Journal Article

Survey Effort Effects on Power to Detect Trends in Raptor Migration Counts

Lewis, Sandra A., Gould, William R.

2000

Overview

Tremendous effort is expended in counting migrant raptors in North America, where over 1,500 count sites exist. The Biological Resources Division of the United States Geological Survey is investigating whether migration counts can serve to monitor population trends. To that end, we investigated the effects of changes in survey duration on power to detect trends using various sample sizes, lengths of surveys, and sampling frequencies within a season based on raptor migration count data collected from 7 sites. As expected, power to detect trends increased as sample size (number of years of counts) increased; its magnitude depended on level of trend and variation in annual counts. Except for cases with extremely low or high power, 5 additional survey years increased power 20%-50%. Changing survey length affected power, but results varied and depended on the migration pattern of species at specific sites. Power to detect trends did not change appreciably when counts were conducted only during 90 days of peak migration (for all species combined). However, when annual counts were based on 30 or 60 consecutive days of peak migration, we found decreases in power for most site-species combinations. Changing the number of days counted during the week resulted in variable changes in power. Our results suggest the number of count days within weeks necessary for sufficient monitoring should be evaluated on a site- and species-specific basis. We determined that a coefficient of variation of 30% or less in annual counts is needed to detect a 3% average annual decrease in counts over 25 years with at least 80% power. This requirement was met at one or more sites for 14 of 20 raptor species. Power to detect increasing trends exceeded power to detect decreasing trends, which is unfortunate from a conservation perspective. Nevertheless, if count data are collected consistently, large-scale population trends for some raptor species may be monitored effectively using migration counts.

• Publisher: The Wildlife Society,Blackwell Publishing Ltd
• Subject: Animal migration behavior / Animal populations / Aviculture / Bird migration / Birds of prey / Breeding / Datasets / Falconiformes / Geological surveys / Hawks / Hemic system / Population trends / Sample size / Seasonal migration / Trends / Wildlife Inventory and Techniques