The brown-headed cowbird and its riparian-dependent hosts in New Mexico

Abstract

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 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.

Vascular Plant and Vertebrate Inventory of Gila Cliff Dwellings National Monument

“…Brown-headed cowbirds were consistent members of the monument's bird community, particularly in the riparian area, but in that area there were 19 species that were more abundant (Table 5.2). Nevertheless, there are two riparian species at the monument that may be particularly susceptible to brown-headed cowbird parasitism: yellow-breasted chat and yellow warbler (see review in Schweitzer et al 1998, Averill-Murray et al 1999). We made one observation of a recently fledged brownheaded cowbird young (Table 5. 4).…”

Avian Density and Nest Survival on the San Pedro River: Importance of Vegetation Type and Hydrologic Regime

“…Parasitism by brown-headed cowbirds is an important mechanism that reduces the productivity of host species in southwestern riparian systems (Schweitzer et al 1998). The ranges and densities of cowbirds have increased in southern Arizona since the early 1900s, concurrent with human settlement (Schweitzer et al 1998).…”

“…Parasitism by brown-headed cowbirds is an important mechanism that reduces the productivity of host species in southwestern riparian systems (Schweitzer et al 1998). The ranges and densities of cowbirds have increased in southern Arizona since the early 1900s, concurrent with human settlement (Schweitzer et al 1998). As such, parasitism by cowbirds represents a novel threat for which local breeding birds may not have developed adequate defense (Hahn and Hatfield 1994).…”

Status, ecology, and conservation of the southwestern willow flycatcher

“…In mid-April to early May, female cowbirds arrive on their breeding grounds (Fleischer et al 1987, Braden et al 1997, although in some years, females may arrive earlier (Darley 1983). Cowbirds begin laying eggs at the end of April and end in mid-July throughout much of the cowbirds' range (e.g., Payne 1976, Yokel 1987, Brown 1994, Braden et al 1997, Schweitzer et al 1998. In some areas, cowbirds may lay in late July (Whitfield, unpub.…”