Growth patterns of common kestrel (Falco tinnunculus) nestlings: effects of biological and habitat factors

Nestling growth reflects the combined effects of developmental constraints, sibling competition and environmental conditions. We quantified relative growth in tarsus length, wing length, head length and body mass of common kestrel ( Falco tinnunculus ) nestlings from a nest-box population in an agricultural landscape of east-central Poland. Trait-specific relative growth rates were summarized by principal component analysis and analysed with linear mixed-effects models that included nestling age, hatching order, brood size, breeding phenology and habitat composition. The first principal component explained 70.2% of variation and represented overall relative growth. Growth declined with nestling age, and this decline differed among hatching orders. Later-hatched chicks, particularly the fourth to sixth nestlings, showed higher relative growth at younger ages but stronger subsequent deceleration than first-hatched chicks. Brood size, laying date and the measured habitat variables received comparatively little support. These results indicate that within-brood developmental dynamics were more important than surrounding habitat composition in shaping nestling growth. Nest-box programmes should therefore evaluate offspring development within broods, in addition to occupancy and breeding success, when assessing conservation outcomes.