Evaluating the sterile male method on Red-winged Blackbirds: effects of the chemosterilant thiotepa on the reproduction of clinically treated birds under field conditions

In 1978 and 1979, spring blackbird flocks composed of Red-winged Blackbirds, Common Grackles, and Brown-headed Cowbirds reached a maximum of 5000 birds in the study area. Red-winged Blackbirds accounted for a majority of birds in these flocks. Flocking lasted approximately 1 month leaving ample time for a field application of the chemosterilization method. Three sets of experiments were used to examine the feasibility and the potential of the sterile male method to reduce productivity of Red-winged Blackbird populations. Birds treated by gavage at a rate of 3.1 mg thiotepa/kg body weight had a hatching rate of 32.7%. This was significantly lower than that found in control groups (79.8 to 90.0%). The second treatment, which consisted of feeding redwings with treated corn for 4 consecutive days, thus causing a mean daily ingestion of 3.58 mg thiotepa/kg body weight, produced a hatching rate of 55.6%. The third treatment, like the second but lasting 10 consecutive days, resulted in a daily ingestion by blackbirds of 4.01 mg thiotepa/kg body weight. The hatching rate of this latter group was 46.3% and was significantly lower than that of the control group. Red-winged Blackbird testes weight remained stationary in the treated group for over 1 month after the beginning of the treatment. These experiments demonstrate that the sterilization approach in field conditions offers hope of reducing populations causing crop damage if proper field application methods are developed.