hemoglobin, erythrocyte count (RBC), mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH), with greater MCV and MCH values (by 20–34%, on average) during brood-rearing than during incubation in the low-quality habitat, and the opposite pattern (by 15–16%) in the high-quality habitat. These hematological responses signify changes in physiological stress associated with parental workloads. Higher MCV and MCH levels (by 22–31%) during the chick-rearing stage in the low-quality compared with the high-quality habitat indicate that parents incurred physiological costs of breeding under ecological trap conditions. No significant effects were found on leucocyte indices (total leucocyte count and heterophil/lymphocyte ratio). We consider the hemoglobin- and RBC-related parameters to be promising indicators of habitat-driven stress associated with prolonged physical exertion, especially when multiple metrics are employed.
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Vol. 119 • No. 2