Specimens of 16 passerine bird species collected in midsummer in the Hubbard Brook forest ecosystems in central New Hampshire were analyzed for major chemical constituents (water, lipid, lean dry matter, ash) and for caloric value. Bird tissue contained on the average 66% water, while the ash content of lean dry tissue was 13.00 + 0.12% as measured in the bomb calorimeter and 14.22 + 0.15% by combustion in a muffle furnace. Ether-extractable lipid averaged 0.165 + 0.007 g/g lean dry weight, a very low value indicating that these birds in summer have no lipid reserve to meet environmentally caused metabolic emergencies. Caloric densities of lean dry tissue, extracted lipids and of whole birds averaged 4616 + 12, 8908 + 28 and 5239 ? 29 calories per g dry weight, respectively. INTRODUCTION Studies of energy flow through ecological systems require information on the caloric densities (calories per gram) of the constituent organisms. For animals, caloric values vary with the major chemical components: water, fat, ash-free lean dry matter (mostly protein) and ash content (Ricklefs, 1974). Analyses of body composition, especially lipid levels, also provide useful indices to the nutritional state and physiological and ecological demands of the organisms (Yarbrough, 1970). For birds, complete data on both caloric densities and body composition for the same species are surprisingly sparse (see Ricklefs, 1974), although information is available for some species during certain parts of their annual cycle, especially migration (e.g., Odum, 1960; Odum et al., 1964, 1965; Johnston, 1968; Hussell, 1969) and winter (e.g., Farrar, 1966; King and Farner, 1966; Helms et al., 1967; Helms and Smythe, 1969). The only thorough data on both body composition and caloric contents of passerine birds in summer are those for marsh wrens (Telmatodytes palustris) by Kale (1965), tree sparrows (Passer montanus) by Mrycha and Pinowski (1970) and meadow pipits (Anthus pratensis) by Skar et al. (1972). Some aspects of body composition, mostly lipid contents, of summer birds are treated by King et al. (1965), Morton et,al. (1973), Yarbrough (1970) and Zimmerman (1965). There is no information on the caloric densities and chemical constituents of species such as thrushes, vireos, warblers and grosbeaks during their breeding season in the eastern deciduous forests of North America. This study examines these characteristics for 16 passerine bird