BLACKBIRD AND STARLING ROOSTING DYNAMICS: MPLICATIONS FOR ANIMAL DAMAGE CONTROL

Each winter an estimated 350 million starlings, red-winged blackbirds (Agelaius phoeniceus), common grackles (Quiscalus quiscula), and brown-headed cowbirds (Molothrus ater) congregate in roosts in the southeastern United States (Meanley 1971, Meanley and Royall 1976). These birds have been of increasing concern because of agricultural damage claims (Stickley et al. 1976, Dolbeer et al. 1978), reputed health hazards (Monroe and Cronholm 1977), and other nuisance problems associated with them. Historical population trends (Dolbeer and Stehn 1979) and the source of winter-roosting blackbirds (Meanley 1971, Meanley and Dolbeer 1978, and Dolbeer 1978) have been summarized, but little information on the number of consecutive nights a bird returns to the same roost (roost fidelity) or the dynamics of a winter roost is available. The purpose of this paper is to present information on roost fidelity and population dynamics needed to better understand and manage winter blackbird and starling roosts.

NATIONWIDE ESTIMATES OF BLACKBIRDS AND STARLINGS

An estimated 538 million blackbirds and Starlings are found in the United States, based on the national cooperative blackbird/Starling winter roost survey conducted by the U.S. Fish and Wildlife Service during the 1974-75 winter period of December 20-February 15. Ap- proximately 74% or 398 million of these blackbirds and Starlings occurred in the Eastern States, including the tier from Minnesota to Louisiana; 26% or 139 million birds were in the West. The national roosting population in 1974-75 was composed of 11 species (Table 1) in the following approximate proportions: 38% Red-winged Blackbirds; 22% Common Grackles; 20% Starlings; 18% Brown-headed Cowbirds; 2% Brewer’s Blackbirds; and less than 1% six species combined (Rusty Blackbirds, Boat-tailed Grackles, Great-tailed Grackles, Tri-colored Black- birds, Yellow-headed Blackbirds, and Bronzed Cowbirds). (Some 2 million robins also were reported in the 1974-75 survey, though not solicited and therefore not tabulated, from 20 of the blackbird roosts in the Southeast.) The 1974-75 species proportions are similar to those found in the last nationwide winter survey (1969-70). In the 1963-64 national winter survey, Redwings made up 33% and Common Grackles 31% of the total population.

EXPERIMENTAL TREE TRIMMING TO CONTROL AN URBAN WINTER BLACKBIRD ROOST

Every fall millions of blackbirds come down the Mississippi Flyway to return to their winter roosts in Arkansas, Louisiana, and East Texas. When these roosts are located in urban areas, public pressure makes the more common chemical means of control impractical. A less destructive and more permanent method of control was sought. At Rice University, in Houston, Texas, there has been a blackbird roost of various sizes and durations since 1956. For the past two years we have had the opportunity both to study roosting blackbird biology and experiment with habitat alteration as a control method. This particular report concentrates on the results and interpretation of the tree- trimming program initiated in August 1974. The birds involved are primarily Brown-headed Cowbirds (Molothrus ater), along with Starlings (sturnus vulgaris), Common and Great-tailed Grackles (Quiscalus quiscula and Cassidix mexicanus), Red-winged Blackbirds (Agelaius phoenicus) and Robins (Turdus migratorius). The campus comprises 121 ha and was planted with live oaks (Quercus virginiana) in 1912. These trees retain their foliage throughout the winter and now form a closed canopy over some 5-6 ha. In the 60s and early 70s most of the birds that came to Houston for the winter roosted in a 64-ha woodlot 10 km north of campus. In January 1970, the U.S. Fish and Wildlife Roosting Survey reported one million birds at this site we call the North Loop. Fifteen- thousand birds were estimated at Rice.