RED-SHOULDERED HAWK BROADCAST SURVEYS - FACTORS AFFECTING DETECTION OF RESPONSES AND POPULATION TRENDS

Forest-nesting raptors are often difficult to detect and monitor becau se they can be secretive, and their nests can be difficult to locate. Some species, however, respond to broadcasts of taped calls, and these responses may be useful both in monitoring population trends and in l ocating nests. We conducted broadcast surveys on roads and at active r ed-shouldered hawk (Buteo lineatus) nests in northcentral Minnesota to determine effects of type of call (conspecific or great horned owl [B ubo virginianus]), time of day, and phase of the breeding cycle on red -shouldered hawk response behavior and to evaluate usefulness of broad casts as a population monitoring tool using area occupied-probability- of-detection techniques. During the breeding seasons of 1994 and 1995, we surveyed 4 10-station road transects 59 times and conducted 76 sur veys at 24 active nests. Results of these surveys indicated conspecifi c calls broadcast prior to hatch and early in the day were the most ef fective method of detecting red-shouldered hawks. Probability of detec tion via conspecific calls averaged 0.25, and area occupied was 100%. Computer simulations using these field data indicated broadcast survey s have the potential to be used as a population monitoring tool.