  We consider statistical patterns of variation in growth rates for over 400
species of breeding birds across North America surveyed from 1966 to 1998. We
report two results. First, the standard deviation of population growth rates
decays as a power-law function of total population size with an exponent $\beta
= 0.36\pm0.02$. Second, the number of subpopulations, measured as the number of
survey locations with non-zero counts, scales to the 3/4-power of total number
of birds counted in a given species. We show how these patterns may be related,
and discuss a simple stochastic growth model for a geographically subdivided
population that formalizes the relationship. We also examine reasons that may
explain why some species deviate from these scaling-laws.
