How do floater male eastern bluebirds benefit by filling vacancies on territories where females already have young?

Abstract

When male eastern bluebirds (Sialia sialis) are killed or desert their mates, replacement males usually take over the territory. We captured 25 eastern bluebird males and held them in captivity to study the behavior of replacement males and their response to young already present in the nest. Of 17 replacement males, most courted the females, and 23% of experimental females renested, a proportion not different from that of control females. No replacement males were infanticidal, and none fully adopted the young. Replacement males spent significantly less time on the territory and defended it significantly less than control males. We discuss breeding options available to a floater male eastern bluebird and conclude that (1) floater male bluebirds that fill territory vacancies benefit by gaining breeding opportunities in the current breeding season, and (2) indifference to the young is the least costly option for replacement male eastern bluebirds.