Transects established to measure whitewing nesting success have revealed that doves are subject to heavy (50-75 percent of the eggs laid) nesting predation. Prior research (Blankinship's study in 1964-1965) has indicated that whitewing (Zenaida asiatica) production can be increased 100 percent if boat-tailed grackle (Cassidix mexicanus) numbers are controlled during the nesting season. This project was initiated in the Lower Rio Grande Valley of Texas in 1967 to determine the effects of grackle control on white-winged dove and grackle nesting success. Work with poisons as a grackle control technique began in 1961. Anticoagulants, 1080, DRC-1339, DRC-1861, and methoxymol were some of the chemicals tried. DRC-1339 (3-chloro-p-toluidine Hydrochloride), a slow acting uremic poison, appeared to have the best chance for success. Cage studies determined that DRC-1339 is lethal to grackles at 1-1.5 mg! kg, while it takes 5.6 mg! kg to kill whitewings. DRC-1339 blocks the passage of wastes by breaking down the cellular tissue in the kidneys (DeCino). The poison poses little threat of secondary poisoning. Death normally occurred from 24 to 96 hours which prevented target species from becoming bait shy. Two areas were used for this study, the Longoria Unit of Las Palomas Wildlife Management Area, which was used for the treatments, and La Paloma Brush, which was used as a "control". The Longoria Unit is 200 acres in size, and La Paloma is 15.5 acres. Both tracts are in Cameron County, Texas, and their vegetation is similar. Whitewing and grackle populations were censused before and after control techniques were applied to determine the effectiveness of the method. Usage of the two areas by both species fluctuated widely in numbers depending upon range conditions and other factors. However, the Longoria Unit has steadily declined in whitewing numbers since the high of 17,000 breeding birds in 1958. Comparative population figures for whitewings and grackles using the Longoria Unit are listed below by years.
