PAIR TERRITORIALITY OF WINTERING STONECHATS - BEHAVIOR, FUNCTION AND HORMONES

Migratory stonechats (Saxicola torquata) spend the winter in the north ern Negev desert in Israel. After arrival in October most birds defend territories throughout the winter either as heterosexual pairs or as single birds. In this population we examined: (1) aggressive behaviour of pairs; (2) the functional significance of pair territoriality; and (3) hormonal correlates of territorial behaviour. Pairs were aggressi ve towards both conspecific intruders and stonechat dummies, particula rly in autumn when territories were established. Males were more aggre ssive than females and aggression was primarily intrasexual. More freq uent than intraspecific interactions were interspecific encounters, wi th 94% involving the local mourning wheatear (Oenanthe lugens). With o ne exception these encounters consisted of attacks of mourning wheatea rs against stonechats, mainly males. Two functions of winter territori ality of migrant pairs are proposed: (a) a reduction of individual def ense costs associated with the maintenance of a territory; (b) an incr ease of the net vigilance to reduce the risk of attacks by competitors or predators. The levels of testosterone (males and females) and estr adiol (females only) were close to or below the lower detection limit of the assay throughout the winter. Therefore, high levels of gonadal steroids are not required for pair formation and aggression in the non -reproductive context. The results indicate that both the function and the proximate control mechanisms of territoriality during the non-bre eding season in winter are different from those during the breeding se ason in spring.