EGGSHELL REMOVAL BEHAVIOR OF AMERICAN AVOCETS AND BLACK-NECKED STILTS

Eggshell removal behavior of American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus) was studied in northern Utah. Both species re moved eggshells from their nests soon after hatching or whenever they found them there. Both male and female avocets exhibited this behavior; female stilts exhibited it (and males probably did also). Although some shells were disposed of by dropping them over land, both species tended to carry shells to nearby water and submerge them. Eggshell removal is prob ably an antipredator behavior in avocets and stilts. Shell removal behavior occurred through out the incubation period (documented on day 3 through hatching in avocets and day 7 through hatching in stilts). Observations of avocets indicated shell removal at a partially depredated nest, removal of one or more entire damaged eggs, and removal of a dead chick. CONDUCTA DE REMOCI? N DEL CASCAR? N DEL HUEVO POR PARTE DE RECURVIROSTRA AMERICANA Y HIMANTOPUS MEXICANUS Sinopsis. Se estudi? la conducta de remoci? n del cascar? n del huevo por parte de indivi duos de avoceta (Recurvirostra americana) y de viudas (Himantopus mexicanus) en una lo calidad de Utah. Ambas especies remueven el cascar? n del nido tan pronto eclosionan los huevos o tan pronto lo encuentran. Ambos sexos de avocetas exhibieron dicha conducta, al igual que dos hembras de viuda y un individuo de sexo desconocido. Aunque algunos cas carones fueron desechados dej? ndose caer sobre tierra firme, ambas especies tienden a llevar ? stos a lugares con agua y sumergirlos. La remoci? n de huevos es probablemente una con ducta anti-depredador. Dicha conducta se llev? a cabo virtualmente a trav? s de todo el peri odo de incubaci? n (del tercer d? a hasta el eclosionamiento en avocetas y del s? ptimo d? a hasta el eclosionamiento en las viudas). En las avocetas se observ? la remoci? n de huevos rotos y de un polluelo muerto, en un nido que fue parcialmente depredado. Many birds remove empty eggshells from the nest shortly after their young hatch (Nethersole-Thompson and Nethersole-Thompson 1942, Skutch 1976). The urge to remove eggshells near the nest is so strong that Stilt Sandpipers ( Calidris himantopus) can be captured in a trap bait ed with an empty shell (Parmelee et al. 1968). At least five hypotheses for the adaptive value of eggshell removal behavior have been proposed (Tinbergen et al. 1962): (1) eggshells that are externally cryptic are white inside, and may be conspicuous enough to lead predators to the nest; (2) later-hatching eggs might become encapsulated, the egg-young thus be coming trapped inside a double shell; (3) sharp edges of shells might injure chicks in the nest; (4) organic material associated with eggshells might lead to growth of bacteria and mold in the nest; (5) hatched shells could interfere with brooding in the nest. In this paper I report observations and field experiments on the egg shell removal behavior of American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus), two members of the family Recurvirostridae. Eggshell removal in these two species was inferred by Hamilton (1975) because he never observed shells in the nest during or 461