RESPONSE OF NESTING DUCKS TO HABITAT AND MANAGEMENT ON THE MONTE-VISTA-NATIONAL-WILDLIFE-REFUGE, COLORADO

We analyzed duck nesting data from the Monte Vista National Wildlife R efuge (MVNWR) in the San Luis Valley (SLV) of Colorado to evaluate the influence of habitat and management on nest density and success. From 1964 to 1990 (except 1977), 4,154 duck nests were found on 16,159 km (10,041 miles) of transect. Strip transects were searched twice annual ly, with a follow-up visit to nest sites to determine nest fate. Durin g 1964-68, 521 km (324 miles) of transect were searched (142 transects ) twice each year, but this was reduced by half during 1969-90 (71 tra nsects). We evaluated information on duck nests in 22 of 24 refuge man agement units. Mallards (Anas platyrhynchos) composed 54.5% of the duc k nests on MVNWR, followed in abundance by teal species (A. cyanoptera , A, discors, A. crecca) and northern pintails (A, acuta). Refuge nest densities ranged from a low of 36 nests/km(2) (93 nests/mi(2)) in 197 9 to a high of 161 nests/km(2) (418 nests/mi(2)) in 1973. Over the 26- year period (1964-90), 52.4% of 4,154 nests were successful. Apparent annual nest success averaged 49.4% and ranged from 27% in 1978 to 72% in 1966. Nest failure resulted from predation (25.8% of total nests), desertion (10.6%), flooding (4.9%), or-unknown causes (10.8%). It was not possible to directly estimate true nest success, but indirect meth ods suggest that true nest success was 60 to 90% that of apparent nest success. Nest success rates were similar among species. Baltic rush ( Juncus balticus) was the most common vegetation on the refuge, followe d in abundance by black greasewood (Sarcobatus vermiculatus), inland s altgrass (Distichlis stricta), and other grass species. Stands of comm on cattail (Typha latifolia), miscellaneous herbaceous vegetation, and Baltic rush were preferred nesting cover, whereas greasewood and salt grass generally were avoided. Nest density declined (P = 0.037) with i ncreasing grazing intensity, with large (38%) declines apparent the fi rst season after only light grazing (0.2 animal unit month/ha [0.5 AUM /acre]). Three years after grazing, nest density was still depressed a t least 17% below estimated pregrazing levels. In addition, nest succe ss declined as grazing intensity increased (P = 0.070). Duck nesting d ensities on the MVNWR are among the highest reported in North America. High duck production can be maintained by applying water in late Marc h; reducing the frequency of grazing and burning treatments; and devel oping wetland management units to achieve their potential.