POWER TO DETECT TRENDS IN WATERBIRD ABUNDANCE WITH CALL-RESPONSE SURVEYS

We broadcast calls of grebes, rails, and bitterns, and passively count ed other species, along 36 survey routes in Mine wetlands during 1989- 92. Our objectives were to determine the magnitude and temporal variat ion in waterbird call-response rates, and to estimate, through compute r simulation, the statistical power of variously configured monitoring programs. Average call-response rates on survey mini-routes ranged fr om 2.8 to 6.0 responses per 10-station mini-route, and temporal variat ion in counts was relatively high (CV in all species > 80%). A power a nalysis, performed by coupling Monte Carlo methods with linear regress ion, indicated that changes in call-response rates of 10% would be dif ficult to detect over a 10-year monitoring period in any species. Chan ges of 25%, however, could be detected reliably (P > 0.90) by surveyin g 40-80 mini-routes on 2-3 occasions annually. Passive counts at call- response stations yielded useful abundance information on many wetland species currently undersampled by existing avian monitoring schemes. We conclude that call-response surveys for selected species, coupled w ith passive counts of other species, are a logistically feasible and s tatistically defensible means of monitoring trends in regional populat ions of wetland birds.