No Seasonal Variation of the Sex Ratio in the Common Kingfisher Alcedo atthis Broods

Sex allocation theory predicts equal offspring sex ratio when costs/benefits of producing each sex are comparable. However, when costs of rearing or fitness values of sexes vary, skewed offspring sex ratio toward the one sex may be beneficial. In this study we analysed seasonal secondary sex ratio (proportion of males) in broods of the Common Kingfisher Alcedo atthis, the socially monogamous, three- to four-brooded (per season) species with biparental care in which adult females are slightly larger than males. Fieldwork was carried out in the Danube river system from Bratislava to Gabcikovo (SW Slovakia) during 2014–2018. In total, 959 nestlings from 149 broods of 78 breeding pairs were sexed, amongst which 483 were males and 476 were females. We considered the following assumptions: 1) no seasonal shift in sex ratio, as studied population occupies habitats rich in food; 2) female biased broods early in the season, as females' rapid maturation enables them to breed in the season they have been born; 3) male biased broods early in the season as several young males have been documented to be nest helpers. However, only a few such cases were observed, so it should be treated with caution. The sex ratio of individual broods ranged from 0 (all female) to 1 (all male), but the mean nestling sex ratio did not deviate from parity. We found no effect of year, brood size, or laying date. Thus, our results agree with outcomes of other avian studies where balanced seasonal sex ratio has been found.