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Explanations of population size often invoke density-dependent reductions in fecundity and survival under conditions of Malthusian growth. An alternative, that territorial bird numbers change arithmetically as a consequence of spatially imposed limits to fecundity was proposed by C. B. Moffat in 1903. Restrictions in the size of annual cohorts set the upper limit of an equilibrium size range for the population deriving from any defined area, while the lower limit is maintained by the buffering effect of floaters (non-breeding adults) in filling breeding vacancies as they occur. Density-dependent floater pressure on reproduction and breeder survival, though not essential to equilibrium, is a likely focus of regulatory adjustment in healthy raptor populations. This essay considers the development of floating population segments and outlines some simple principles of modeling bird populations which contain floaters. Estimates of population size at equilibrium and the floater-to-breeder ratio offer particularly clear measures of the potential of defined populations to act as sources of recruits to the larger landscape and are more informative than traditional growth rate estimates in assessing population health and stability of territorial species.
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