Components of Avian Breeding Productivity

ABsTl•CT.--Numbers of nestlings fledged per pair per season were calculated for 35 species of passerine birds in four localities from data on length of breeding season, clutch size, nesting mortality, and length of nest cycle. Mean numbers of nestlings fledged was 6.42 for 12 woodland and edge species in Kansas, 4.33 for 8 species in lower Sonoran desert habitat in Arizona, 2.36 for 6 species in humid forest and second-growth habitats in Costa Rica, and 4.80 for 9 species in a desert scrub habitat in Ecuador. The data were analyzed by multiple correlation and stepwise multiple regression to determine the contribution of the variables to variation in the number of young fledged from one species to another in each locality. Variations in season length, clutch size, and breeding success were important components in variation in numbers of young fledged in Kansas, but length of nest cycle, being positively correlated with both clutch size and nest success, had the strongest correlation with the number of young fledged. In Arizona and Costa Rica, variation in breeding success held the key to predicting variation in number of young fledged, whereas in Ecuador season length and clutch size were the critical variables. The analyses suggest hat data on production of young and the factors that cause production to vary from one species to another would add immensely to our understanding of the population ecology of birds.•Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19174. Accepted 1 July 1975.