Effects of roadside transect width on waterfowl and wetland estimates

Strip transects located along roads are commonly used to estimate waterfowl populations and characterize associated wetland habitat. We used data coll ected in May and early June, 1995, on forty-five 40-km(2) plots in North Da kota to evaluate bias of 800-m and 400-m wide roadside transects for sampli ng wetlands relative to a larger (40-km(2)) scale and to compare duck abund ance at the two widths. Densities of all basins combined and of seasonal ba sins considered alone were biased high for both transect widths, but mean b ias did not differ from zero for temporary or semipermanent basins. Biases did not occur when excavated seasonal and temporary basins (i.e., road ditc hes) were excluded from the sample. Mean basin density was higher for the i nner (400-m) transect width than for the outer transect width (area remaini ng of the 800-m transect, outside of center 400-m width) for all basins com bined and for seasonal and temporary basins. We detected an area-related bi as in the occurrence of basins in transects: smaller basins (0.08-1.6 ha) w ere over-represented in transect samples by 2.9-6.5%, and larger basins (gr eater than or equal to 11 ha) were under-represented in 800-m transects by 7.3% and in 400-m transects by 16.3%. We compared the distribution of ducks relative to water conditions in the inner and outer transect widths to eva luate whether they were affected by proximity to the road. Mallards (Anas p latyrhynchos L.), northern pintails (A. acuta L.), and gadwall (A. strepera L.) responded to water conditions equally in the inner and outer transect widths, but northern shovelers (A. clypeata L.) and blue-winged teal (A, di scors L.) responded more strongly to wetlands on the inner than the outer t ransect width, indicating that estimates of these species would be higher f rom a 400-m wide transect than from an 800-m wide transect. Differences in an adjustment index, used to account for the portion of basin obscured from view, were highly variable between inner and outer transect widths but did not indicate that use of wider transects was hampered by visibility. Biase s of transect sampling need to be carefully considered when extrapolating w etland basin or duck densities from transects to larger landscape scales.