Components of Avian Breeding Productivity

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 that 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. Ornithologists have pondered the significance of clutch size in birds for nearly three decades without fully resolving the factors responsible for its variation among species and regions. Although Skutch (1949) linked the evolution of clutch size to adult mortality and population turnover rate, population biologists have only re- cently placed clutch size in a demographic context. For example, not until 1961 did Drury point out that the production of young by a pair of birds during a breeding season depends on the number of broods raised as well as clutch size. Cody (1971) and Ricklefs (1973) treated avian demography more fully and demonstrated the use of life table analyses to compare species. Although breeding biology and mortality have been investigated in scores of de- tailed field studies, complete life tables have been constructed for only a handful of species. Of the statistics required to complete a life table, the annual production of young is one of the most difficult to obtain. Observation of individually marked birds yields relatively few data per hour of effort (for example see Nice 1937, Snow 1958). Furthermore, estimates of mean annual productivity obtained from direct observa- tion have large probable errors (wide confidence limits) owing to the great variation between individuals. Annual productivity can be calculated indirectly from clutch size, breeding success, nest cycle length, and length of breeding season (Ricklefs 1970). These data are more readily obtained than direct measures of breeding pro- ductivity. Furthermore, data from different studies may be combined to calculate the annual production of a species. In this paper we calculate annual production (fledglings per pair of adults) for various passerine birds in two temperate and two tropical localities. The data are