The degree of embryonic development influenced some eggshell characteristics of wild red-tailed hawks (Buteo jamaicensis borealis)

The influence of embryonic development on eggshell characteristics of wild birds, particularly raptors, is not well studied. Red-tailed hawk (Buteo jamaicensis borealis Gmelin 1788) eggs (n = 66) collected in central New York State during the late 1800s and early 1900s were examined to determine if eggshell mass, eggshell thickness, and a thickness index corrected for egg shape and blowhole size were influenced by the degree of embryonic development at the time of collection. Changes in these characteristics were examined with linear mixed models with year of collection and clutch size specified as random variables. Unexpectedly, length, breadth, and mass of eggshells were greater in eggs with advanced embryonic development than eggs with less developed embryos, perhaps due to sampling error or collector bias. Eggs containing well developed embryos were 13% thinner and had a thickness index 7% smaller than eggs with poorly developed embryos. Eggs with larger embryos had larger blowhole diameters; this relationship may be useful in determining degree of embryonic development in eggs of other species where development information is not explicit. Given the historical and current interest in eggshell thinning due to pollutants, degree of embryonic development should be accounted for in studies of eggshell characteristics.