Some aspects of the weights of over 5,500 birds of 79 species, mostly migrants, captured in spring at an oasis on the northern edge of the Sahara, are discussed. Mean weights for five species were 26–44% below those of fat birds (those containing at least 30% lipid) weighed about the same time on the south side of the desert. Weights of fresh arrivals in Britain are higher than those in North Africa, but the differences are not great, suggesting that these have also made a long journey across Europe. Peak arrival times of day-migrants differed from those of night-migrants and the weights of some species varied with the time of day. There was no great advantage to migrants in remaining at the oasis; there were few weight changes greater than those normally occurring within an individual's daily range. Several species lost weight steadily during a cold spell, and many died. Male Swallows are heavier and larger than females, and in both sexes weight increases with wing-length.