Offspring sex ratio and partially asynchronous hatching in the Little Bittern (Ixobrychus minutus) population in south-western Slovakia

Abstract

Birds are one of the best-studied animal groups in Europe. Despite this, there are several bird species, including the Little Bittern (Ixobrychus minutus), of which the breeding biology is still little known. Here, I examine the sex of offspring in relation to hatching order, the degree of hatching asynchrony and some other aspects of nesting biology (such as breeding phenology, clutch size, egg and nestling survival) of the Little Bittern in the south-western Slovakian population. Egg-laying occurred from 5 May to 10 July, but the majority of clutches were initiated in May. Mean clutch size was 5.2 ± 0.92 (SD) eggs. Parents initiated full incubation predominantly after laying the third egg (86.4% of cases) of the clutch, which led to only partially asynchronous hatching of eggs. However, no evidence was found for offspring sex ratio adjustment according to hatching order. Nevertheless, survival rates of eggs and nestlings were relatively high (80.9% and 94.3 %, respectively). The possible causes of observed patterns are discussed.