Brown Booby (Sula leucogaster) Science Article 4
Pseudo-eggs, foreign round objects, notably eggs of other speciesor pebbles, are common in some ground-nesting species, and havebeen reported especially in larids (Sugden 1947, Twomey 1948,Coulter 1980, Conover 1985). As many as 10% of Ring-billedGull Larus delawarensis nests in Washington, USA includedpseudo-eggs (Conover 1985). The nests of some larids that usuallylay three eggs sometimes have a pebble and two eggs. Pebblescould be an important stimulus for incubation in gulls and terns.
ERIC MELLINK, Marine Ornithology 30: 43-44 (2002)