The breeding ecology of eastern Willets (Catoptrophorus semipalmatus) was studied over a 3-yr period in a salt marsh on the Atlantic coast of Virginia. During the study, 171 adults were color-marked. Willets are strictly monogamous and show strong site and mate fidelity from year to year. Normally a pair bond dissolves only after one member dies or fails to return in the spring. With an estimated annual adult mortality of 15%, the average life of a pair bond is 3 yr. The sexes share in most aspects of breeding, including brood-rearing, but females abandon both mate and brood about 2 weeks after the chicks hatch. Males apparently incubate at night and sometimes relieve the female at the nest during the middle of the day. In this area Willets defend separate nesting and feeding territories, though nest-site defense decreases through the incubation period. When mates are lost, males retain their original nesting and feeding territories, while females pair with available mates near their original nesting territory. Females compete for mates, and males select the most aggressive females.

Nesting was particularly synchronous in lowland marsh sites, where the onset of egg laying was slightly earlier than in upland sites. Nest density was the highest reported for any scolopacid, and the highest densities were in sites closest to the marsh. Nest spacing is achieved through agonistic behavior, but there is evidence of a counter-tendency leading to nest clumping. Several features of the Willet social system resemble characteristics typical of colonial species of birds. These are discussed along with other aspects of social organization and the selective forces operating on it.