Daily movement of the brown-headed cowbird in south-central Texas during winter

Abstract

The daily movements of brown-headed cowbirds on wintering grounds and ecological factors affecting these movements were examined. Study methods included mark-recapture and direct observation of behavioral patterns. Brown-headed cowbirds that winter in Texas breed throughout the Mississippi River watershed. Southward migration begins in August, with flocks from northern-most breeding regions appearing to stop at preliminary wintering grounds in Tennessee, Arkansas and Oklahoma. Central and southern breeding cowbirds tend to migrate to the gulf coast states by October. When severe weather strikes the preliminary wintering grounds in November and December, those cowbirds concentrated in that region migrate further south to the gulf coastal states and join birds already present. Cowbirds appear to migrate in flocks segregated with respect to age and sex, but such segregation breaks down on the wintering grounds. Upon arrival in the study area, cowbirds either establish a roost or utilize an established roost with earlier migrating and/or resident cowbirds. Only one roost was used at a time within the study area. Cowbirds from two or more roosts within 50 miles of each other did not share the same feeding sites. Cowbirds did not change roosts, except as a populational unit, during the study. Changing of roosting sites within the study area (a Type I movement) occurred once each year and shifts of cowbird populations out of or into the study area (abandonment of feeding sites previously used) occurred four times during the study. These shifts were termed Type II movements. ...