SOURCE-USE DECISIONS BY HOARDING GRAY JAYS - EFFECTS OF LOCAL CACHE DENSITY AND FOOD VALUE

By analogy with patch-use decisions made by animals foraging among pat ches, animals storing food items among scattered sites must make decis ions such as how long to persist in exploiting a known source before l eaving to search for alternative sources. If the survival of scattered caches is density-dependent, then there should be a ''source-departur e threshold,'' beyond which additional caching would reduce the long-t erm average rate of storage of recoverable food. The hoarder would pro fit maximally by leaving the source at that threshold, even though the source still contains food. We experimentally examined how two factor s influence various source-use decisions of Gray Jays Perisoreus canad ensis exploiting locally abundant food sources. First, we examined the behavior of jays exploiting a food source during an initial ''encount er'' and during another encounter 3 days later. The jays made fewer ca ches during subsequent encounters with a source at a given location, a round which the local density of previously made caches was high. They also compensated by spacing out their caches more widely and by incre asing the rate at which they retrieved and redistributed previously ma de caches. Second, we investigated the effect of the composition of a food source (i.e., the value of its food items) on the jays' source-us e decisions. The jays cached substantially more food items (raisins) f rom a large-item (($) over bar X=491) than from a small-item (328) sou rce when sources of each type were made available on different days. T he cumulative number of caches made by individual jays (over 10 h) was more nearly asymptotic in the smaller-item treatment. We propose that the jays' tendency to recache previously made caches accounts for the ir failure to cease hoarding from our experimental sources. This two-s tage process of food storage (i.e., initial placement and subsequent r ecaching) may yield long-term average rates of storage throughout the territory exceeding those predicted by simple rate-maximization, patch -use models.