Adaptations for marsh nesting in common and Forster's terns

Salt marsh colonies of common terns (Sterna hirundo) appear to be increasing along the east coast of the United States, particularly in areas where there is heavy use of their traditional barrier beach habitat by humans. The potential for successful marsh nesting by common terns was examined by comparing their nesting requirements with those of marsh-nesting Forster's terns (S. forsteri). The comparison focused on nest survival of tidal inundation, the major source of nest loss for marsh-nesting birds. Both species typically build short nests, which are usually elevated on mats of dead vegetation. Forster's tern nests were on lower ground, on higher mats, and in taller grass than were common tern nests. The lower ground at Forster's tern nests is offset by higher mats, and total nest height above mean high water is the same for both species. The high grass and low ground at Forster's tern sites resembles those of other typically marsh-nesting species, and offers more protection from tides that exceed nest height, because high grass helps to hold the mat and nest in place. Forster's terns renested more quickly after flooding, produced larger replacement clutches, and hatching occurred at a higher proportion of late nests compared with common terns. These differences suggest that Forster's terns are better adapted to the frequent necessity to renest in the salt marsh. Species differences in nest sites do not appear to be the result of interspecific competition, but rather may reflect different responses of the two species to water around the nest site during the period of nest site selection.