THE CONSEQUENCES OF PARTIALLY DIRECTED SEARCH EFFORT

Search effort is undirected when a forager has a stereotypical searchi ng behaviour that results in fixed encounter rates with its prey (e.g. diet choice models), and is directed when the forager can bias its en counter with a 'chosen' prey. If the bias is complete, search is total ly directed (e.g. habitat selection models). When the bias is incomple te (i.e. search modes are not exclusive to a single prey type), search is partially directed. The inclusion of a prey type in the diet is th en the result of two decisions: (1) which prey to search for and (2) w hich prey to handle. The latter decision is determined by the ratio of energy to handling time and the abundance of the preferred prey. The former decision is a function of the encounter probabilities and densi ties of all potential prey types in addition to their ratio of energy to handling time. Assuming two prey types, there are three distinct be havioural strategies: (1) search for the preferred prey/forage selecti vely; (2) search for the preferred prey/forage opportunistically; and (3) search for the non-preferred prey/forage opportunistically. If pre y are depletable (i.e. prey occur in resource patches), the forager ma y switch search modes such that prey are depleted to the point where t he marginal values of the search modes are equalized.