AVIAN REMOVAL EXPERIMENTS - DO THEY TEST FOR HABITAT SATURATION OR FEMALE AVAILABILITY

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.