It is thought that this recent expansion will continue, with new recruits being added from their population strongholds in Arizona. Once a tremendous river system with luxuriant and extensive forest, the Sacramento River is now a prime example of the effects of long-term abuse that face nearly all riparian areas in the West. Willow Flycatcher, Warbling and Bell's Vireos have become extirpated from the valley. Among the causes are loss of habitat through farmland conversion, stream channelization, and the building upstream of Shasta Dam. With the expansion of agriculture, cowbirds have expanded their range into California, entering the state along the lower Colorado River Valley in the 1870's and reaching the northern Sacramento Valley by 1950.
Cowbirds feed in open areas on the grain and waste of farms.
With the reduction of large patches of riparian forest to narrow strips, songbirds that formerly nested with impunity have not been able to escape the heavy nest parasitism of cowbirds. This is the final blow to many riparian species. Endangered Birds in Riparian Ecosystems. Ohmart, R.D. 1994. Changes in Riparian Avifauna.