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 respond 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 energy storage by (1) shifting the time of their daily body mass accumulation toward earlier in the day and (2) increasing the overall level of their fat reserves. During the winter of 1995–1996, we tested these predictions with a caching species, the tufted titmouse (Parus bicolor). Each of six experimental birds was fed a diet of uncachable sunflower seed powder for 6 days, preceded and followed by 6-day control periods during which they were fed cachable sunflower seeds. The daily pattern of body mass gain was unaffected by the opportunity to cache. Furthermore, when unable to cache, the birds did not increase either their 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 against the hypothesis of a trade-off between fat reserves and food caches in tufted titmice, and suggest that fat reserves are managed independently of external food caches. Received: 9 April 1997 / Accepted after revision: 30 August 1997