Nestling growth, sibling competition, and prey delivery rates of the Eleonora’s falcon (Falco eleonorae)

Context In altricial birds, nestlings hatch in a dependent state, making their size and growth key indicators of resource availability and parental investment. Parents may enhance fitness by regulating brood size through selective food provisioning, often disadvantaging later-hatched (marginal) chicks. Eleonora’s falcon serves as a model for studying these dynamics as the fate of the marginal chick (particularly vulnerable to starvation and sibling aggression under poor environmental conditions) is crucial to overall reproductive success and fledgling sex ratio. Aims We aimed to identify the critical stage for Eleonora’s falcon nestling survival by examining variations in clutch morphometrics and growth rates in relation to hatching sequence and brood size and the potential role of parental favoritism through skewed food provisioning. Methods During 2022, we analyzed measurements of 63 eggs laid in 32 nests and growth data from 79 nestlings hatched in 39 nests. In 2023, we examined 17,000 behavioral events in video recordings from 13 nests. Nestling growth patterns were modeled using non-linear regression functions relating time to morphometrics. Parental care was evaluated through focal video sampling. Egg traits were examined using generalized linear models with generalized estimating equations, while interactions among nestling age, hatching sequence, and feeding rates were analyzed using generalized linear mixed models. Key results Clutch size averaged 2.4 ± 0.58 eggs. Eggs averaged 42.7 ± 1.25 mm in length, 33.9 ± 1.16 mm in breadth, and 22.7 ± 2.77 g in weight, with third-laid eggs being smaller, more spherical, and lighter in color. Egg weight increased with size but declined as clutch size grew, suggesting a resource tradeoff. Nestling growth followed logistic curves: first-hatched chicks grew fastest, second-hatched were slightly smaller, and third-hatched showed delayed growth. Brood size did not affect feeding frequency, and third-hatched chicks were fed similarly to their siblings during the critical second week for survival. Conclusions Parental investment appears equal early on, but the growth and survival of marginal nestlings are ultimately limited by physical disadvantages and sibling competition rather than active parental favoritism. Implications Understanding how brood hierarchy and sibling rivalry shape nestling development requires integrating growth patterns with parental provisioning, especially under changing environmental conditions.