The body composition of young American kestrels (Falco sparverius) hand-fed either a protein-rich diet (day-old cockerels Gallus domesticus) or a fat-rich diet (laboratory mice Mus musculus) was determined one day after fledging. Mouse-fed fledglings (n = 16) had significantly greater fat deposits than cockerel-fed birds (n = 15), while the crude protein content of the carcass was unaffected by the diets. At fledging, mouse-fed birds showed a significantly greater wingloading than cockerel-fed birds. Larger fat reserves (as in mouse-fed birds) might be mobilised in the event of a food shortage and thus these birds would be at an advantage in relation to fledglings with smaller reserves. On the other hand, large fat deposition, which alters wingloading, might impair the flight performance of the fledglings.