THE INFLUENCE OF DENSITY ON FREQUENCY-DEPENDENT SELECTION BY WILD BIRDS FEEDING ON ARTIFICIAL PREY

Previous work has demonstrated frequency-dependent selection by wild g arden birds when feeding on green and brown pastry 'baits'. When the d ensity of baits is low the common colour is eaten disproportionately m ore than the rare colour (apostatic selection), and when the density i s very high, the rare colour is eaten disproportionately more than the common (anti-apostatic selection). We explored the relationship betwe en frequency-dependent predation and density in an experiment at 16 se parate sites, using four levels of density and two frequencies of gree n and brown. Analysis of estimates of log relative risk ratios showed little evidence for frequency-independent selection, but frequency-dep endent selection changed gradually from apostatic at low density to an ti-apostatic at high density The validity of these conclusions in term s of individual bird behaviour was confirmed by Monte-Carlo simulation s. We thus conclude that selection by wild birds feeding on green and brown artificial prey is frequency dependent, and that the strength an d direction of this selection changes with prey density in a gradual a nd predictable manner.