What swallow was Darwin talking about?

"As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera. 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." Charles Darwin, Origin of Species, 2nd edition

Not sure what species this refers to (yes, I'm ending the sentence this way). This could be Barn Swallows moving with human settlement. Could it be referring to Tree Swallows, moving with humans as we put up bird houses. I'm just not sure and I'd like to hear some opinions. Darwin was in a race to get Origins out and left our footnotes and references.