This past Sunday, which happened to be Easter, I went out to do a little birding near my house in Boulder, Colorado. I was hoping to cash in on the incredible wave of migrants that had swamped the city over the previous couple of days, generating sightings of rarities ranging from Yellow-throated Vireo and Yellow-throated Warbler to Hudsonian Godwit and Sedge Wren. After a fairly uneventful morning of recording Mallards, Downy Woodpeckers, and Black-capped Chickadees, I walked past a bridge over a small canal and suddenly heard what I was looking for — the unmistakable sound of a bird rare in the county — the song of an Eastern Phoebe.
Almost immediately, though, I could tell that something was wrong. The bird didn’t sound quite right — its phrases were clearly variable, not stereotyped like a spring phoebe song should be — and it was missing the really strong burr at the end of the “fee-BRRR” song phrase that is the hallmark (and probable namesake) of the Eastern Phoebe. I realized, with growing excitement, that it could well be a hybrid.
As I’ve reported earlier, hybrid Black × Eastern Phoebes are on the increase, and have shown up along the Front Range in Colorado before. There was also a hybrid Black × Say’s Phoebe in California a couple of years ago. Hybrid phoebes are among the most interesting of all birds, because they allow us a unique window into the genetic control of song.
While recording audio of its song, I got some decent binocular looks at the singing bird. It looked quite typical of Eastern Phoebes, with a dark brown head; lighter brown upperparts and wings with a slightly grayer tone; faint brownish wingbars; and underparts that were mostly pale, but slightly darker on the upper breast, fading gradually down to the belly, which had a slightly yellowish cream color. From the front the bird had the very vague “vested” appearance caused by the slight contrast between darker flanks and yellow central belly, but this is not unusual on pure Eastern Phoebes. Unfortunately, I can’t recall the precise color of the throat or the level of contrast between throat and face, which would have been a useful mark. Ultimately, if I had seen the bird and never heard it, I might never have suspected anything amiss.
I think there are two lessons to learn here: 1) hybrid phoebes are becoming more and more common in Colorado; and 2) not all of them may be visually identifiable, but the voice gives them away. All the more reason to start recording audio!