Turning eggs to fossils: A natural experiment in taphonomy

Ashfall from Mount St. Helens’ May 18, 1980 eruption covered eggs and nests of two species of colonial, ground-nesting gulls. While some members of both species excavated their eggs from beneath the ash, most eggs and nests remained buried. One year after the ashfall, habitats known to contain pre-ashfall nests were transected and the ash layer was measured and removed. Eggs, nests, and associated materials were counted. Entire hollow eggshells and eggshell fragments were found, as well as rodent bones and teeth, bird bones, beetle carapaces, and vegetation. No fetal bones were found with eggs. Fossilization potential for nests and eggs was related to intra- and interspecific behavioral differences among parents, as well as to temporal and spatial environmental factors. Gulls and other ground-nesting birds are implicated as accumulators of biological material for potential fossilization.