Figure 1. The ultimate prerequisite for successful migration in many migrant birds is fat as fuel for their long flights. Before departure on such flights, a large amount of fat is stored in thick subcutaneous deposits. The relatively lean garden warbler (left) weighing about 18 g would not be able to undertake long flights, in contrast to the fat bird (right) weighing about 30 g and well prepared for its trans-Saharan journey to Afrotropical wintering grounds. Photographed by F. Bairlein.

Figure 2. Seasonal pattern of body mass owing to fat deposition in redstarts, black redstarts and in hybrid offspring of both species. After Berthold et al., (1996).