A field test of the distance sampling method using Golden-cheeked Warblers

Abstract

Criteria for delisting Golden‐cheeked Warblers (Dendroica chrysoparia) include protection of sufficient breeding habitat to ensure the continued existence of 1000 to 3000 singing males in each of eight recovery regions for ≥10 consecutive years. Hence, accurate abundance estimation is an integral component in the recovery of this species. I conducted a field test to determine if the distance sampling method provided unbiased abundance estimates for Golden‐cheeked Warblers and develop recommendations to improve the accuracy of estimates by minimizing the effects of violating this method's assumptions. To determine if observers could satisfy the assumptions that birds are detected at the point with certainty and at their initial locations, I compared point‐transect sampling estimates from 2‐, 3‐, 4‐, and 5‐min time intervals to actual abundance determined by intensive territory monitoring. Point‐transect abundance estimates were 15%, 29%, 43%, and 59% greater than actual abundance (N= 156) for the 2‐, 3‐, 4‐, and 5‐min intervals, respectively. Point‐transect sampling produced unbiased estimates of Golden‐cheeked Warbler abundance when counts were limited to 2 min (N= 154–207). Abundance estimates derived from point‐transect sampling were likely greater than actual abundance because observers did not satisfy the assumption that birds were detected at their initial locations due to the frequent movements and large territory sizes of male Golden‐cheeked Warblers. To minimize the effect of movement on abundance estimates, I recommend limiting counts of singing males to 2‐min per point. Counts for other species in similar habitats with similar behavior and movement patterns also should be limited to 2 min when unbiased estimates are important and conducting field tests of the point‐transect distance sampling method is not possible.

Amount and Type of Forest Cover and Edge Are Important Predictors of Golden-cheeked Warbler Density

“…The density model's performance at the intensively monitored sites also could reflect a positive bias. Field evaluations of the distance-sampling method have reported positive biases equal to or greater than those we observed and attributed them to location error (Alldredge et al 2008) and the species' behavior and movement patterns (Buckland 2006, Cimprich 2009, Peak 2011b. Alternatively, the model's performance at the intensively monitored sites may not be representative of its performance across the sampling frame because of sampling error; the two sites represent only 2% of the sampling frame.…”

Landscape and local effects on occupancy and densities of an endangered wood-warbler in an urbanizing landscape

“…Recent work has shown golden-cheeked warblers respond to differences within forest types and forest structure broadly considered to be goldencheeked warbler habitat. Golden-cheeked warbler density and productivity were greater in mixed juniper-oak forest than in juniper-dominated forest on Fort Hood, Texas (Peak andThompson 2013, 2014). Golden-cheeked warbler density on Balcones Canyonlands National Wildlife Refuge, Texas, peaked at a local *70:30 ratio of juniper to broadleaf canopy cover (Steve Sesnie, USFWS, unpublished data).…”

“…Obtaining unbiased and broadly applicable estimates of density and habitat associations of goldencheeked warblers, and songbirds in general, has been hampered by the difficulty in applying models to counts of breeding birds (Peak 2011;Hunt et al 2012;Warren et al 2013). Monitoring forest songbirds often requires counting primarily acoustic signals from individual territorial breeding males to estimate male density in an area (Brewster and Simons 2009;Hunt et al 2012).…”

“…However, evaluation of model performance is typically non-existent or based on simulated data because the underlying quantity being estimated (i.e., true abundance or density) is usually not known (Cassey and McArdle 1999;Royle 2004;Amundson et al 2014). Peak (2011) and Hunt et al (2012) demonstrated point-count surveys may be unreliable predictors of actual observed density of golden-cheeked warblers. Thus, any estimate of density should be interpreted with caution and careful attention should be paid to model validation.…”

Age-specific survival of male Golden-cheeked Warblers on the Fort Hood Military Reservation, Texas

“…All birds captured were uniquely marked using both U.S. Geological Survey aluminum bands and a distinctive combination of plastic, colored leg bands. Monitoring efforts occurred every five days for at least two hours in an effort to resight birds that were marked in previous years, calculate density estimates via spot mapping, and collect behavioral and reproductive data (Peak 2011b).…”