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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.

Sexual selection in juvenile vertebrates is a rare and seldom studied phenomenon. Bearded Reedlings are passerines that form stable, life-long pair bonds within a few weeks after fledging. We investigated the degree to which juvenile Bearded Reedlings are sexually dimorphic and whether they possess condition-dependent ornaments. We found significant intersexual differences in the bill reflectance, reflectance and size of eumelanin-based plumage traits (lore and back patch) and morphological traits. Females that were in better body condition had longer tails and bills with greater red chroma, and lore size was negatively related to the occurrence of chewing lice. In males, body condition was positively related to melanic back patch length. Moreover, a positive relationship between bill color and feather mite load in males suggests a symbiotic, rather than parasitic, relationship. Our study provides the first evidence that sexually dimorphic traits in immature birds with pair bonds have the potential to signal condition and resistance to parasites.

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Egg size is an important determinant of offspring weight and survival. However, despite extensive research on factors affecting egg dimensions, causes of egg variation are not fully understood. Between 2000 and 2016 we studied a population of little bitterns in western Poland, and examined a total of 529 eggs from 88 clutches. Furthermore, we conducted an extensive literature search for data on clutch and egg size from other populations of the species to test hypotheses explaining the relationships between these variables and latitude. Little bitterns in western Poland laid big eggs but also produced large clutches compared to most other studied populations of the species. The ratio of largest to smallest egg calculated for egg volume was 1.75 for all studied eggs, and up to 1.35 within clutches, suggesting a huge variation between and within clutches in the studied population. Egg dimensions were highly intercorrelated, but egg size was not related to clutch size in the study population. The comparison of data from different populations of the species revealed that egg volume increased significantly with latitude and was positively correlated with clutch size. These results support the embryonic temperature hypothesis and clutch cooling rates hypothesis, but contradict the optimal egg dimensions hypothesis.