Frequency-Dependent Selection by Birds When the Density of Prey is High

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Abstract
Previous experiments on frequency-dependent selection at high prey densities are inadequate because too few replicates have been carried out. Using several experimental designs, populations of pastry baits, colored brown or green, were exposed at densities of both 1.1 and 0.08/cm2, to wild passerine birds. Overall, there was frequency-independent selection against brown plus frequency-dependent selection against rarity. There is some evidence that predators other than passerines also eat an excess of the rarer forms when feeding on dense prey populations. These results contrast with the usual situation at low prey densities, when predators tend to eat a disproportionate excess of common forms. Selection against rarer forms will tend to lead to monomorphism of species that live in flocks.

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