When Louis Aggasiz Fuertes drew these birds they were all the same genus, Oporornis.  This made sense because Connecticut, mourning and MacGillivray’s warblers are similar in appearance and habits.  All three breed in northern forests where they are shy, secretive skulkers, nesting and feeding on the ground.

The Connecticut warbler (at top) is the hardest to find, so hard that his nest wasn’t discovered for 70 years after the species was described.  His breeding grounds in the bogs and moist forests of Minnesota, Wisconsin, upper Michigan and central Canada are protected by mosquitoes!   Birds of North America says, “Its secretive behavior and preference for breeding habitat in remote areas with abundant insect life has made it very difficult to study.”  No kidding!

The mourning warbler (middle) has a wider distribution.  He breeds in second growth forests from British Columbia to Newfoundland and into the northern tier of Pennsylvania.  He’s one of the few warblers that benefits from human disturbance, preferring to nest in clearcuts 1 to 10 years old.  I usually see him during spring migration at Magee Marsh, Ohio.

MacGillivray’s Warbler (bottom) prefers second growth too, but he breeds at low to moderate elevations in the Rockies and Sierras.  I saw my first MacGillivray’s warbler (Life Bird!) in Glacier National Park in burned areas that are the dry mountain equivalent of a clearcut.

For many years the Oporornis genus calmly hummed along until two discoveries upset the apple cart.

Everyone thought these species never met on their breeding grounds … and they don’t … except for one spot in the Peace region of British Columbia near Dawson Creek where in 2009 Irwin et al. discovered that mourning and MacGillivray’s warblers hybridize.

Then in 2010 DNA evidence split the Oporornis genus.  Now the Connecticut warbler stands alone, though many websites and field guides have not caught up.

• Old: Oporornis = [Connecticut, mourning, MacGillivray’s and Kentucky warblers].  Geothlypis = [common yellowthroat]
• New: Oporornis = [Connecticut].  Geothlypis = [common yellowthroat, mourning, MacGillivray’s and Kentucky warblers]

In appearance and ancestry, these birds are variations on a warbler theme.

(illustration by Louis Aggasiz Fuertes in National Geographic, public domain from Wikimedia Commons)