Common Ground-Dove Columbina passerina
     Eastern C. p. passerina
     Western C. p. pallescens
These two populations differ only in plumage color saturation. While differences can be obvious there are many intermediate individuals and many cannot be assigned to subspecies.

Ruddy Ground-Dove Columbina talpacoti
     Eastern C. t.
     Western C. t.
As in Common Ground-Dove these two populations differ only in plumage color saturation. While differences can be obvious, and should be taken into account when identifying these similar species, similar differences exist in many other species (e.g. Mourning and White-winged Doves) but do not affect field identification and are not illustrated.

Eastern Screech-Owl Otus asio
     Typical O. a. asio et al.
     Great Plains O. a. maxwelliae
     Mexican O. a. mccalli
There are several other subspecies differing slightly from those illustrated but not forming clear groupings.

Western Screech-Owl Otus kennicottii
     Pacific O. k. kennicottii
     Great Plains O. k. aikeni
     Mojave O. k. yumanensis
     Mexican O. k. suttoni
There are several other subspecies differing slightly from those illustrated but not forming clear groupings.

Great Horned Owl Bubo virginianus
     Eastern B. v. virginianus
     West Taiga B. v. wapacutha
     Pacific B. v. pacificus
     Southwest B. v. pallescens
Variation in Great Horned Owl (as in some other widespread species e.g. Song Sparrow) is clinal and generally follows the rule of darker colors in more humid climate, paler in more arid climate. There are several other subspecies not illustrated that are intermediate in plumage between the ones shown.

Northern Pygmy-Owl Glaucidium gnoma
     Pacific G. g. californicum group
     Interior West G. g. pinicola
     Mexican G. g. gnoma
These three subspecies differ in size, voice, and slightly in plumage. Several studies have suggested splitting into two or three species.

Burrowing Owl Athene cunicularia
     Florida A. c. floridana
     Western A. c. hypugaea
These two subspecies differ consistently in all plumages and may differ slightly in voice. The breeding ranges are isolated and a split might be proposed if it were not complicated by the existence of other subspecies throughout the West Indies and South America.

Spotted Owl Strix occidentalis
     Pacific (Northern) S. o. occidentalis group
     Interior West (Mexican) S. o. lucida
Two subspecies differ subtly in plumage but not in voice or size. Genetic research found very little contact between them, not surprising considering that they are sedentary birds with widely separated breeding range. Proponents of the Phylogenetic species concept have suggested splitting.

Short-eared Owl Asio flammeus
     Northern A. f. flammeus
     Caribbean A. f.
Two subspecies differ consistently in plumage and average size, may also differ in voice. Taxonomic decisions are complicated by the existence of other subspecies worldwide, including some very distinctive forms on the Galapagos and Hawaiian islands.

Northern Saw-whet Owl Aegolius acadicus
     Typical A. a. acadicus
     Queen Charlotte A. a. brooksi
These two subspecies differ in size and plumage, as well as diet (Queen Charlotte birds eat a lot of marine amphipods picked up on beaches).

Common Nighthawk Chordeiles minor
     Pacific/Eastern C. m. minor et al.
     Southwestern C. m. henryi
     Northern Plains C. m. sennetti
These populations differ only slightly in plumage and all variation is clinal. Differences are most pronounced in juvenal plumage.

Whip-poor-will Caprimulgus vociferus
     Eastern C. v. vociferus
     Arizona C. v. stephensi
These two populations differ significantly in voice and do not overlap in range. Splitting into two species has been proposed and seems reasonable despite the near identical appearance.

Allen's Hummingbird Selasphorus sasin
     Northern S. s. sasin
     Southern S. s. sedentarius
These two subspecies differ mainly in bill length.

Acorn Woodpecker Melanerpes formicivorus
     Coastal M. f. bairdi
     Interior M. f. formicivorus
These two subspecies differ slightly in size, bill length, and plumage, and do not overlap in range. More study is needed to check for differences in voice or other characteristics. More distinctive subspecies occur in Mexico.

Red-bellied Woodpecker Melanerpes carolinus
     Northern M. c. 'carolinus' group
     Florida M. c. 'perplexus'
All differences are variable and these two populations are apparently not consistently distinguishable (although my experience is that birds in the Miami and Keys area are consistently different from birds in the northeastern states). All subspecies have been merged by recent authors (e.g. Short, 1982). However the same authors suggest that the birds of the Florida Keys may be distinctive enough to warrant subspecies recognition and require more study.

Red-breasted Sapsucker Sphyrapicus ruber
     Northern S. r. ruber
     Southern S. r.daggetti

Downy Woodpecker Picoides pubescens
     Eastern P. p. pubescens group
     Interior West P. p. leucurus group
     Pacific P. p. gairdnerii group

Hairy Woodpecker Picoides villosus
     Eastern P. v. villosus group
     Interior West P. v. septentrionalis group
     Pacific P. v. harrisi group

Three-toed Woodpecker Picoides tridactylus
     East Taiga P. t. bacatus
     West Taiga P. t. fasciatus
     Rocky Mountains P. t. dorsalis

Northern Flicker Colaptes auratus
     Red-shafted C. a. cafer group
     Yellow-shafted C. a. auratus group