Searching for the Missing Notes

An interesting question was presented to me when I gave a bird song program for the Columbus Audubon Society last month. One of the members said that she had been hearing a Chickadee that alternated between Carolina and Black-capped Chickadee songs, and she’d heard this bird for the past couple of years as well.

Columbus is south of the contact zone between Black-capped and Carolina Chickadees.  I’ve recorded some intriguing songs right along the contact zone, but have only heard Carolina songs when I’ve gone to the Columbus area.  

I am no expert in Carolina Chickadees.  I live in Cleveland and I don’t get to go as far south as Columbus very often, especially when spring semester is still in session.   

Still, switching between the songs of two species seemed unlikely.  This woman knows her birds, so what could she have been hearing?

In the course of my Columbus Audubon program, I shared a little experiment I did with the songs of the Black-cappeds and the Carolinas.  I added high notes from Black-capped Chickadee calls to their two-note song, creating a reasonably good likeness of the Carolina Chickadee song. 

Here's the Black-capped song followed by Black-capped calls.  See those high pitches at about 7000 Hz?  When I took those high notes and added them to the Black-capped song, I came up with...my quasi-Carolina.

Next, I subtracted the high notes of the Carolina Chickadee’s four-note song to create a passable Black-capped. It has just the two pitches a step apart, and there's a tiny space between them where one of the upper notes had been.

When a Carolina Chickadee is not very close or there's also noise in the environment, sometimes that's exactly how they sound.  

I’ve occasionally heard people tell me that they can’t hear the difference between the two species, and in some cases I think they may not be hearing the two higher frequency pitches of the Carolina’s songs.  Those high pitches can be in the range of Cedar Waxwings, Brown Creepers, and Blue-gray Gnatcatchers, which are birds that some people find difficult (or impossible) to hear.

Here's an example of those three birds followed by the Carolina Chickadee.  

This is where all the high notes are located: right between 6000 and 8000Hz. You can listen to these pitches while you look at the sonogram.

Also, people who are just learning bird songs sometimes won’t connect the higher sounds in a song to the lower pitches that are louder and more clear to our ears.  For example, it may seem hard to believe that the first few notes of a Ruby-crowned Kinglet’s song are part of the same song that includes the rolling, bubbling jumble of pitches in its lower register.  You can see those high pitches way above the rest of the song in this sonogram.

What if people can hear those pitches if the bird are closer or the environment relatively quiet, but those pitches are lost at a greater distance or in the context of human noise?  

I noticed that when I was in Columbus Metro Parks that were near freeways or busy urban/suburban main roads, I couldn’t always hear those high notes.  Add in a train, an ambulance, and some irate Blue Jays, and those top pitches were increasingly difficult to pick out of the sound texture.  When I couldn’t hear those top notes, there was a tiny gap between the two lower pitches that sounded just like the modified Carolina Chickadee recording in my bird song program. 

Here are a couple of examples from my trips down to the Columbus area during spring break earlier this month. Some birds seem to really cut through the sound texture with their top notes, such as the one you listened to at the beginning of this post (and recorded at Highbanks Metropark).

But now listen to this interaction at the Hoover Nature Preserve on the Hoover Reservoir just north of Columbus.  I stopped at the Mud Hen Marsh parking area, and a heated Chickadee confrontation was in progress.  There was also a lot of traffic, and this is what I heard.

When I was able to get really close to those two Chickadees, I heard the high notes of their songs.  I've edited the traffic noise so that you can hear them, too.

There’s the upper part of the song – and each bird is singing his four-note challenge on top of the other’s proclamation.  Yet I could not hear those pitches when I first got out of my car.

I still have some questions, though. Why can I hear the top notes when some Carolina Chickadees sing at a distance, but not others?  The sonograms always reveal the top pitches, whether I can hear them at the time or not.  It seems that sometimes those upper pitches are higher than others, which would be just enough to make them a lot harder to hear, and there’s always the varying frequency and intensity of human noise.

Central and Southern Ohio, I’d like to hear from you!  What have you observed, and what do you think is going on?

Many thanks to Paula Ziebarth, who raised the question, and Darlene Sillick, who invited me to speak to Columbus Audubon.