I’ve seen white-winged crossbills before, especially in the winter of 2009, but this year they’ve eluded me. People send news of them to PABIRDS but when I travel to their reported location they aren’t there. True to their irruptive nature crossbills are always on the move. Dang!
Last week I ran into Claire Staples while on my lunch break in Oakland. We exchanged bird sightings and Claire said she’d experienced the same problem finding crossbills until quite recently when she heard them near her home in Squirrel Hill.
The clue is their sound. Claire says they sound like typewriters, a useful tip as I actually do remember what typewriters sound like. Shows how old I am!
So now on my walks I’m trying hard not to look for crossbills as I don’t want to jinx my chances of seeing them. But I’m listening for the sound of typewriters.
Have you ever noticed how hard it is to find a bird when it’s making an alarm call? And how easy it is to find when singing?
It’s not just that birds hide when alarmed and sing out in the open. They change their tune to conceal or reveal. They know that the “physical structure of a sound affects the ease with which a listener — predator or neighbor — can locate its source.” (*)
Northern cardinals are a great example of this principle.
When they’re hiding in a thicket, their call is a thin, faint, high note. The alarm call’s narrow frequency range makes it really hard to pinpoint. Click here for an example.
By contrast, when they’re announcing their presence or guarding their territory the sound is rich and variable in a wide frequency range. This gives it a lot more “hooks” for our ears to grab onto. Here’s an example of their song and contact calls.
So when birds are warning each other of danger, there’s a reason why you can’t find their location. They’re hiding by voice.
Earlier this month I watched a flock of robins and starlings feast on the Bradford pears near Heinz Chapel. Birds usually don’t sing in the fall but this flock was muttering and whisper-singing. Three birds in particular caught my ear.
A robin sang softly.
A starling mimicked the robin.
A mockingbird mimicked the starling mimicking the robin.
By the time the robin’s song came out of the mockingbird’s mouth it was nearly unrecognizable. (Click here for the robin’s song.)
European starlings, on the left above, are considered mimics but they have wiry voices that distort whatever they say. Here’s a typical starling song. At the 00:32 mark he does a good imitation of a house sparrow. I couldn’t find an audio clip of a starling mimicking a robin.
Northern mockingbirds, on the right, are much better mimics than starlings. They can follow a robin’s tune and cadence but miss the melodious thrush harmony. They brazenly mask this deficiency: “I meant to sing the tune without the harmony.” Click here to hear a mockingbird mimicking many birds, including robins.
The mockingbird at Heinz Chapel clearly copied the starling’s wiry song including his poor imitation of the robin.