Eggshell thickness in marine birds in the New York Bight--1970s to 1990s.

Maintaining eggshell thickness is critical for birds, as thin eggshells result in breakage during incubation, with subsequent hatching failure. Beginning in the 1960s, eggshell thickness has been used as a biomarker of exposure to chlorinated hydrocarbon pesticides, as a measure of avian population health, and as a predictor of potential reproductive failures. In this study, eggs were collected from four coastal bird species nesting in the New York Bight (Cedar Beach, NY to Barnegat Bay, NJ) in the early 1970s, early 1980s, and early 1990s, and eggshell thickness was measured. We tested the hypothesis that decreasing use of chlorinated hydrocarbons, and subsequent decreased levels of these pollutants in the New York Bight estuarine food web, should have resulted in increased eggshell thickness from the 1970s to the 1990s. Most of the variation in eggshell thickness was explained by decade and species. Eggshell thickness increased from the early 1970s (or the 1980s for some species) to the early 1990s for all four species examined: common tern (Sterna hirundo), Roseate tern (S. dougallii), least tern (S. antillarum), and black skimmer (Rynchops niger). For common terns and black skimmers, eggshell thickness increased by nearly 50% from the 1970s to the 1990s, whereas in the smallest species, the least tern, eggshell thickness increased only by 12%. In the 1990s, least terns with the smallest eggs had the thinnest eggshells, and black skimmers with larger eggs had the thickest eggshells.