Causes and consequences of pre-laying weight gain in a food-caching bird that breeds in late winter

Animals that reside at high latitudes and altitudes year-round often use cached food to survive over the winter months, but a few species also rely on stored food to sustain them during the breeding season when the nutritional requirements of females are higher than normal. Gray jays Perisoreus canadensis rely on perishable cached food during the winter and females begin breeding in late winter when fresh food is rarely available. To examine pre-laying patterns of weight gain, as well as the causes and consequences of weight gain among individuals, we weighed females regularly throughout the pre-laying period. Females began increasing their weight approximately nine days prior to their first egg date, and on average increased their body weight by 25%, which is on par with other bird species that rely on non-cached food. Final pre-laying weight was positively influenced by the percent of conifers on territories, providing some support for previous results showing that coniferous trees are better able to preserve cached food. We also found that both final pre-laying weight and the rate of weight gain were positively related to female age, supporting the hypothesis that female caching ability improves with age. With increasing final weight, females tended to lay larger clutches and hatched more nestlings, despite the fact that final weight was not influenced by weight at the beginning of the weighing period. Our results confirm that gray jays are able to reach breeding condition while relying primarily on food stored before winter, and suggest a novel mechanism by which habitat-mediated carry-over effects and female age may influence reproductive performance in a food-caching animal.