Olive-sided Flycatcher Contopus borealis
     Northern
     Pacific
I've distinguished these two populations based on differences in song between coastal northern California and Taiga breeders, but this geographic variation does not quite match with the currently named subspecies. Southern California breeders (C. b. marjorinus) average slightly larger with darker underparts than the widespread subspecies (C. b. borealis/cooperi??). Obviously more study is needed to reconcile the named subspecies with variation in song.

Willow Flycatcher Empidonax traillii
     Eastern   E. t. traillii group
     Southwestern  E. t. brewsteri group
Variation in this species is complex and extremely subtle, with many published revisions by different researchers over the last 50 years. No differences in voice are known, the differences in plumage are clinal and variable.

Pacific Slope Flycatcher Empidonax difficilis
     Mainland  E. d. difficilis
     Channel Islands   E. d. insulicola

Brown-crested Flycatcher Myiarchus tyrannulus
     Arizona   M. t. magister
     Texas   M. t. cooperi

Tropical Kingbird Tyrannus melancholicus
     Western   T. m. occidentalis
     Eastern  T. m. satrapa
These two populations differ very slightly in appearance according to Pyle: Eastern birds have a slightly deeper tail fork on average, and slightly richer colors overall. Possible differences in voice that are described in the guide need more study.

Fork-tailed Flycatcher Tyrannus savana
     South American  T. s. savana group
     Central American  T. s. monachus

Eurasian Skylark Alauda arvensis
     European A. a. arvensis
     Asian  A. a. pekinensis
The introduced population in British Columbia originated in the British Isles (A. a. arvensis), but it is quite possible that plumage features have changed during the subsequent xx years in the very humid environment of Vancouver Island. Vagrants in Alaska (and once in California) are the Asian subspecies A. a. pekinensis.

Horned Lark Eremophila alpestris
     West Arctic  E. a. arcticola
     Pacific Northwest  E. a. strigata
     Interior West  E. a. occidentalis
     Central California   E. a. sierrae
     Southeast Arizona   E. a. adusta
     South Texas   E. a. giraudi
     Northeast   E. a. alpestris
These subspecies are examples of the range of variation found across North America, where 21 subspecies are listed in Pyle 1997. There are trends in the variation as described in the book, but not clear groups of subspecies.

Purple Martin Progne subis
     Eastern   P. s. subis
     Western  P. s. hesperia group
These two populations differ in voice, size, plumage, migration timing, and do not overlap in range.

Cliff Swallow Hirundo pyrrhonota
     Northern  H. p. pyrrhonota
     Southwestern  H. p. swainsoni
     These two species differ in size and plumage color. Two other subspecies in the southwest are intermediate to some extent, and all subspecies blend at edges of their range.

Cave Swallow Hirundo fulva
     Mexican   H. f. pelodoma
     Caribbean  H. f. fulva
These two populations do not overlap in breeding range. They differ in size and slightly in overall plumage color. Possible differences in voice need more study.

Barn Swallow Hirundo rustica
     American   H. r. erythrophthalmus group
     Eurasian   H. r. rustica group
Eurasian subspecies rustica and gutturalis have both been identified in Alaska. American population is difficult to distinguish, as the underparts color varies from whitish to rusty.

Gray Jay Perisoreus canadensis
     Taiga   P. c. canadensis
     Rocky Mts   P. c. capitalis
     Pacific   P. c. obscurus
Several other subspecies are intermediate between these three populations.

Steller's Jay Cyanocitta stelleri
     Pacific  C. s. stelleri group
     Queen Charlotte Islands   C. s. carlottae
     Interior   C. s. macrolopha group

Blue Jay Cyanocitta cristata
     Northern   C. c. bromia
     Southern    C. c. cristata
These two subspecies more or less match the characteristics illustrated in the book, but their distribution does not match my limited observation. The population at Cape May, NJ seems to be subtly dimorphic, including both pale and dark blue birds, while birds in the Adirondacks are darker blue than even the darkest NJ birds. More study is needed.

Western Scrub Jay Aphelocoma californica
     Pacific   A. c. californica group
     Interior West   A. c. woodhouseii group

Gray-breasted Jay Aphelocoma ultramarina
     Texas   A. u. couchii
     Arizona    A. u. arizonae

Black-billed Magpie Pica hudsonia
No subspecies are described within North America, although subtle clinal size differences are known. The Northern and Southern examples illustrated may simply represent the ends of clinal variation. More study is needed.

American Crow Corvus brachyrhynchos
Currently accepted subspecies are based on subtle clinal variations in size and do not match the vocal differences described in the book. Populations from Washington to California (and perhaps farther east as well) sound distinctly different from Atlantic coast birds.

Black-capped Chickadee Parus atricapillus
     Eastern   P. a. atricapillus group
     Rocky Mountain   P. a. septentrionalis group
     Pacific   P. a. occidentalis group

Carolina Chickadee  Parus carolinensis
     Northeast    P. c. extimus
     Southwest    P. c. agilis
These two subspecies represent the extremes of color variation in this species. Other subspecies are intermediate.

Mountain Chickadee Parus gambeli
     Pacific   P. g. baileyae
     Rocky Mountains    P. g. gambeli
Differences between these subspecies are slight, and there are several other intermediate subspecies in the interior west.

Boreal Chickadee Parus hudsonicus
     Alaska   P. h. columbianus group
     Typical   P. h. hudsonicus group

Chestnut-backed Chickadee Parus rufescens
     Typical   P. r. rufescens
     Central California Coast   P. r. barlowi
Marin County, CA is occupied by an intermediate subspecies P. r. neglectus.

Tufted Titmouse Parus bicolor 
     Tufted   P. b. bicolor
     Black-crested  P. b. atricristatus group

Bushtit Psaltriparus minimus
     Pacific   P. m. minimus group
     Interior   P. m. plumbeus group
     Black-eared   P. m. melanotis group (the subspecies occurring in North America is P. m. dimorphicus).

White-breasted Nuthatch Sitta carolinensis
     Eastern   S. c. carolinensis group
     Mountain   S. c. nelsoni group
     Pacific   S. c. aculeata group

Pygmy Nuthatch Sitta pygmaea
     Interior   S. p. melanotis
     Central California   S. p. pygmaea

Brown Creeper Certhia americana
     Eastern   C. a. americana group
     Western   C. a. occidentalis group
     Mexican   C. a. albescens

Cactus Wren Campylorhynchus brunneicapillus
     Interior    C. b. couesi group
     Coastal Southern California   C. b. affine group (the subspecies found in California is C. c. sandiegense).

Carolina Wren Thyrothorus ludovicianus
     Mexican   T. l. lomitensis
     Northern    T. l. ludovicianus group

Bewick's Wren Thryomanes bewickii
     Eastern   T. b. bewickii
     Southwestern   T. b. eremophilus group
     Pacific   T. b. drymoecus group

House Wren Troglodytes aedon
     Eastern    T. a. aedon
     Western   T. a. parkmanii
     Brown-throated    T. a. cahooni group (the intermediate birds in Arizona have been given the name T. a. 'vorhiesi')

Winter Wren Troglodytes troglodytes
     Eastern   T. t. hiemalis group
     Pacific   T. t. pacificus group
     Pribilofs   T. t. alascensis group

Marsh Wren Cistothorus palustris
     Eastern   C. p. palustris group
     Worthington’s   C. p. griseus
     Western    C. p. paludicola group