Winter Abundance Patterns of Some Songbirds near the 100th Meridian in the Southern United States

Abstract

-This study examines winter abundance patterns of three groups of song birds across the south-central United States-the phoebes (Sayornis), thrashers (Toxostoma and Oreoscoptes), and bluebirds (Sialia). The analysis was done on 15 years of National Audubon Society Christmas Bird Count data that were collected in an area encompassing most of Texas, Oklahoma, and part of New Mexico. There was no statistically significant east-west variation in group abundances, but a transition from to forms within each group occurred between the 98th and 102nd meridians, and the transition was abrupt. A significant biogeographic boundary appears to exist within two degrees on either side of the 100th meridian. Biogeography traditionally has involved the study of species' distributions over the surface of the earth. Data usually are not available to determine more than simple outline maps of species' ranges (Udvardy, 1969; Rotramel, 1973). A great wealth of avian biogeographic data has been accumulating since 1901 in the form of National Audubon Society Christmas Bird Counts. These data have been used infrequently until recently. Bock et al. (1977) used 10 years of data, which they grouped into five-degree latitude by fivedegree longitude blocks, to add the dimension of geographic variation in winter abundances for some North American bird They found that these abundance patterns showed a major biogeographic boundary in the Southern Great Plains near the 100th meridian. The purpose of our study was to use Christmas Count data on a finer spatial scale to examine changes in winter bird abundance patterns across Texas and Oklahoma, near the 100th meridian. The groups studied were the phoebes (Eastern Phoebe-Sayornis phoebe, Black Phoebe-S. nigricans, and Say's Phoebe-S. saya), the thrashers (Brown Thrasher-Toxostoma rufum, Curve-billed Thrasher-T. curvirostre, Crissal Thrasher-T. dorsale, and Sage Thrasher-Oreoscoptes montanus), and the bluebirds (Eastern Bluebird-Sialia sialis, Western Bluebird-S. mexicana, and Mountain Bluebird-S. currucoides). Each group consists of which are common in our study area, and each group is composed of and counterparts. For the purpose of our analysis we refer to the Eastern Phoebe, the Brown Thrasher, and the Eastern Bluebird as eastern species and all other as western species. The Long-billed Thrasher (T. longirostre), which occurs in our study area, was not included because the majority of its range is in Mexico (A.O.U. Checklist, 1957). We examined Christmas Count data collected in our study area over a 15year period for the above to test the hypotheses that 1) abundance patterns of the three groups show no significant east-west variation; 2) a E SOUTHWESTERN ATURALIST 26(2):95-100 MAY 2 , 1981 This content downloaded from 207.46.13.169 on Sat, 01 Oct 2016 06:07:34 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist