MANAGEMENT OF FAT RESERVES IN TUFTED TITMICE (PARUS BICOLOR) - EVIDENCE AGAINST A TRADE-OFF WITH FOOD HOARDS

Caching species can manage their energy supply by adjusting body fat, number of caches, or both. It has been hypothesized that because body fat has a higher fitness cost than caches, small food-hoarding birds r espond to increased starvation risk by increasing the number of their caches rather than their fat load. This hypothesis predicts that when birds cannot cache they should compensate for the loss of external ene rgy storage by (1) shifting the time of their daily body mass accumula tion toward earlier in the day and (2) increasing the overall level of their fat reserves. During the winter of 1995-1996, we tested these p redictions with a caching species, the tufted titmouse (Pai us bicolor ). Each of six experimental birds was fed a diet of uncachable sunflow er seed powder for 6 days, preceded and followed by 6-day control peri ods during which they were fed cachable sunflower seeds. The daily pat tern of body mass gain was unaffected by the opportunity to cache. Fur thermore, when unable to cache, the birds did not increase either thei r mean daily body mass, body mass in the middle of the day, or evening body mass compared to the two control periods. These results argue ag ainst the hypothesis of a trade-off between fat reserves and food cach es in tufted titmice, and suggest that fat reserves are managed indepe ndently of external food caches.