I'm having a hard time visualizing this. Here is a link to specimens showing all races of Dark-eyed Junco and I think you can see that the outer tail feathers are mostly, if not all white. The central tail feathers are dark, but there is also dark on the inner webs of some of the tail feathers, more as you progress inward. These dark margins of the inner webs may be more apparent when the birds fan their tails as they try to distract you away from their nests. They do that around here a lot. In fact I just found a nest last week with eggs which must be a second brood as we've had lots of juveniles around.
But the Blue-gray Gnatcatcher has the same pattern with dark margins on the inner webs of the white outer rectrices.
My guess is that you are seeing a somewhat different pattern because the juncos may fan their tails more flamboyantly than a gnatcatcher. But I don't think there is a real difference in the tail patterns between the two species.
Anyway, a gnatcatcher and a junco are completely different kinds of birds. The junco with its stubby pink bill and ground-foraging and bulkier appearance is night-and-day different from a the slim, acrobatic gnatcatcher with its tiny bill and bold white eyering. Even if there were a difference in their tail patterns, I would think that there are much easier ways to tell the two appart than such an ephemeral difference, a difference if it exists which depends on the amount the tail is fanned in flight.
Joseph Morlan
Fall Birding Classes in San Francisco start September 9
