1. Energy measurements were made over 4 d on groups of three ducklings (aged from 5 to 22 d), and three broiler chickens (aged from 11 to 32 d) offered high- or low-energy diets. 2. Food, metabolisable energy (ME) and water intakes were significantly higher for ducklings than for chickens. The ratio of water:food was 4-2:1 and 2-3:1 for ducklings and chickens, respectively. The food conversion ratio differed between diets but not species. Performance was generally better for both species on the high-energy diet. 3. Heat production, energy, fat and protein retentions were higher for ducklings than chickens, and ducklings retained 0.44 of their energy as fat compared with 0.37 for chickens. Overall the ratio of protein (g) to fat (g) retention was 2.2:1 and 2.8:1 for ducklings and chickens respectively. 4. For ducklings, metabolisability of the high-energy diet declined from 0.774 to 0.747, and to a lesser extent of the low-energy diet, as they aged. There was no such decline for chickens. Net efficiency of utilisation of ME for gain was 0.64 for ducklings compared with 0.50 for chickens. 5. Fractional retention of dietary nitrogen (N) was 0.62 for ducklings and 0.55 for chickens. Gaseous ammonia-N was 4.5 and 2.2%, respectively, of N retained. 6. In a second experiment groups of ducklings only, were offered high- and low-protein diets from 12 to 22 d of age. Comparisons among four diets showed that food and energy intake was lower on the low-protein diet than on the other three. Energy retention on the high-energy diet was greater (P less than 0.05) than on the other three diets. 7. It was concluded that a high-energy diet is important for ducklings and chickens for maximum biological performance during the first 4 weeks of life.