The classical test for habitat saturation and the existence of floater
s in birds involves removing territorial individuals and documenting w
hether replacement occurs. In most experiments with birds, only males
are removed. An alternative explanation for the observed reoccupation
of vacant territories by males is that they are attracted by the prese
nce of unmated (widowed) females, and not necessarily by availability
of habitat. We performed experiments with a migratory passerine bird,
the Black-throated Blue Warbler (Dendroica caerulescens), at the Hubba
rd Brook Experimental Forest, New Hampshire, in which we removed (1) o
nly males, (2) only females, and (3) both males and females from breed
ing territories. In all cases when only males were removed, new or nei
ghboring males colonized the vacant territory, whereas when both sexes
were removed only one of nine territories was reoccupied; when female
s were removed, there was no replacement. Our results suggest the impo
rtance of considering female availability, in addition to habitat avai
lability, in studies of habitat distributions and population dynamics
of birds and other species.