Kentucky Warbler(Geothlypis formosa)

Kentucky Warbler (Geothlypis formosa)
San Pedro Terrace Rd and Coastal Trail, Pacifica, San Mateo County, CA
12 May 2020
Joseph Morlan

While birding the area adjacent to the Linda Mar Care Center, I heard an unfamiliar loud, insistent song comming from the alders and willows along San Pedro Creek. I investigated and eventually was able to get quite close to the song which continued every 20 to 30 seconds. I used the video mode on my camera (Canon PowerShot SX60 HS) to make audio recordings. At the time I assumed the bird was probably some common species giving a very unusual song. As I was leaving, I met Chris Hayward and Malia DeFelice of Half Moon Bay who had been birding at nearby San Pedro Valley Park. I mentioned the unusual song to them and offered to send them audio recordings.

I extracted the audio in mp3 format from the video clips using WinX HD Video Converter Deluxe software. I sent the recordings first Dan Singer who suggested the possibility of Kentucky Warbler. After listening to the song of Kentucky Warbler online at: https://www.allaboutbirds.org/guide/Kentucky_Warbler/sounds, I started to get more enthusiastic. It sounded virtually identical to what I had heard and recorded. Next I sent the recordings to Al Jaramillo asking if he thought it might be a Kentucky Warbler. He replied very quickly>.

Joe,
I would say yes. That does sound like a Kentucky. I created a spectrogram (attached) using one of your recordings (first song in the image), and a Kentucky song from Ohio. Everything is dead on - frequency, note shapes, spacing of syllables. It is as good as it gets other than seeing it!
Alvaro

Here is a copy of Alvaro's spectrogram comparison:

I also posted the three sound clips on Xeno-Canto: https://www.xeno-canto.org/set/6019?view=3

One respondent (Bobby Wilcox) noted that my recordings were an "almost perfect match" with this known Kentucky Warbler:

After these confirmations, I emailed Malia DeFelice who returned to the site with Chris Hayward and Kent Forward. They were able to get additional recordings which were posted on eBird:

https://ebird.org/checklist/S68958490?view=audio
https://ebird.org/checklist/S68971924?view=audio
https://ebird.org/checklist/S68956078?view=audio
https://ebird.org/checklist/S68957263?view=audio
https://ebird.org/checklist/S68935945?view=audio

My eBird report with three sound clips and a copy of Al Jaramillo's comparison sound spectrum are at: https://ebird.org/checklist/S68940561?view=audio

Although none of us were able to get visual confirmation, we all agreed that the song sounded right. The site is a steep embankment without any reasonable access inside the riparian corridor. I understand one birder waded the creek in an unsuccessful attempt after the singing had stopped. It is illegal to wade San Pedro Creek because it hosts endangered Steelhead Trout.

I received the following comment from Steve Rottenborn:

I saw your post about the Kentucky Warbler, and I listened to the recordings. It sure sounds like a Kentucky to me. My one word of caution is that I once heard what I was absolutely certain was a Kentucky Warbler along Gazos Creek Road, and when I finally got a look at it, it was a MacGillivray’s. In no way am I saying you had, or might have had, a Mac – I’m just letting you know.....

I also received comment from Jim Steel stating that he sometimes hears MacGillivray's Warblers sing a Kentucky-like song. However he noted that this sound-alike song is not exactly the same as my recording which he believes to be a real Kentucky Warbler. Here is my recording of an alternate MacGillivray's Warbler song which sounds somewhat like a Kentucky However I think the waveform is off, particularly the shape of the lower-pitched elements.

Mourning Warbler also has an alternate song that is similar to Kentucky Warbler, although again not quite an exact match. An example from New York State is at: https://macaulaylibrary.org/asset/120463 Again I think the wave form is off with the lower pitched elements not forming an inverted "V" and being off-time with the higher pitched elements.

And the following comment from Curtis Marantz:

Sounds about right, but maybe a little on the short side? There was a bird in Massachusetts that was identified by some as a Mourning x Common Yellowthroat hybrid that to me sounded like a Kentucky Warbler (I therefore thought it was more likely a Mourning x Kentucky Warbler hybrid). As such, I'd do my best to see it...

Here is a link to the song of this hybrid: https://macaulaylibrary.org/asset/113796 Although sounding much like a Kentucky, and having the distinct inverted "V"s for the lower pitched elements, the higher pitched elements are not as steep, nor do they reach the highest frequency of typical Kentucky.

I requested comment from Nathan Pieplow. He responded that the song was definitely Kentucky Warbler, but noted that song is learned in Parulidae. He cited a Black-throated Blue Warbler giving a Black-throated Green Warbler song and two cases involving Chestnut-sided Warbler. In one laboratory experiment, the warbler learned the song of Common Yellowthroat. In another a Chestnut-sided Warbler sang an Indigo Bunting song in the wild. These cases require "tutoring" by exposure to the song of the other species. Our bird would have to learn from an actual Kentucky Warbler either within its normal range or from a vagrant Kentucky Warbler in California. Neither scenario seems reasonably likely in my opinion.

I also received comment from Michael Retter and Tom Baxter, both of whom agreed it was a Kentucky Warbler. If deemed acceptable, this would be a first San Mateo County record.