Choosing Where to Feed: Choice within Fields

Abstract

Most natural food resources are patchily distributed, either as discrete entities, such as rotting tree stumps and bunches of berries, or as statistical heterogeneities in an otherwise continuous distribution, such as the occurrence of earthworms in a field. Whichever form it takes, patchiness in food resources has important consequences for the foraging strategies we should expect predators to adopt (e.g. Pyke et al. 1977, Krebs 1978, 1979, Krebs and McCleery 1984). At a simple level, food patchiness should affect the way predators spend time in different parts of their environment. If they are to maximise their feeding efficiency, we should expect them to spend most time where food is most abundant, i.e. in the richest patches. In socially-feeding predators, the amount of time spent in a given area by different individuals translates into the number of individuals we might expect to encounter in that area at any given time (e.g. Barnard 1980b). In the last chapter, we saw that gulls and plovers prefer to feed in old pasture, where earthworms are most abundant. Choice of field on the basis of worm availability is a response to patchiness at one level. As we have seen, however, differences in patchiness can also be measured within fields. In this chapter we shall look at the way birds respond to prey patchiness at this second level by examining their distribution within fields.