The influence of density on frequency–dependent selection by wild birds feeding on artificial prey

Abstract

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.

Interactions among predators and the cascading effects of vertebrate insectivores on arthropod communities and plants

“…We propose that vertebrate insectivores may, in fact, release herbivores from consumption by intermediate predators, but these relatively large and highly mobile vertebrates in turn switch among trophic groups opportunistically and dynamically. Previous studies suggest that many avian and mammalian insectivores show frequency-dependent pressure on prey groups to the extent that those prey are abundant relative to other potential prey (41). If vertebrate insectivores dynamically switch to the most abundant trophic groups (42), this frequency-dependent predation may equalize their effects on predatory and herbivorous prey, resulting in little net effect on the trophic composition of arthropod communities.…”

Evolutionary consequences of a search image

“…In contrast, search images have been proven to exist in a large range of predators (Tinbergen, 1960;Pietrewicz and Kamil, 1979;Bond, 1983;Melcer and Chiszar, 1989;Allen et al, 1998;Jackson and Li, 2004;Bond and Kamil, 2006). A possible explanation would be that in real systems predators are unable to completely control the prey densities, due to temporal and spatial heterogeneity in the environment, prey variability, multiple predator species etc.…”

“…It is well known that polyphagous predators can form search images for certain prey (Tinbergen, 1960;Allen et al, 1998;Bond and Kamil, 2002). Although the effect of search images on the evolution in prey has been studied (Bond and Kamil, 2006), little has been done on the evolution of a search image in predators.…”

“…Although the formation of a search image has historically been associated with bird species (Tinbergen, 1960;Pietrewicz and Kamil, 1979;Bond, 1983;Allen et al, 1998;Bond and Kamil, 2006), there is a strong indication that predators from different classes form search images. For example, Jackson and Li (2004) showed that hunting spiders also form search images.…”

“…If we make the reasonable assumption that a search image would be formed for the more abundant prey type, this leads to a disproportionally high predation rate on the more abundant prey type and, thus, to prey switching (Murdoch, 1969;Oaten and Murdoch, 1975;Van Leeuwen et al, 2007). Search images cause prey switching by a predator, which leads to coexistence between prey species even if the prey species are in direct competition (Murdoch, 1969), such behaviour could also lead to polymorphism in prey species (Allen et al, 1998;Bond and Kamil, 2006). While many predators are able to form a search image, it seems that there is a trade off.…”