East vs West or North vs South

Juncos are the first birds I see on these cold mornings. The dark heads and chests of this slate-colored form of Dark-eyed Junco are black spots against the white of this new snow.

Dark-eyed Junco, Junco hyemalis

Dark-bodied, dark-eyed Junco against new snow

Slate-colored form of the dark-eyed Junco against new snow

Just a few days ago, I was taking photos of the western subspecies of this fellow, in sunny California, the Oregon Junco.  What a difference a few days (and a few miles) makes.

Oregon Junco, western race of the Dark-eyed Junco

Oregon Junco, western race of the Dark-eyed Junco

oregon junco

Same dark eye, same pinkish bill, but the Oregon Junco has browner sides and back in place of the slate gray color of the our eastern and midwestern race.  The gray in the Oregon Junco is confined to a hood over its head, rather than an over-all background color.  That is, the two forms of Junco are quite morphologically distinct.

As noted before in an earlier post, Dark-eyed Juncos (Junco hyemalis) are actually a diverse collection of six subspecies.  What I didn’t know when I wrote that post is that Juncos are a very rapidly evolving group, with each of the subspecies occupying a slightly different geographic range and migratory pattern so there is little contact between them — the perfect conditions for evolution of new species.

And…this has all happened in just the last 10,000 years since the last glacial retreat, as Yellow-eyed Juncos moved northward from Mexico to invade new territory in northern North America.  The subsequent local evolutionary changes due to mutations and geographic separation have resulted in a distribution of subspecies that looks like the map below.  This diversification is one of the fastest rates recorded for vertebrates. Evolution in action!

JU stands for Junco in the map below.  The different types are SC (slate-colored), WW (white-winged), OR (Oregon), PS (pink-sided), GH (gray-headed), and RB (red-backed).  YEJU is the ancestor of all of these — the Yellow-eyed Junco of Mexico and Central America.

Geographic distribution of Junco subspecies

Geographic distribution of Junco subspecies from Mila et al. 2007.  Proc Royal Soc Britain 274: 2653-2660.

7 thoughts on “East vs West or North vs South

  1. Interesting article about juncos. We have the slate-colored juncos here and they are also among the first birds to the feeders each morning. They have a distinctive hierarchy and it is fun to watch the interaction between members of their flock.

    • They are fun to watch, but have you seen how some of the birds will dominate others as if to force them away from good feeding spots? They might seem highly social, but there is some fierce competition going on between members of the flock, too.

  2. The Slate-Colored Dark-Eyed Junco was one of the first “new” birds I learned when I moved from downtown Minneapolis (no backyard) to St. Paul (backyard). So does that mean that these birds have broadened their range considerably since 2007? I mean, if they are in your backyard and they are in my backyard, and plentiful at that, they must be forming permanent footholds farther south. But the above map shows their range as stopping roughly at the Canadian border.

    • Sorry, I forgot to mention that the map represents breeding ranges. The SCJ breeds north of us, but winters here and points south, presumably where there is less snow and easier to find food.

  3. Pingback: Mystery Squirrel | Back Yard Biology

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