Long-term hoarding in the Paridae: A dynamic model

Abstract

Using stochastic dynamic programming we modeled the hoarding and foraging behavior of tits and chickadees, Paridae, that are resident in the boreal forest at high latitudes. Here autumns have a rich supply of seeds and temperatures are relatively mild, while winters are cold with short days and a low food supply. We assumed that parids have a memory of limited duration and that forgotten seeds accumulate in a bank that adds to the general food supply in the hoarder's territory. Our model predicts that birds should start 'high-intensity' hoarding in early autumn, but not before that. Because of mass-dependent costs the birds will keep their fat levels low during the autumn. When winter arrives they will carry more body fat, both for the long winter nights and to hedge against the large effects of weather variations in winter. After increasing the fat level at the start of winter, fat should gradually increase even more, to compensate for the diminishing food supply. Most hoarding occurs in autumn as a way of building up the supply of long-term stores. Remembered, or short-term caches, may hedge against stochastic events in the environment. Even though conditions are not beneficial for hoarding in winter, the birds still stored in winter to maintain larger short- and long-term hoards if environmental variation increased. Almost all time in winter that not was spent foraging was spent perching, mainly to avoid predation.