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We compared nonbreeding finch densities (biomass) with seed abundance in arid and semiarid sites in Kenya, the United States, Brazil, and Argentina. Our objectives were to examine whether densities are limited by food, and whether a given seed abundance results in similar finch densities on different continents. We also wished to estimate the relative roles of food and other biotic factors in determining absolute density, by comparing the observed densities to those that might potentially be achieved on a given food supply. We used data from the finches of the Galapagos Islands as an approximate measure of the food-imposed upper limit to finch density because food there is short, emigration is restricted, and predation is reduced.

Half the variation in finch densities across sites around the world could be explained by standing seed abundance, strongly suggesting that finches on mainlands are food limited. Densities on different continents were roughly similar when seed supplies were the same, despite the different phylogenetic origins of finches present. Nevertheless, average densities on mainlands were only about one-quarter the densities found on Galapagos islands. Low mainland densities possibly result from competition with other granivores (e.g., rodents and ants). However, mainland finches restrict their foraging to areas of relative safety, where seed densities may be lower than elsewhere, hinting that predation may also reduce densities. We suggest that predation and possibly other biotic factors limit mainland populations jointly with food, and do not simply reduce densities below a point where food is limiting.