An extreme type of brood overlapping in wild-living birds

Abstract

Brood overlapping is a breeding strategy that aims at shortening the time between successive broods. Most typically, each brood is raised in separate nest and parents share care between broods. If successive overlapping broods are located in the same nest, the second brood is usually initiated after nestlings from the previous brood have left the nest. The cases in which a female lays eggs in the same nest but before the nestlings from the previous brood have fledged (hereafter, Single Nest Brood Overlapping, SNBO) are rare and seldom studied. Based on a literature research, we revise data on SNBO occurrence. To date, about 37 cases of SNBO have been recorded in eight bird species and occurs mainly nest box breeders. Moreover, we surveyed SNBO in a wild-living passerine, Great Tit (Parus major). We found evidence of SNBO in 10.1% of the 69 first broods that we monitored using camera traps installed inside nest boxes. Females started the second brood when first brood nestlings were 17–19 days old, up to 6 days prior to fledging. This type of brood overlap occurred more often in nests that had smaller initial number of fledglings. The timing of the first brood, the clutch size and duration of the nestling phase had no significant effect on SNBO occurrence. We discuss some potential causes and fitness consequences of this unusual breeding strategy.