Stored fat is a main source of energy for birds in winter and it seems reasonable that fatter birds should survive better. Small birds wintering at high latitudes, however, carry less fat than their capacity permits, suggesting a predation cost of excessive fatness. The birds seem to maintain their fat depots at a level balancing the risks of starvation and predation. The thesis examines this ecological trade-off by looking for relationships between levels of fat depots on the one hand and factors involving the risks of starvation and predation on the other.
A basic assumption is that birds manage their level of reserves rather than constantly maximizing their intake rate. Support for this was found when great tits, Parus major, were able to rapidly compensate for a low level of reserves caused by an increase in overnight energy expenditure. The response suggests that birds make state-dependent foraging decisions based on the current level of reserves and the time of day. Further support for the assumption was found in other studies where birds adjusted their levels of reserves to experimental changes in environmental parameters.
The reliability of a food supply is likely to be one important component of the starvation risk. It seems probable that such reliability may differ between species and individuals within species. This was studied in wintering willow tit, P. montanus, flocks where it was found that dominants carried less reserves than subordinates. Subordinates, with the added risk of being unable to forage because of dominants, may be forced to carry more fat. A causal relationship between rank and body reserves was found when the removal of dominants resulted in subordinates lowering their level of reserves. The predictability of the food supply was studied further by comparing the daily patterns of reserve accumulation between three bird species that store food and great tits which do not. In contrast to theoretical predictions, the daily routines found in the hoarding species looked as if the birds were uncertain of their food source, and the non-hoarding great tit seemed relatively more secure of its food.
Changes in the perceived risk of predation influenced levels of body reserves carried by greenfinches, Carduelis chloris, and yellowhammers, Emberiza citrinella. In accordance with theoretical predictions, the body mass of greenfinches varied in relation to different levels of the predation risk. The presence of a predator also affected the daily routines of gaining body mass. Generally, foraging stopped and the birds lost weight until feeding resumed at an increased rate, in concordance with theoretical expectations. In contrast to greenfinches, yellowhammers increased their evening body reserves when facing an increased predation risk. These different responses may be a result of a natural species difference. While greenfinches lowered their evening reserves, perhaps yellowhammers were gaining reserves in preparation to leave a dangerous situation. The results suggest that a response to a predator may differ between species or the circumstances the birds are in.
In conclusion, it was found that small birds actively manage their levels of reserves, probably reflecting a trade-off between the risks of starvation and predation. This relationship may, however, be more complicated than previously thought.
Stockholm: Department of Zoology, Stockholm University , 1997. , 29 p.