Reliability of Singing Bird Surveys: Effects of Song Phenology during the Breeding Season

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Abstract
Singing bird surveys are frequently used to estimate the difference in density between 2 or more populations of a species detected almost entirely by vocalizations. Typically, the statistical analysis assumes that individuals in the populations have the same average probability of being detected. How much difference in average detectabilities that might occur due to variation in singing rates during the breeding season was estimated. Presence or absence of song proved to be the best measure of detectability. Among 125 house wrens (Troglodytes adedon) in central Ohio [USA], the probability of singing at least once during a 3-min period was 0.7 before mating, 0.5-0.6 from mating to the completion of egg-laying, 0.7 during incubation and 0.5 or less thereafter. These probabilities were not detectably affected by season (1 June-15 July), time of day (first 5 h of daylight), or number of other nearby males (1-6). Random phenological differences, such as an earlier season in 1 yr as compared to another, might cause an error of up to 25% in the estimate of relative density. If several years of data are available, then the estimates of relative density will usually be more accurate. Substantial errors may still occur if changes in song phenology are density-dependent. Several methods have been designed to overcome the problem of unequal detectabilities, but most of them require the assumption that all birds close to the observer are detected; assumption is seriously in error for House Wrens. In most cases, singing bird surveys should not be relied upon to produce accurate estimates of relative density.