Embryonic eggshell thickness erosion: A literature survey re-assessing embryo-induced eggshell thinning in birds.

Although eggshell thinning has been described mainly in the context of environmental pollution, it can also be the effect of reproductive changes induced by a developing embryo. On the basis of a literature survey of 25 bird species (26 published papers) we reviewed data on embryo-induced eggshell thinning (EET) in three groups of birds: precocials, semi-precocials and altricials. The average EET at the equator of the eggs was 6.4% (median = 4.7%). Our review did not confirm a general prediction of elevated EET at the egg equator in precocial species: altricial birds exhibited the highest EET (average = 12.0%), followed by precocials (7.6%) and semi-precocials (4.2%). We make certain critical recommendations based on the results of this study. Studies aiming to assess variation in eggshell thickness should examine intrinsic factors affecting shell properties of avian eggs, like thickness, which are the result of anatomical or reproductive changes.