Choice between better and worse prey items
It is deduced that the choice should be influenced by the availability of the most preferred item, but not by the availability of the least preferred item. (Davey, 1989; p. 241)
This has been checked with shorebirds in the field (redshank) choosing large or small worms (Goss-Custard, 1977) and pigeons choosing immediate or delayed rewards in the laboratory (Lea, 1979)
Choosing where to forage (“Patch selection”)
One theory (the “marginal value theorem”) says that an animal should leave a particular patch when its energy intake drops to the average level for the surrounding habitat. This involves the concept of an “expected rate of return”. (Davey, 1989; p245).