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.

(photo by Cris Hamilton. Inspiration for this Tenth Page is from page 220(*) of Ornithology by Frank B. Gill.)