Polybrominated diphenyl ether (PBDEs) concentrations are increasing exponentially in biota. We studied the growth of American kestrel (Falco sparverius) nestlings exposed in ovo and during development to environmentally relevant PBDE congeners and concentrations. Eggs within each clutch, divided between groups by laying sequence, were injected into the air cell at 19 days of incubation with safflower oil or penta-BDE congeners BDE-47, -99, -100, and -153 dissolved in safflower oil (18.7 microg total sigmaPBDEs/egg), approximating current levels in Great Lakes herring gulls. The measured proportions of BDE congeners found in the dosing oil were 56.4% of BDE-47; 27.2% of BDE-99; 24.8% of BDE-100; and 0.6% of BDE-153. For 29 days, nestlings were orally gavaged daily with the same sigmaPBDE mixture (15.6 +/- 0.3 ng/g body weight/day). Relative congener abundances in the dosing mixture compared to the carcasses suggest biotransformation of BDE-47; BDE-183 was also detected. PBDE exposure did not affect hatching or fledging success. PBDE-exposed nestlings were larger (weight, bones, feathers) as they gained weight more quickly and ate more food, the latter in association with their SigmaPBDE body burdens. BDE-100 was most influential on nestling growth, being positively associated with size, weight gain, and food consumption. Increasing concentrations of BDE-183 and -153 were related to longer bones, and BDE-99 to longer feathers. The larger size of the PBDE-exposed birds may be detrimental to their bone structure and have excessive energetic costs. The repeated relationships with BDE-100 and growth may be important for wild Falconidae, since this is the predominant penta-BDE congener in these raptors.