Two possibilities have been put forward. The first, Orange-crowned Warbler, seems unlikely. I've heard literally hundreds of OCWA in the past month or two, the parks I bird in and run in are filled with them. But all of them sound more or less the same, and furthermore none has ever (to my memory) graced my property. The likelihood that the first one to show up at my house would be one with a strange variant song seems improbable.
Which leaves the most common suggestion, Bewick's Wren. We know BEWR have variable songs, and furthermore they live year round and breed in our front or back yards. In 30+ years here I've never heard this particular song, but I guess we'll have to say this was the first time. Other birds that breed here include Chickadees and (probably) Titmice, both of which (especially the latter, which also has a variety of songs) could possibly (but not probably) be the singers, but Bewick's Wren certainly seems by far the most probable.
And, until and unless I hear the song again and actually see the singer, that's where it will remain.
From: Steven Patt <stevenpatt...>
Sent: Tuesday, April 18, 2017 8:58:38 AM
To: South Bay Birds
Subject: Unknown backyard bird song