Dynamic use of wetlands by black ducks and mallards: Evidence against competitive exclusion
The decline of the American black duck ( Anas rubripes ) has been attributed to competition from mallards ( A. platyrhynchos ) that led to exclusive use of fertile wetlands by mallards. Data from annual breeding waterfowl surveys provide instantaneous, single observations of breeding pairs, which are used to estimate breeding population size and evaluate the condition of habitat. Data from these surveys have been used to document habitat use by black ducks and mallards. We used quiet-observation surveys from elevated platforms to study sympatric black ducks and mallards in northern Maine during the breeding season. Our objectives were to document occupancy of wetlands by breeding black ducks and mallards throughout the day during prenesting and early nesting periods to determine whether 1) wetlands were occupied by only a single species, 2) pairs of the same species occupied wetlands throughout the period, and 3) single observations of short duration adequately determine numbers and species using a wetland. We observed ducks at 5-minute intervals from elevated platforms on wetland margins to determine numbers and species of indicated pairs using each wetland over time. We visited 80% of the wetlands ≥2 times, with mean total time per wetland averaging 267 minutes. For each wetland we determined the most frequently observed grouping of black ducks and mallards from all combinations recorded during all intervals (e.g., 1 black duck [BO] pair during 9 intervals; 2 mallard [MA] pairs and 1 BO pair during 22 intervals; 0 pairs during 3 intervals). A single pair, a lone male, or no ducks were recorded during 34% of the 5-minute intervals. For wetlands with >2 hours of observations ( n =65 ), all but 2 were used by ≥2 different combinations of ducks. On most wetlands, the most frequent grouping was observed during <40% of the intervals. To simulate aerial surveys, we randomly selected 1 5-minute interval for each wetland. On average, the number of indicated pairs recorded during random 5-minute intervals was less than half of the total black duck pairs (2.0 vs. 4.4, P = 0.009 ), total mallard pairs (1.1 vs. 2.6, P= 0.0001), and pairs of both species combined (3.2 vs. 7.0, P= 0.0001) determined for each wetland based on total observations. On wetlands used by both species, random counts detected one or both species 49% of the time. Although 53 of the 65 wetlands observed ≥2 hours were used by both species, random visits detected both species on only 27 wetlands. Our data do not support assertions that the mallard has caused the decline of black ducks through interspecific competition for habitat, or that wetlands are occupied continuously by single pairs that aggressively exclude conspecifics. Our data indicated that single, short-duration visits with disturbance to wetlands are unreliable and inappropriate to document seasonal use of wetlands by breeding black ducks and mallards.