A few days ago, I found this passage in the second edition of Origin "We see this in the recent extension over part of the United States of one species of swallow having caused the decrease of another species."
I wasn't familiar with this story and I had no idea what species were involved. As a wonderful coincidence, I happened to discuss my blog with another Darwin-phile and she was able to uncover the species identity by referring to Darwin's expanded version of Origins.
Here's the same part in the expanded version:
“If one species of Swallow were to increase we might expect that other Swallows would suffer more than other birds; & so it seems to be, for with the late curious increase in parts of the United States of the Hirundo fulva, the Barn swallow has decreased.”
And the footnote associated with this is: Dr. Brewer in N. American Journal of Science. vol. 38 p. 392.
Unfortunately, I don't have access to this online but Hirundo fulva is the Cliff Swallow. So we have the species identified but I have not heard of competition between Cliff and Barn Swallows. As their name implies nesting habitat is mostly different. Anyone?
And I realize I don't have pictures of either species. Blugh.