Identification of nest predators: a sampling perspective

Videotaping is currently recognized as the most reliable method of predator identification at active bird nests but it is relatively expensive and labour intensive. While the number of published studies has increased over the past 10 years, the mean sample size is not increasing. Thirty-one case studies (n>5 events) reported 6-70 (median=22) predation events by 2-14 (6) species of predators. The number of predator species increased with a 0.50±0.09 (SE) power of sample size across studies (0.54±0.06 with the present study included). This relationship was consistent across single- and multi-species studies and corresponded well with that found within the present 5-year study (176 events, 20 species) where neither the annual nor the pooled-sample species accumulation curve reached an obvious asymptote. The species accumulation curve was smooth and fell within the confidence limits of the rarefaction curve over the entire range of sample sizes, suggesting homogeneous sampling. In 31 case studies the dominant predator accounted for 21-96% (38%) of total predation and this proportion did not correlate with the sample size across studies. In this study the observed proportion fluctuated widely until the cumulative sample size reached about 50 records and stabilized thereafter at the final value of 37%. Because the regional pool of potential nest predators is usually high, a complete enumeration of the local predator community is difficult with an acceptable nest monitoring effort. Correct identification of the dominant predators is likely even with small samples, but quantification of their share is uncertain when based on <50 records. Researchers are encouraged to increase their sampling effort above the current level and to consider contingency of results upon sample size.