Frequent Copulations Despite Low Sperm Competition in White Storks (Ciconia Ciconia)

Abstract

White Storks Ciconia ciconia paired for ca. 30 days before laying a clutch. During this period, mates copulated frequently (160 copulations/pair; 0.4 copulations/daylight h), but copulation rate was drastically reduced a week before laying of the eggs. Both fewer copulation attempts by males and lower female receptivity accounted for this reduction. This pattern was the same regardless of whether pairs nested solitarily or in colonies. Colonial as well as solitary males spent more time at the nest before egg-laying while the opposite trend was found for females. Consequently, females were more likely to remain alone at their nests while ovulating. Colonial birds had ample opportunities for engaging in extra-pair copulations (EPC) during the female fertile period, but these were very infrequent (0.4% of all successful copulations) and involved recently-paired birds exclusively. This suggests that sperm competition in this species is of little relevance for explaining patterns of pair copulations. Accordingly, males did not guard their female mate and the timing of copulation was poorly tuned to chances of fertilizing the female's eggs. However, it remains to be explained why storks copulated so much and for a prolonged period when the risk of EPC was so low. It is suggested that copulations may be part of a signalling system by which males advertise and females assess the physical condition of the male, which is likely to correlate with the ability of males to forage efficiently for them and their offspring. In support of this possibility, males who copulated frequently fed chicks at a higher rate during the nestling period.