Note 45 — The Birds of Washington (continued 7)

“The present example of an isolated colony of a particular form, or what must be regarded as the same form in the absence of obvious distinctive characters, is one of several instances which are very troublesome to both the systematist and the student of geographic distribution. The birds of this species occurring, exclusively, in the area defined above are clearly intermediates between P. a. septentrionalis, a form larger and paler than P. a. atricapillus, which occupies the region immediately eastward, and P. a. occidentalis, a form smaller and darker than P. a. atricapillus, which inhabits the region immediately westward. It thus happens that, while these puzzling birds are practically, if not absolutely, indistinguishable from P. a. atricapillus they can hardly be considered exactly the same, since they are everywhere widely cut off from the latter by the very extensive area occupied by P. a. septentrionalis.”—Ridgway.