Drivers of reproductive investment in the Sooty Falcon Falco concolor: habitat, density and weather fluctuations

Abstract

The Sooty Falcon Falco concolor is a specialised long-distance migrant breeding within the arid landscapes of North Africa and the Middle East. Identifying the drivers of reproductive success is critical for effective conservation, yet the relative roles of habitat type, population density, and meteorological variability remain poorly understood. We analysed 44 breeding events across the species’ range to evaluate factors influencing mean clutch and brood sizes. To address multi-collinearity, 15 environmental variables were reduced via principal component analysis (PCA), and linear mixed-effects models (LMMs) were employed to test ecological and demographic predictors. Our results document an ‘island advantage’ that contrasts with the classical ‘island syndrome’ pattern: insular populations produced significantly larger clutches and broods than inland desert populations, an advantage that persisted from laying through to early chick-rearing. In contrast, clutch size declined with increasing local population density, suggesting negative density-dependence during initial reproductive investment. While the species appeared resilient to the primary regional thermal gradient, short-term weather fluctuations – specifically summer rainfall pulses and associated cooling – significantly favoured larger clutches, underscoring sensitivity to environmental instability during the critical laying period. These findings identify offshore islands as primary reproductive sites, likely due to the convergence of migratory prey and reduced predation. Consequently, conservation strategies under the International Single Species Action Plan should prioritise the protection of insular habitats, as inland populations may face higher exposure to the combined effects of lower productivity and climatic variability.