Differences in population density and energy use between birds and mammals: A microecological perspective

1. Data from 364 mammal and 564 bird species were used to compare these two taxa in the patterns of variation in population density and energy use as a function of body mass. 2. This study demonstrates previously unappreciated quantitative differences between mammals and birds. Over a wide range of sizes, population densities and rates of energy use are at least one order of magnitude higher in mammals than in birds of similar size, The highest population densities are found in species that weigh about 100 g in mammals and 30 g in birds. 3. Comparisons between mammals and birds from the same dietary category indicate that mammals maintain higher densities and use more energy than birds. Insectivorous mammals and birds maintain the lowest densities. 4. Flying mammals and birds reach lower densities and use more energy than non-flying forms. 5. These findings reveal relationships between the morphology, physiology and behaviour of individual organisms and the ecological performance of these endothermic vertebrates in populations, communities and ecosystems: relationships that have not previously been appreciated.