Eggshell thickness can be an indicator of environmental pollution in wild birds and shell quality in wild and domestic birds, but it is difficult to measure calcite eggshell thickness due to the presence of the adherent outer eggshell membrane. Eggshells of 13 waterbird species were divided in half longitudinally and the outer membrane was removed from one of the halves. Subsequently, we measured eggshell thickness, both with and without the outer eggshell membrane, using a Hall-effect thickness gauge to the nearest 0.001 mm along the equator of each eggshell half. Outer eggshell membrane thicknesses ranged from 0.014 to 0.073 mm. Caspian Tern (Hydroprogne caspia) and California Gull (Larus californicus) had the thickest eggshell membranes (0.056 and 0.073 mm, respectively), and Green Heron (Butorides virescens) and Killdeer (Charadrius vociferus) had the thinnest membranes (0.014 and 0.022 mm, respectively). The eggshell membrane, as a percent of the total eggshell and membrane thickness, varied among the 13 species and ranged among species from 7.9% to 20.6%. The outer membrane comprised a greater percent of the total eggshell and membrane thickness for Black Skimmer (19.3%; Rynchops niger), California Gull (20.5%), and Forster’s Tern (20.6%; Sterna forsteri) than for Green Heron (7.9%), Double-crested Cormorant (10.4%; Phalacrocorax auritus), and Western Grebe (10.6%; Aechmophorus occidentalis). Within species, the outer membrane thickness was not correlated with egg morphometrics but, for a subset of species, there was some indication that the calcite eggshell thickness decreases with embryo development (age). We discuss several reasons for conducting future eggshell thickness measurements without removing the membrane.

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