The rate at which birds can deposit fat (before migration starts, and at stopover sites), is a major factor determining the overall speed of migration. The existence of an upper limit to daily metabolisable energy intake (DME(max), Kirkwood 1983), implies that there also is an upper limit of fat deposition rate (FDR(max), the daily gain in fat mass as a percentage of lean body mass). FDR(max) in birds of different sizes is on theoretical grounds predicted to be proportional to M-0.27, where M = lean body mass. An examination of literature data on rapid body mass increases in migrating birds, reveals that the highest values for individuals and populations are proportional to body mass as M-0.25 and M-0.31, respectively. Studies of foraging ecology at stopover sites indicate that migrants exploiting very abundant food sources may regularly reach the DME(max) ("metabolically limited migrants"). The fat deposition rates of these birds will mainly depend on their daily energy expenditure. In other species, DME(max) may never be reached and fat deposition rates are determined by foraging efficiency and time available for foraging ("foraging-time limited migrants"). As far as maximum speed of migration is concerned, larger birds are generally more efficient flyers than smaller birds and can therefore partly compensate for their lower maximum fat deposition rates. Maximum speed of migration is predicted to be proportional to M-0.14.
