I've long speculated that the reason that black-caps are so rare on the
islands here is that they have arrived from back east relatively recently
and have not yet succeeded in colonizing the islands. The fact that all
black-caps from coast to coast are considered a single sub-species (I
believe they are monotypic) supports such a contention. They have also not
yet colonized as far south in California as have the chestnut-backeds. Also
they are adapted to more frequently disturbed habitats, e.g., riparian
woodland.

Gene Hunn
enhunn at attbi.com.