Egg-capping: een lotgeval bij de Buizerd Buteo buteo en evolutionaire aspecten

In a Common Buzzard nest in the Reichswald, a forest in western Germany, an egg cap of a hatched egg was found to be stuck over the remaining unhatched egg (a phenomenon called egg-capping). The fully-grown embryo had been unable to hatch, and consequently had died. The cause of death may have been oxygen deficiency or energy shortage. Egg-capping has been encountered before, but records in the literature are rare and those few report a frequency of occurrence in 0.2-3.0% of nests of various species. Theoretically, many factors may influence egg-capping, but these remain speculative in the face of the lack of experiments. Here, two possible factors are highlighted: egg-capping frequency is likely to be reduced by (quick) egg-shell removal, and by asynchronous hatching since this leaves fewer egg-shells to be removed from the nest at a time (opposing the idea stated in the literature). However, synchronous hatching is supposed to improve hatchability of capped eggs, since embryo growth would be affected less. Raptor researchers are encouraged to note and publish observations of egg-capping, as very little is still known about egg-capping and its evolutionary consequences.