THE MATING SYSTEM OF THE WHITE-THROATED MAGPIE-JAY CALOCITTA-FORMOSA AND GREENWOOD HYPOTHESIS FOR SEX-BIASED DISPERSAL

Greenwood explained the different sex bias in dispersal of birds (usua lly female biased) and mammals (usually male biased) by a difference i n mating systems: male birds primarily defend resources while male mam mals primarily defend females. The White-throated Mag pie-jay Calocitt a formosa is unusual among birds in that females are philopatric and j ointly defend permanent resource territories while males disperse befo re they are 2 years of age. One female in a group is the primary breed er, One male joins the group permanently as her mate, Males that do no t have a permanent breeding position circulate among groups and attemp t to mate with both the primary breeding female and other group female s. Other females feed the primary breeder and her offspring and also p ursue other reproductive behaviour, including secondary nesting in the territory and egg dumping into the primary breeder's nest. I argue th at the unusual dispersal pattern in this species is a result of the al ternative reproductive strategies that can be pursued by males and fem ales excluded from being primary breeders, The White-throated Magpie-j ay conforms to Greenwood's predictions: males pursue a mate defence ra ther than resource defence mating system and they are the dispersing s ex. The primary factor influencing alternative reproductive tactics ma y be asynchronous reproduction among groups during the long breeding s eason arising from frequent renesting in an area of high nest predatio n.