The impact of pesticides on the mortality rates and recruitment rates of nongame birds during the last 25 years was evaluated by studying the population dynamics of 16 species. A mathematical model showing the relations between population parameters that yielded stable populations was developed. The information needed for the model included (1) mortality rate schedule (obtained from recoveries of banded birds), (2) recruitment rates, and (3) the age of sexual maturity. The rate of recruitment necessary for a stable population and/or the annual rate of change (increase or decrease) in population levels were estimated. Population parameters were compared to determine whether changes had occurred between time periods (i.e., 1925-45 vs. 1946-65). The great horned owl, red-shouldered hawk, sparrow hawk, osprey, barn owl, Cooper's hawk, red-tailed hawk, great blue heron, blackcrowned night heron, brown pelican, barn swallow, chimney swift, blue jay, blackcapped chickadee, cardinal, and robin were subjected to this analysis. No increase in postfledging mortality rates in any of the species was detected during the last 25 years (since 1945). Since there was no evidence of increased mortality rates it was concluded that accelerated declines in several of the species studied resulted from lowered reproductive success. Mortality rates were found to have decreased in the Cooper's hawk, sparrow hawk, great blue heron, and brown pelican and this was associated with a decrease in shooting pressure. Evidence of lower recruitment rates was found in the brown pelican, osprey, Cooper's hawk, red-shouldered hawk, and sparrow hawk. No changes in recruitment rates were noted in the red-tailed hawk, great horned owl, great blue heron, or barn owl. Information on recruitment rates was not available for comparison with the other species although rates of recruitment essential for a stable population were estimated. This work will provide the basis for making comparisons in future studies. No change in recruitment rates was apparent among species feeding primarily on mammals. Species exhibiting a lowered reproductive success since 1945 were those whose major food items consisted of fish, reptiles, amphibians, or birds. Lowered reproductive success was accompanied by a decrease in eggshell thickness. Other investigators have reported that sparrow hawks and mallard ducks fed a diet of DDE and dieldrin have produced thin eggshells under laboratory conditions, and exhibited a lower, reproductive success. Many of the bird species that have declined are those that consume food in which chlorinated hydrocarbon pesticides have been concentrated through a series of transfers along food chains. The chlorinated hydrocarbon pesticides are believed responsible.