Protein loss during long-distance migratory flight in passerine birds: adaptation and constraint.

During long-distance flights, birds catabolize not only fat but also protein. Because there is no storage form of protein, protein catabolism entails a structural or functional loss. In this study, we investigated which organs were most reduced in lean mass during different phases of fat store loss and whether protein loss can be regarded as adaptive or as a constraint. Body and organ composition were analysed both during the autumn migration over continental Europe (sample from Switzerland) and after a long-distance flight over the Sahara and the Mediterranean Sea in spring (sample from Ventotene, Italy) in four species of passerine bird: pied flycatcher Ficedula hypoleuca, willow warbler Phylloscopus trochilus, garden warbler Sylvia borin and barn swallow Hirundo rustica. Large variations in protein mass occurred when long non-stop flights were performed. After a long-distance flight, birds showed a marked increase in net protein loss when fat stores were nearing depletion (analogous to the late phase of endurance fasting when the rate of protein catabolism is increased). When fat reserves were above approximately 5-10 %, protein was derived from all organs, but particularly from the breast muscles. When fat stores diminished further and protein catabolism increased, the mass of the digestive organs was reduced fastest. When the decrease in breast muscle mass during flight was regarded in terms of potential flight performance, it appeared that the use of breast muscle protein with decreasing body mass can be regarded as adaptive as long as fat stores did not reach a critical level. Below approximately 5-10 % body fat, however, protein loss reduced flight performance. This demonstrates that the phase of fasting (the size of the remaining fat stores) is an important condition for understanding the occurrence and effects of protein loss during endurance flights.