Spatio-temporal analysis of Gray Jay (Perisoreus canadensis) population dynamics: A test of the “hoard rot hypothesis”

Background/Question/Methods

Gray Jays (Perisoreus canadensis) are widespread boreal-breeding birds. On the southern edge of its range in Algonquin Park, Ontario, Canada, this species has shown steady declines since the 1960s, and currently only 30% of their historical range in the park is occupied. Gray Jays are territorial year-round, and adults cache food in the summer and fall for the overwintering period.  Recent experiments have shown that: (1) coniferous trees, such as black spruce and pine, are better able to preserve cached food than deciduous trees; and (2) high fall temperatures are associated with poorer preservation of cached food.  These observations were generalized as the “hoard rot hypothesis” (Waite and Strickland 2006), which predicts that reproductive success depends importantly on the quality and abundance of food hoards.  Here we test three predictions of the hoard rot hypothesis using 108 years of Christmas Bird Count data. 

Results/Conclusions

We find that in contrast to the hypothesis predictions populations are not declining more rapidly at the southern limit of their range than at higher latitudes; and populations are not declining more rapidly in deciduous broadleaf forest than in coniferous evergreen.  However we do find that year-to-year decreases in bird counts are associated with warm Fall temperatures, and that this signal is stronger in deciduous broadleaf forest.  These results suggest that the response of Gray Jays to climatic change will not be uniform across the species’ range, but will be mediated by forest cover and land cover change.