MANAGEMENT OF FAT RESERVES AND FOOD CACHES IN TUFTED TITMICE (PARUS BICOLOR) IN RELATION TO UNPREDICTABLE FOOD-SUPPLY

In the temperate zone, permanent-resident birds and mammals that do no t hibernate must survive harsh winter conditions of low ambient temper ature, long nights, and reduced food levels. To understand the energy management strategy of food-hoarding birds, it has been hypothesized t hat such birds respond to increased starvation risk by increasing the number of their hoards rather than by increasing their fat reserves an d that they cache early in the day and retrieve their caches later to achieve fat reserves necessary to survive the night. We tested these h ypotheses by observing the responses in captivity of a caching bird, t he tufted titmouse (Parus bicolor), to the combined influences of redu ced predictability of food and naturally occurring ambient temperature and photoperiod. When the food supply was unpredictable, birds signif icantly 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 fo od and to fly significantly less. Half the birds also increased their dawn and mean daily body mass. All birds tended to forage, gain body m ass, 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 mor ning. Our results do not support the hypothesis that caching birds inc rease caching rate but not body mass under an unpredictable food regim e. Instead fat reserves and food caches are both important complementa ry sources of energy in food-hoarding birds. Energy management by wint ering birds occurs in response to a number of biotic and abiotic facto rs acting simultaneously; thus future models must incorporate independ ent variables in addition to the state of the food supply and time of day.