Management of fat reserves and food caches in tufted titmice ( Pants tricolor ) in relation to unpredictable food supply

In the temperate zone, permanent-resident bird* and mammals that do not hibernate must survive harsh winter conditions of low ambient temperature, long nights, and reduced food levels. To understand the energy management strategy of foodhoarding birds, it has been hypothesized that such birds respond to increased itarvation risk by increasing the number of their hoards rather than by increasing their fat reserves and that they cache early in the day and retrieve their caches later to achieve fat reserves necessary to survive the night We tested these hypotheses by observing the responses in captivity of a caching bird, the tufted titmouse (Pants tricolor), to the combined influences of reduced predictability of food and naturally occurring ambient temperature and photoperiod. When the food supply was unpredictable, birds significantly increased both internal fat reserves at dusk and external food caches. Initially leaner birds tended to increase their fat reserves to a greater extent and initially fatter birds tended to cache more food and to My significantly less. Half the birds also increased their dawn and mean daily body mass. All birds tended to forage, gain body mass, and cache food at significantly lower rates in the morning and at significantly higher rates in the evening. Cache retrieval showed the opposite trend, with birds retrieving most of their caches in the morning. Our results do not support the hypothesis that caching birds increase caching rate but not body mass under an unpredictable food regime. Instead fat reserves and food caches are both important complementary sources of energy in food-hoarding birds. Energy management by wintering birds occurs in response to a number of biotic and abiotic factors acting simultaneously; thus future models must incorporate independent variables in addition to the state of the food supply and time of day.