INFLUENCE OF REDUCED FOOD AVAILABILITY ON GROWTH OF CAPTIVE AMERICAN KESTRELS

Captive American kestrels (Falco sparverius) were used to model the ef fect of low prey availability on growth and survivorship potential in nestling raptors. The experimental design consisted of 4 treatments: n estlings were fed ad libitum (100% diet) or on increasingly restricted diets (90, 80, and 70% of the ad libitum diet). Nestlings fed a reduc ed diet grew significantly more slowly than those fed the ad libitum d iet, as shown by the body mass growth constant. The restricted diet ha d no signifi cant effect on the asymptotic size of the kestrels, i.e., body mass and tarsus and antebrachium lengths. However, young kestrel s fed reduced diets (80 and 70%) had a lower body mass at fledging tha n those fed ad libitum and the 90% diet, although they fledged at the same age. The growth of the ninth primary was slower for birds on rest ricted diets than for those fed ad libitum. Sexual dimorphism was obse rved for body mass and antebrachium growth constants, with males growi ng faster than females. However, females reached a higher asymptotic b ody mass and antebrachium length, as well as a higher body mass at fle dging, than males. Thus, when prey availability is reduced by as much as 30%, nestling kestrels grow more slowly and store less fat, which c ould lead to poorer postfledging survivability.