Control and Context of Year-Round Territorial Aggression in the Non-Migratory Song Sparrow Zonotrichia melodia morphna

Abstract

A sedentary population of Song Sparrows Zonotrichia melodia morphna in western Washington State shows year-round terroriality, but in different contexts. During the breeding season (March to August), all defend multiple purpose territories as monogamous pairs, or alone, but only about 30% of breeding pairs remain on the same territory during the non-breeding season (September to February). The rest either move to wintering areas close by (i.e. within a kilometer) and form local alliances that are territorial toward other Song Sparrows, or defend a wintering territory alone. Complex social groups may form in which some individuals defend a discrete territory and others float among several territories. Territorial aggression is generally thought to be activated by gonadal hormones, particularly androgens. During the breeding season, territorial behavior is tightly correlated with circulating levels of luteinizing hormone and testosterone, but identical behaviors during the non-breeding season appear to be expressed independently of gonadal hormones. Castrated male Song Sparrows defend territories equally well as intact males even through onset of the breeding season. It is suggested that although territorial behavior may be expressed year-round, the context, neural, and hormonal bases of territoriality may change dramatically.