Two hypotheses are evaluated to explain the annual changes in the size of breeding populations of ptarmigan (Lagopus spp.) from one breeding season to the next: H1, fluctuations are caused by density-dependent changes in the mortality of birds 4 months old and older that are excluded by territorial behaviour; and H2, populations fluctuate through annual changes in breeding success during the previous season. In 11 populations reviewed here, changes in the size of breeding populations were positively correlated with the previous season's breeding success. Significant mortality in three populations of birds > 4 months of age occurred during the winter before the birds became spaced on territories in the spring. In 9 of the 11 ptarmigan studies reviewed, sizes of spring populations were linearly correlated with the size of the fall populations the previous year, indicating that little compensatory, density-dependent mortality between the breeding seasons, a test implication of the territorial hypothesis (H1), had occurred. The data presented and reviewed suggest that changes in breeding success are sufficient to explain the dynamics in population size. Territorial behaviour appears to be a breeding tactic to space birds relative to environmental requisites, as argued by D. Lack, and does not result in significant numbers of nonterritorial, surplus birds with short life expectancies.
