Background Overlapping of successive broods is a relatively rare breeding strategy that allows individuals to make effective use of the available reproductive window.
Methods In this paper we analyse the occurrence of overlapping breeding attempts in the Bearded Tit (Panurus biarmicus), a non-migratory passerine species, whose peripheral populations vary enormously in numbers.
Results The colour-ringed population of Bearded Tits was studied in western Poland in 1990, 2012-2013 and 2015-2017. Overlapping broods were found only during years with low population sizes and low densities (1.2-3.6 pairs per 10 ha), and pairs with such broods constituted 11.1-20%. In years with higher population densities (8.7-13.44 pairs per 10 ha) overlapping broods were not recorded. Pairs started building the next (overlapping) nests when their young were 3-10 days old. They divided their duties while rearing two broods simultaneously: females were occupied only with the new clutch, while males, in contrast to other species with brood overlap, not only fed the young from an earlier brood, but also helped the females with the building of a new nest and incubation of a new clutch.
Conclusions Laying overlapping clutches enabled pairs to shorten their average breeding cycle by 15-21 days and produce more offspring. It is possible that overlapping breeding attempts is a density-dependant strategy, enabling the population to restore after severe declines, however, alternative explanations are also possible. Future studies are needed to better understand mechanisms underlying the occurrence of this phenomenon.
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