Point transect sampling for monitoring passerine birds in Denali National Park and Preserve: An assessment of 2002-2008 pilot data
Dates
Year
2002
Citation
Hoekman, Steven T., and Lindberg, Mark S., 2002, Point transect sampling for monitoring passerine birds in Denali National Park and Preserve: An assessment of 2002-2008 pilot data: National Park Service: Fort Collins, Colorado, v. NPS/CAKN/NRTR—2012/589, 54 plus pages-54 plus pages.
Summary
Passerine birds (passerines) in Denali National Park and Preserve (Denali) have been selected for long-term monitoring as part of the National Park Service's (NPS) Inventory and Monitoring Program. Monitoring goals include relating bird distributions to habitat characteristics and detecting severe population declines over 20 years. Point transect surveys were implemented to monitor breeding passerines because distance sampling methods adjust counts for probability of detection, require only 1 visit to each sampling point during a breeding season, and allow monitoring of multiple species because males are easily identified by their songs. Our objectives were to analyze pilot data to 1) assess adherence to assumptions of distance sampling, [...]
Summary
Passerine birds (passerines) in Denali National Park and Preserve (Denali) have been selected for long-term monitoring as part of the National Park Service's (NPS) Inventory and Monitoring Program. Monitoring goals include relating bird distributions to habitat characteristics and detecting severe population declines over 20 years. Point transect surveys were implemented to monitor breeding passerines because distance sampling methods adjust counts for probability of detection, require only 1 visit to each sampling point during a breeding season, and allow monitoring of multiple species because males are easily identified by their songs. Our objectives were to analyze pilot data to 1) assess adherence to assumptions of distance sampling, 2) examine factors influencing detection probabilities, 3) estimate density across years and habitats, and 4) examine statistical power to detect future population declines. Sampling was conducted during June 2002-2008 at a systematic grid of locations in Denali north of the Alaska Range. At each grid location, point transects were centered on 25 sampling points separated by 500 m. Using standard point transect methods, observers enumerated and estimated distance to all passerines detected during a 10 minute interval. During 2002-2008, 35 separate grid locations were sampled, with some sampled in up to 5 years. Twenty three observers surveyed 1,331 point transects, with 12,623 detections of 80 species. Of 14 species with sufficient samples for analyses, we found that all 5 species of thrushes as well as Arctic Warblers, Fox Sparrows, and Dark-eyed Juncos (see Table 4 for scientific names) showed a relative surplus of detections at intermediate distances (~40-70 m). This pattern likely arose from bird evasion of observers and/or error in distance estimation. Resulting violations of critical assumptions of distance sampling were uncorrectable; hence, these species were unsuitable for distance sampling analyses. Estimated detection functions for 6 remaining species adequately met assumptions when data were pooled across years. However, we found large variation in detection functions relative to year, habitat type, and wind speed. Resulting functions often severely violated assumptions of methods, were biologically implausible, and were inconsistent across species and with our predictions. We also found consistent differences in detection functions relative to individual observers. Observers often had surpluses or deficits of observations at specific distances, most likely as a result of error and bias in distance estimates to birds detected only by auditory cues. We concluded estimates of detection probability and hence density were likely subject to large bias and variability. Furthermore, we failed to identify satisfactory remedies and felt density estimates from these data would be uninformative. Even if we ignored potential bias in year-specific estimates, power analyses suggested current methods and levels of sampling effort would be unlikely to meet the monitoring goal of >80% power to detect a 50% population decline over 20 years for multiple species. We concluded that characteristics of Denali made distance sampling methods inappropriate for monitoring populations of passerines and that impediments would be difficult or impossible to overcome. Instead, we recommended survey methods allowing estimation of probability of detection that do not rely on estimation of distance to birds and that will be relatively insensitive to evasive movements by birds.
