Variation in growth patterns of Marsh Harrier Circus aeruginosus nestlings: effects of hatching order, nestling subperiod, brood size and weather conditions

Post-natal growth has long-term consequences for survival and fitness in birds and is subject to strong directional selection. Highly variable patterns of nestling development are influenced by an array of extrinsic and intrinsic factors. This study analysed the impacts of hatching order, nestling subperiod, brood size and weather conditions, all of which could influence the growth rate of Marsh Harrier Circus aeruginosus nestlings. It was conducted over a period of 6 years, during which measurements of 173 nestlings from 66 nests were made at fishpond complexes in eastern Poland. Relative growth rates (RGR) were calculated on the basis of tarsus and wing lengths during two subperiods of nestling development. Linear mixed models showed that the hatching order and nestling subperiod influenced nestling growth, whereas brood size did not affect wing or tarsus relative growth rates. The effect of hatching order differed between the two traits and between subperiods. Marginal nestlings in broods of five exhibited poor wing growth and perished before fledging. In contrast to wing growth, tarsus growth rates were similar in all the siblings in the first subperiod. During the second developmental subperiod, the growth rates of both locomotor traits exhibited the same pattern, i.e., higher values in the later-hatched chicks than in the earlier ones. Precipitation, but not temperature, had a negative impact on growth in the second subperiod. Our results show a variation in growth patterns between nestmates, which was the most evident in the rate of wing growth. The study enhances our understanding of brood reduction and cainism in birds of prey.