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Estimating Nest Success: When Mayfield Wins

1990; Oxford University Press; Volume: 107; Issue: 3 Linguagem: Inglês

10.1093/auk/107.3.595

1938-4254

Douglas H. Johnson, Terry L. Shaffer,

Animal Ecology and Behavior Studies

-The Apparent estimator of nest success may be severely biased because unsuccessful nests are less likely to be found than are successful nests. The Mayfield estimator is a preferred alternative. The situation is somewhat different for nests in colonies or on islands because of greater visibility of nests, higher synchrony of nesting, and often higher hatch rates than dispersed mainland nests. Also, destruction is more likely to occur catastrophically, which violates an assumption of the Mayfield method that the mortality rate is constant. By simulation we investigated the performance of the Apparent and Mayfield estimators under a variety of circumstances. We found that when mortality rate was constant, the Mayfield estimator generally performed well regardless of whether or not nesting was synchronous or whether mortality was high or low. The Apparent estimator required more searches and higher detectability of nests. When mortality was mostly catastrophic, the Mayfield method performed poorly. The Apparent method was better, but high levels of detectability were needed for accurate estimates. We reached similar conclusions for attempts to estimate the number of nests initiated. Received 31 July 1989, accepted 5 January 1990. ORNITHOLOGISTS have come to realize that the traditional estimate of nest success (the fraction of observed nests that are successful) may be severely biased. This unwelcome result exists because unsuccessful nests may be active only briefly and are less likely to be observed than are successful nests, which persist for the entire laying and incubation period. Although Snow (1955), Hammond and Forward (1956), and others alluded to the bias, it was not formally dealt with until Mayfield (1961, 1975) proposed a solution. Johnson (1979) provided the statistical underpinnings of the method, which is similar to estimating an exponential survival function with censoring (e.g. Gross and Clark 1975). Problems that require a Mayfield treatment are somewhat different for nests on islands or in nesting colonies for four reasons, some of which Ely and Raveling (1984) identified. First, nest success is often much higher on islands than on mainlands. This reduces the bias of the Apparent hatch rate, and minimizes the need for the Mayfield method. Second, nesting on islands or in colonies is often fairly synchronous, which facilitates nest finding in early stages, before many of them are destroyed. Third, mortality of clutches in island or colonial nests is more often catastrophic than in mainland or isolated nests. The Mayfield method assumes that a constant mortality rate applies. Fourth, the small size of the nesting area, and high density and high visibility of nests, often permit many destroyed clutches to be found, which reduces the bias in the Apparent hatch rate. Additionally, nests destroyed before discovery are not used in the Mayfield calculation. In island or colonial situations these may be numerous, and biologists are tempted to include them to enhance sample sizes.