A total of 210, 1-d-old Ross 308 male broiler chickens were used in an experiment to investigate the effects of a supplementary mixture containing 5% carvacrol, 3% cinnamaldehyde, and 2% capsicum on dietary energy utilization and growth performance. The 2 diets were offered ad libitum to the chickens from 0 to 21 d of age. These included a maize-based control diet and the control diet with 100 g/t of supplementary plant extracts. Dietary apparent ME, N retention (NR), and fat digestibility (FD) coefficients were determined in the follow-up metabolism study between 21 and 24 d of age. Feeding the mixture of carvacrol, cinnamaldehyde, and capsicum increased weight gain by 14.5% (P = 0.009), improved feed efficiency by 9.8% (P = 0.055), and tended to increase (P = 0.062) carcass energy retention and reduce (P = 0.062) total heat loss compared with feeding the control diet. There was a 16.1% increase (P = 0.015) in carcass protein retention but no difference in carcass fat retention. Feeding plant extracts improved dietary FD by 2.1% (P = 0.013) but did not influence dietary NR. Supplementation of plant extract resulted in a 12.5% increase (P = 0.021) in dietary NE for production (NEp), while no changes in dietary ME were observed. The experiment showed that although dietary essential oils did not affect dietary ME, they caused an improvement in the utilization of energy for growth. Plant extracts may affect metabolic utilization of absorbed nutrients. Studies that have focused solely on the effect of plant extracts on ME alone may well have not detected their full nutritional value.