Territory Sizes and Patterns of Habitat Use by Forest Birds Over Five Decades: Ideal Free or Ideal Despotic?

Relations among territoriality, abundance and habitat suitability are fundamental to the ecology of many animal populations. Theory suggests two classes of possible responses to increasing abundance in territorial species: (1) the ideal free distribution (IFD), which predicts smaller territory sizes and decreased fitness as individuals adaptively pack into suitable habitats, and (2) the ideal despotic distribution (IDD), which predicts stable territory sizes and fitness in preferred habitats for dominant individuals and increased use of marginal habitats, reduced fitness and changes in territory sizes for subordinate individuals. We analysed the territory sizes and locations of seven migratory songbird species occupying a 10-ha plot in the Hubbard Brook Experimental Forest, New Hampshire, USA over a 52-year period. Species varied in abundance over years from twofold to 22-fold, and all species displayed clear patterns of habitat preference within the study plot. Consistent with IFD, and contrary to IDD, territory sizes decreased with local abundance for all species, irrespective of habitat preferences. There was at least a twofold variation in territory size within years. Conformity of territory size to predictions of the IFD argues for the efficacy of territorial defence in songbirds and has general consequences for population dynamics.