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Counting Warblers and Vireos

Friday, April 20, 2012 10:34 AM | Anonymous member

How many golden-cheeked warblers and black-capped vireos are on Balcones Canyonlands NWR? That's a tough question. Because singing territorial males of both species are relatively easy to find, our estimates are typically based on the number of territorial males. We know that researchers identified more than 90 black-capped vireo territories each of the last two years and some were certainly missed, so I think a conservative estimate for black-capped vireos is 100 territorial males. As for golden-cheeked warblers, our best estimate comes from former refuge biologist Chuck Sexton who estimated about 800 territorial males in 2010. 

     Recent scholarly papers have shown that population estimates of golden-cheeked warblers using the best methods are usually not very good. However, we think we can do a much better job estimating how much of the refuge is occupied by these species. Thus, I have been working with Scott Rowin, Roland Davis, Emily Haeuser, David Morgan, and Susanne Shipper to determine presence, or occupancy, of these species on the refuge. Today we are completing our second survey of each of 250 points located randomly across the refuge. We will survey each point two more times. These surveys will provide a robust estimate of occupancy across the refuge and allow us to see how things like canopy cover, canopy height, and other habitat features are related to the probability of occurrence. And, as the refuge continues to grow habitat for these species, we will have reliable estimates of how our management has affected the proportion of the refuge that is occupied by territorial males. We think we are on the right track and look forward to seeing if the birds agree.

 


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