Why do hoarding birds gain fat in winter in the wrong way? Suggestions from a dynamic model
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In winter, small birds should be fat to avoid starvation and lean and agile to escape predators. This means that they face a tradeoff between the costs and benefits of carrying fat reserves. Every day they must gain enough fat to survive the coming night. Food-hoarding species can afford to carry less fat than nonhoarders because they can store energy outside the body. Furthermore, hoarders should avoid carrying excessive fat during the day because they can gain fat fast by retrieving food late in the afternoon. With no stored supplies, nonhoarders face more unpredictable access to food, and they should start gaining fat earlier in the day. The predicted pattern is then that nonhoarders gain fat early and that hoarders gain fat late in the day. Recent field data show the opposite pattern: hoarders gain relatively more fat reserves in the morning than nonhoarders do. Using a dynamic model that mimics the conditions in a boreal winter forest, I investigated under which conditions this pattern will arise. The only assumption of those investigated that produced this pattern was to relax the effect of mass-dependent predation risk. I did this by introducing a limit under which fat reserves did not affect predation risk. Hoarders then started the day by gaining fat in the morning. Later; when they had reached a safer (but still not risky) level, they switched to hoarding. The pattern I searched would only occur if either. not all food was possible to store, or if retrieval gave less energy than foraging in good weather conditions. If I assumed that low levels of body fat also increased predation risk, hoarders would cache in the morning when they carried least fat. I discuss empirical evidence for how body fat affects predation risk. In summary the factors that produced the pattern I searched were a change in the predation-mortality function combined with restrictions on hoarding.