During the winter, small birds face harsh conditions and react by putting on fat. There are likely to be costs of fattening, both through acquiring and maintaining such reserves. It has recently been asserted that the cost of acquiring fat is sufficient for lite long-term regulation of fat, even if there are no maintenance costs. We show both computationally and analytically, that this is not the case, and that in the long term the cost of acquiring fat becomes insignificant in determining fat levels. We argue that winter should be considered a long time period for a small bird. Thus, since small birds do not keep reserves at the maximum possible level, we infer that there are mass-dependent costs to small birds in winter.