The influence of a developing bird embryo on eggshell thinning is well known, however this phenomenon has been described mainly for poultry, and in wild birds is poorly understood. This study focussed on Capercaillie ( Tetrao urogallus), in which the effect of a developing embryo on the change of eggshell thickness has not been described. However, problems relating to a large number of dead embryos and eggs cracking during incubation are observed in captive breeding centres giving us the opportunity to test how the developing embryo affects the eggshell and thereby its properties, including eggshell strength and thickness. In total, 196 unhatched eggs, including 146 infertile and 50 eggs with embryos which died at various stages of development, were analysed in relation to eggshell shape, thickness and strength. Moreover, 102 posthatched eggshells were also examined. Although the strength of eggshell was affected by egg shape and shell thickness, contrary to expectations, the stage of embryonic development had no significant influence on eggshell strength. As has been demonstrated in other species, etching of eggshells by the developing embryo reduces shell thickness in Capercaillie but it has minimal effect on the strength of the eggshell. Egg viability is not compromised during the later stages of development by contact incubation with the female. The eggshell of infertile eggs or eggs containing early-dead embryos were 10% thicker than posthatched eggshells and eggs with late-dead embryos, confirming that eggshell thinning in Capercaillie is typical for precocial species.
Raptors are still affected by environmental pollutants: Greenlandic Peregrines will not have normal eggshell thickness until 2034
