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

Abstract

The classical test for habitat saturation and the existence of floaters in birds involves removing territorial individuals and documenting whether 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 presence 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 Hubbard Brook Experimental Forest, New Hampshire, in which we removed (1) only males, (2) only females, and (3) both males and females from breeding territories. In all cases when only males were removed, new or neighboring males colonized the vacant territory, whereas when both sexes were removed only one of nine territories was reoccupied; when females were removed, there was no replacement. Our results suggest the importance of considering female availability, in addition to habitat availability, in studies of habitat distributions and population dynamics of birds and other species.