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UBC Theses and Dissertations

Effects of food addition on a population of grey jays Delehanty, Brendan


Grey jays (Perisoreus canadensis). in the southwest Yukon were provided with supplemental food on three study areas to study the effects of food on territoriality and foraging strategies. Territory sizes and overlap were measured in 1993 and 1994. Territories of food addition grids were 3 0% smaller than those on grids without access to added food. There was only slightly more overlap between territories on food addition grids compared to controls, and the difference was not statistically significant. Time spent foraging, the rate at which jays made caches, and the weights of jays were measured. These data were used to test four hypotheses about the factors that limit overwinter body masses of jays. Birds on food addition grids made three times as many caches as control birds in a similar amount of time spent foraging. In winter, grey jays with added food spent less time foraging, yet they were able to maintain higher body condition than control birds. These results were consistent with the hypothesis that overwinter weights are limited by both a food shortage and by costs associated with increasing weight. Seasonal trends in body condition differed between birds on food addition grids and those without supplemental food. Birds with added food were in better condition year round, and were able to increase in weight between summer and fall. Control birds lost weight between summer and fall, but then increased dramatically in winter. These different weight trends may represent different caching or cache retrieval strategies.

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