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

SUMMARY (1) Previous experiments on frequency-dependent selection at high prey densities are inadequate because too few replicates have been carried out. (2) Using several experimental designs, we exposed populations of pastry baits, coloured brown or green, at densities of both 11 and 0.08 per square centimetre, to wild passerine birds. (3) Overall, there was frequency-independent selection against brown plus frequencydependent selection against rarity. (4) There is some evidence that predators other than passerines also eat an excess of the rarer forms when feeding on dense prey populations. (5) These results contrast with the usual situation at low prey densities, when predators tend to eat a disproportionate excess of common forms. (6) Selection against rarer forms will tend to lead to monomorphism of species that live in flocks.