The Alder Flycatcher and the
Willow Flycatcher are "sibling species." Sibling species look alike, but are separate
in other ways, which almost always includes ecology and vocalizations. These two species,
collectively known as "Traill's Flycatcher," were not recognized as distinct species until the mid-
twentieth century, as I detailed on 7/25. Notice that this Alder sings with no variety, because he
has only one song-type, while the Willow uses three song-types. Sibling species are usually also "sister species,"
meaning closest relatives, but that doesn't mean they have split recently. It just means that
their lineage is morphologically conservative. The entire genus Empidonax is morphologically conservative,
as is the entire genus Phylloscopus (leaf warblers), as is the entire genus Poecile (chickadees).
The ancestral Traill's Flycatcher split into Alder and Willow around two million years ago.
Into the weeds: Geographic variants of a single lineage don't qualify as sibling species. They are sometimes described as
subspecies. Two subspecies just haven't had time to become different enough to be separate species, not even sibling
species. Remember, sibling species are unequivocally separate species, they just look alike, to us. In Empidonax,
subspecies of the same species almost always sound alike. The Southwestern Willow Flycatcher shows a little
differentiation, and two subspecies of the Western Flycatcher were different enough vocally that they were recognized as species.
Alder and Willow are literarlly unequivocal species, and there you have it.