CONTINUED RANGE EXPANSION BY BRONZED COWBIRDS IN THE SOUTHWESTERN UNITED STATES

Abstract The brood-parasitic bronzed cowbird (Molothrus aeneus,) continues to expand its range in New Mexico and Texas. The current breeding range is 280 km outside of the historic breeding range. In Texas, spread rate increased from 6.22 km per year prior to 1956 to 13.75 km per year after 1956. Spread rate in New Mexico was constant at 12.72 km per year. Flight-range estimates based on fat indices of spring migrant cowbirds trapped at their northern range boundary in Texas suggest the potential for migration 341 km further north. We discuss factors that might limit or promote further range expansion.