Previous work has demonstrated frequency-dependent selection by wild garden birds when feeding on green and brown pastry`baits'. When the density of baits is low, the common colour is eaten disproportionately more than the rare colour (apostatic selection), and when the density is very high, the rare colour is eaten disproportionately more than the common (anti-apostatic selection). We explored the relationship between frequency-dependent predation and density in an experiment at 16 separate sites, using four levels of density and two frequencies of green and brown. Analysis of estimates of log relative risk ratios showed little evidence for frequency-independent selection, but frequency-dependent selection changed gradually from apostatic at low density to anti-apostatic at high density. The validity of these conclusions in terms of individual bird behaviour was con¢rmed by Monte-Carlo simulations. We thus conclude that selection by wild birds feeding on green and brown arti¢cial prey is frequency dependent, and that the strength and direction of this selection changes with prey density in a gradual and predictable manner.

We compare the results of four experiments, conducted at different times and with different protocols, that explored the relationship between frequency-dependent selection and prey density in wild birds feeding on artificial populations of coloured baits. One (experiment 4) used pastry baits that differed only in the presence or absence of a red stripe, and this experiment provided no evidence for any kind of selective behaviour. The other three experiments used green and brown baits, and they all provided evidence for a trend towards increasing anti-apostatic selection with high densities (>100 baits m). However, one of these (experiment 3) provided no evidence for frequency-dependent selection at low densities (0.5-20 baits m), while the other two experiments concurred in suggesting a trend towards increasing apostatic selection with low densities (down to 2 baits m). Together, these experiments both support and qualify the published findings of experiment 1 that frequency- dependent selection by wild birds on bait populations is modified by density. Experiment 4 indicates that frequency-dependent selection may break down entirely if bait types are too similar, while experiment 3 indicates that some details of this trend with density will depend either on the protocol used or on exogenous changes in the birds' feeding behaviour.