Extreme number of second clutches in Common Kestrels (Falco tinnunculus) in Hungary

Between 1995 and 2024, during a long term population monitoring of partially migratory Common Kestrels in Hungary, we identified potential second brooding pairs on several occasions. In 2020, we were able to conclusively prove the occurrence of second broods within the same breeding season. Following these findings, in 2023, during an exceptionally strong vole outbreak year, we identified the commencement of second broods in several waves. In 2023, a total of 379 Common Kestrel breeding attempts were observed, of which 42 involved second broods. While replacement broods were exclusively identified in closed kestrel-specific nesting boxes, second broods occurred both in artificial colonies and solitary sites, with breeding taking place in kestrel-designed nest boxes, as well as three cases in natural nests. In our study, we found that after a mild winter and a food-rich year, the earliest breeders were those that later attempted a second brood. Second-breeder Common Kestrels did not search for alternative nesting sites, and without exception, they reused the same nest used for their first brood. It is noteworthy that most second-brooding pairs started laying eggs for their second clutch before the young from the first clutch had actually fledged, meaning there was overlap between the two breeding attempts. On average, the initiation of the second clutch occurred two days earlier than the fledging of the last chick from the first clutch. Our observations suggest that Common Kestrels engage in second broods as an active strategy to enhance their reproductive success, rather than as a compensatory response to a low-success first clutch. We provide recommendations for adjustments in the monitoring of rodent-hunting raptor species that may exhibit similar adaptive responses.