Notes on Color Variation in Downy Caspian Terns

Abstract

Variation in the color of the downy plumage is known to occur in many species of terns. Descriptions of the downy young which indicate polymorphism or considerable individual variation in down color can be found in the literature for at least the following species: Gelochelidon nilotica (Witherby et al. 1949; Bannerman 1962), Hydroprogne caspia (Bergman 1955), Sterna hirundo (Bent 1947; Witherby et al. 1949), S. paradisaea (Bergman 1955), S. forsteri (Bent 1947), S. dougallii (Bent 1947; Witherby et al. 1949), S. aleutica (Bent 1947), S. fuscata (Murphy 1936; Bent 1947; Witherby et al. 1949), S. albifrons (Murphy 1936; Tomkins 1959), Thalasseus maximus (Chamberlain 1939; Bent 1947), T. sandvicensis (Witherby et al. 1949; Bannerman 1962), and Anous stolidus (Watson 1908; Murphy 1936; Dorward and Ashmole 1963). Species apparently monomorphic include Chlidonias spp. (Witherby et al. 1949), Anous tenuirostris (Cullen and Ashmore 1963), and Gygis alba (Dorward 1963). The degree of color variation and the extent to which different color forms or morphs intergrade with each other appear to vary from species to species. Despite the widespread occurrence of color variation in tern chicks, few species have been described carefully, and relatively little is known concerning the ecological significance of this variation. This paper deals with the color variation,, the frequencies of the various color types, and possible selective pressures influencing these frequencies in downy Caspian Terns (Hydroprogne caspia) at a breeding colony in the south end of San Francisco Bay, California.