Northern parula warblers (Setophaga americana) will be migrating through western Pennsylvania in the next few weeks.  They’re on their way to northern breeding grounds, but plenty of them nest south of Pennsylvania.  Why don’t they nest here, too?

My guess is that they used to.

Northern parulas are very versatile about climate.  Their range map shows they breed from the Gulf of Mexico to southern Canada but there’s a gap in the Rust Belt states, New Jersey, and New England that divides their northern and southern populations.

Breeding parulas are hard to find in western Pennsylvania because they’re picky about nesting material.  They look for a site near water with Old Man’s Beard (Usnea lichen) or Spanish moss where they hollow out a cup in the hanging mass and line it with soft fibers. (On rare occasions they choose other hanging material such as flood debris in trees.)

Shown below at left is old man’s beard lichen (Usnea species), at right is Spanish moss (Tillandsia usneoides).  Both are epiphytes that grow on other plants but they aren’t parasitic. They get their nutrition from the air.

Spanish moss is a southern plant whose northern limit is in coastal Virginia. Usnea grows in North American and European forests where the air is clean, but Old Man’s Beard has been missing from western Pennsylvania for more than a century, killed by our air pollution.  Though Pittsburgh’s air isn’t as bad as it used to be, it’s still too polluted for a plant that lives on air.  Without Old Man’s Beard, the northern parula passes us by.

So, yes, northern parulas probably used to nest here … and they might come back.  Pennsylvania’s forests have regrown since deforestation a century ago, and the air in the mountains is clean enough for lichens.  Breeding northern parulas have increased in the Allegheny and Appalachian mountains and on the high plateau.

When our air is clean enough for Old Man’s Beard we’ll have northern parulas, too.

(photo of northern parula by Steve Gosser. Photos of Old Man’s Beard lichen and Spanish moss from Wikimedia Commons. Click on these links to see the original Wikimedia photos)