Why Are Cowbirds a Particular Problem for Kirkland's Warblers

Why are cowbirds a particular problem for Kirkland's warblers?

A) The cowbirds have only recently started parasitizing the warbler nests, so they haven't had time to adapt yet, and the warblers are rare to start with.
B) The cowbirds bring bison into warbler territory, and they tend to trample the warbler nests because they're on the ground.
C) Cowbird eggs look a lot like warbler eggs, so the warblers don't recognize they aren't their own.
D) Warblers and cowbirds prefer the same sort of habitat.
E) Kirkland's warblers tend to get stuck in traps designed for the cowbirds.