When production costs or fitness returns vary between sons and daughters, selection is expected to favor females that adjust sex ratios accordingly. However, to what extent birds can do so remains debatable. Here we analyze the secondary offspring sex ratios in Marsh Tits Poecile palustris breeding in the primeval, strictly protected, part of Białowieża National Park (north-eastern Poland). We collected data on parent birds (age), their breeding performance (laying dates, clutch size, nesting success), and the offspring sex ratio in two types of old growth habitats over three years. The individual broods varied from all male to all female, but no significant deviation from parity was detected at the population level. The inter-brood variation could be accounted for neither by environmental (season, habitat) nor parental (female age, laying dates, clutch size, and brood size) variables analyzed. The primary sex ratios (at laying) did not differ from the secondary (brood) sex ratios. Thus, there was no indication of post-hatching sex ratio adjustments, either. Together, our results do not support the hypothesis of adaptive sex ratio adjustments in the studied Marsh Tit population. We suggest that possible fitness benefits could be insufficient to exceed the costs conferred by sex ratio manipulation in this species.
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