An analysis of the population dynamics of selected avian species

Graduation date: 1971The impact that pesticides may have had on the mortality rates and productivity rates of non-game birds during the last 25 years was evaluated by studying the population dynamics of 16 species. A mathematical model showing the relationships 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) productivity rates, and (3) the age of sexual maturity. Production requirements for a stable population and annual rate of change (increase or decrease) in population levels were estimated. Population parameters were compared to determine if changes had occurred between time periods (i.e, 1925-1945 vs. 1946-1965). The great horned owl, red-shouldered hawk, sparrow hawk, osprey, barn owl, Cooper's hawk, red-tailed hawk, great blue heron, black-crowned night heron, brown pelican, barn owl, chimney swift, blue jay, black-capped chickadee, cardinal, and robin were subjected to this analysis. No increase in post-fledging mortality rates in any of the species has occurred since 1945. Therefore, accelerated decline in the species studied must have resulted from lowered reproductive rates. Mortality rates have decreased in the Cooper's hawk, sparrow hawk, great blue heron, and brown pelican. A decrease in shooting pressure was associated with decreased mortality rates. Evidence of declining reproductive rates were found in the brown pelican, osprey, Cooper's hawk, red-shouldered hawk, and sparrow hawk. No changes in reproductive rates were noted in the red-tailed hawk, great horned owl, or barn owl. Information on productivity rates was not available for comparison with the other species although rates of productivity essential for a stable population were estimated. This work will provide the basis for making comparisons in future studies. No change in reproductive 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 bird species which consume food in which chlorinated hydrocarbon pesticides have been concentrated through a series of transfers along food chains have declined. The chlorinated hydrocarbon pesticides are believed responsible