Point counts of Michigan forest birds: improving sampling efficiency for species of concern

TitlePoint counts of Michigan forest birds: improving sampling efficiency for species of concern
Publication TypeThesis
Year of Publication1996
AuthorsHall KR
Academic DepartmentSchool of Natural Resources and Environment
DegreeMaster of Science
Number of Pages75 pp.
UniversityUniversity of Michigan
CityAnn Arbor, MI

For many years, researchers have suggested that populations of many species of neotropical migrant birds in the United States are declining. Point counts are the method most commonly used in monitoring plans for estimating the relative abundance of birds; they consist of an observer collecting data at a series of locations (or "points) for a specified period of time. Monitoring populations using point counts plays a critical role in the conservation of bird species, as these data may signal species declines. I examine methods for improving the efficiency of point count sampling in wetland and upland forests in northern lower Michigan. First, using data for two groups of species, neotropical migrants and short-distance migrants and residents, I examine the effects of (1) changing the duration of point counts and (2) changing the area covered by each count (count radius) on the number of individuals detected in wetland and upland ecosystems. Next, I compare different combinations of methods to see which produce the highest expected cumulative count per hour (a measure of sampling efficiency). For neotropical migrants in wetland sites, I found that an 8-12 minute, 100m radius point counts would be most efficient. This radius size would also be most effective for sampling the residents and short-distance migrants using long count durations (8-12 minutes), although similar efficiency levels could be attained using shorter duration (5-6 minute) 50 m radius counts. In the uplands, the neotropical migrants had the highest expected cumulative counts with 12-minute, unlimited radius samples; residents could be sampled most efficiently using either the unlimited radius or 100 m radius size, and long counts (10-12 minutes). These results suggest that what is most efficient varies both by forest type and by migratory gorup in northern Michigan forests. Next, I address species-specific effects of using different point count radii (with 10 minute count lengths) on sampling efficiency in the same ecosystems, focusing on species of conservation concern. I also examine how the mean detection rate influences the number of samples needed to detect a 40% decline in the relative abundance of a species, using different levels of statistical power. In the wetlands, two neotropical migrant species and most of the short-distance migrants and residents would all be most efficiently sampled using the unlimited radius count, while over half of the neotropical migrants, including four of conservation concern (Veery, and Nashville, Canada, and Blackburnian Warblers) would be most efficiently sampled using 100 m radius counts. In the uplands, most of the neotropical migrants, including two of conservation concern (Eastern Wood-Pewee, and Veery) would be most efficiently sampled using an unlimited radius count. The number of sampling days required to detect a 40% decline using a one-sided test (alpha=0.1, beta=0.2) varied widely for the species of concern. Using the count radii found to be most efficient for each species, detecting this large difference in relative abundance would rquire approximately 8 days of data collection for the Eastern Wood Pewee and Nashville Warbler (in upland and weltand sites, respectively), 17 and 52 days, respectively for the Canada Warbler and Blackburnian Warblers in the wetlands, and 25 days for the Veery in uplands. The last species of concern, the Great Crested Flycatcher, was detected at very low rates in both systems, and as a result, detecting declines would be very difficult.