Management of fat reserves in tufted titmice Baelophus bicolor in relation to risk of predation.

Fat reserves are an important energy source for animals wintering in temperate zones. Long nights, low ambient temperature and often unpredictable food all increase the probability of death from starvation, and to survive, animals carry energy reserves as fat. Based on the assumption that extra weight makes birds more vulnerable to diurnal predators, it has been hypothesized that predation risk places an upper limit on avian fat reserves. The hypothesis leads to the prediction that birds should decrease their mass in response to increased risk of predation. We tested this prediction with a resident species, the tufted titmouse, Baelophus bicolor. During a 3-day period, we presented a taxidermic mount of a sharp-shinned hawk, Accipiter striatus (predator treatment) or a mourning dove, Zenaida macroura (control treatment) to seven titmice housed individually with food ad libitum. Even though vigilance and delay in time to resume foraging were significantly higher after exposure to the hawk model, the body mass of titmice after exposure to the hawk increased rather than decreased, a result opposite to that predicted. In particular, the birds significantly increased their evening body mass and mean daily mass gain during the treatment period. Our results suggest that risk-dependent foraging appears to regulate avian body fat reserves by limiting foraging time. When food is unlimited, as in our experiment, birds may be able to obtain high energy reserves even while maintaining intense vigilance for predators, and under such conditions, the energetic costs of flying may control upper level energy reserves.Copyright 1998 The Association for the Study of Animal Behaviour.