Schweitzer, Sara H.; Finch, Deborah M.; Leslie, Jr., David M. 1998. The brown-headed cowbird and its riparian-dependent hosts in New Mexico. General Technical Report RMRS-GTR-1. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 23 p. Numbers of brown-headed cowbirds (Molothrus ater) are increasing in some regions of North America, while certain populations of long-distance, neotropical migratory songbirds (NTMs) are declining. In the Southwestern

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... United States, several species of NTMs nest only in riparian habitats. The significant decline of two species of NTMs dependent upon riparian habitat, the southwestern willow flycatcher (Empidonax traillii extimus) and the least Bell's vireo (Vireo bellii pusillus), is of great concern. Brood parasitism by the brown-headed cowbird and loss of riparian habitat may be the primary causes of the decline of these populations. Extant data on the distribution, abundance, density, and rates of parasitism of the brown-headed cowbird in New Mexico have not been synthesized and interpreted. Our goal was to collect and review existing data on the brown-headed cowbird in New Mexico, compare them to data from adjacent western states, and interpret the findings. We hypothesized that increased human use of riparian habitats in New Mexico had resulted in increased abundance of brown-headed cowbirds and their parasitism on riparian-dependent NTMs. Our results suggest that quantitative studies should be conducted to determine the distribution, abundance, density, and rates of parasitism of brown-headed cowbirds in New Mexico's riparian habitats because existing data are inadequate. Results of such studies will allow conclusions to be made about the multiplicative effects of riparian habitat use and modification by human activities on cowbird and rare NTM populations. Cover photos (left-right): Brown-headed cowbirds, Middle Rio Grande (slide collection of Deborah Finch); blue grosbeak nest with 3 cowbird eggs (speckled) and 4 grosbeak eggs found at Rio Grande Nature Center, Albuquerque, NM (photo by Wang Yong); male blue grosbeak captured at mist-net station, Rio Grande Nature Center, NM (photo by Wang Yong). You may order additional copies of this publication by sending your mailing information in label form through one of the following media. Please send the publication title and number.