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Publication Date

6-1-2003

Abstract

Since arriving in Texas in the latter half of the 19th Century, the Bronzed Cowbird (Molothrus aeneus) has expanded its range to include approximately two‑thirds of the state. Lubbock and the northern edge of the Edward’s Plateau currently demarcate the northern limits of Bronzed Cowbird breeding range in Texas. Area occupied by Bronzed Cowbirds within Texas has increased over time, but this increase has not been quite linear, indicating that rate of spread has varied over time. Post‑1956, spread was more rapid, occurring at a mean rate of 10.16 km/yr. Pre‑1956, mean rate of spread was 4.15 km/yr. More rapid spread post‑1956 was likely facilitated by human‑induced habitat changes. Potential for further range expansion exists. Observational and trapping data indicate increasing populations at the northern limits of their breeding range. In addition, fat indices of cowbirds trapped during spring migration at the northern edge of their range were relatively high (mean = 2.96, range = 1–5), suggesting that these birds potentially could have migrated further north. Continued range expansion is a management concern as the Bronzed Cowbird is a generalist brood parasite that may elevate risk of songbird nest failure due to increased risks of egg damage and parasitism.

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