photograph of a Western Meadowlark (top) by Rohan Kamath; photograph of an Oak Titmouse (formerly called Plain Titmouse) (bottom) by Tom Grey
What do Eastern and Western Screech-Owls, Plain and Tufted Titmice, and Eastern and Western Meadowlarks have in common? They are species that diverged from one another in isolation rather recently, and have remained largely or entirely geographically separated. Taxonomists group such closely related species that are allopatric (that is, with nonoverlapping distributions) into superspecies.

Superspecies are especially interesting because they represent a "snapshot" of the process of speciation -- evolution caught in the act, as it were. There is no sharp dividing fine between very well-differentiated subspecies and members of a superspecies, so designation of superspecies is usually tentative and sometimes controversial. In regions where their mapped ranges approach one another, it is important to look for evidence of members of a superspecies occurring together (being partially "sympatric" as opposed to allopatric). Birds often change their distributions quickly, and in many regions there are relatively few observers. If you are fortunate enough to find such a situation, you should be on the alert for the formation of mixed-species pairs or even successful hybridization. If the two forms overlap with little or no interbreeding, taxonomists would consider them separate species; if there is extensive interbreeding they would be given subspecific rank.

Following is a list of somewhat more than 100 North American species that are now considered members of 53 superspecies of North American birds. We have not listed numerous cases where North American species are members of superspecies whose other members breed only outside of our area. The list is based primarily on the judgments of the 1983 American Ornithologists' Union (AOU) checklist. The high frequency of question marks indicates the difficulty of taxonomically pigeonholing organisms at various stages within the continuous process of differentiation. In many cases where there is a narrow zone of overlap and a slight degree of hybridization, as in Ladder-backed and Nuttalls Woodpeckers, most (but not all) taxonomists would prefer the two forms to be considered allospecies within a superspecies. When the ranges of two very closely related forms do not overlap so that one cannot determine the degree of natural hybridization (if any), as in the case of Pygmy and Brown-headed Nuthatches, some taxonomists will claim they should be considered subspecies of the same species, and others that they should be members of superspecies. In the gulls, the situation is so complex that our classification is somewhat arbitrary. The bottom line is that it really makes little difference exactly how they are categorized -- understanding their biology is the crucial point. This list serves to "flag" situations where allopatric speciation seems to be reaching its terminal stages, and where you might make a contribution to understanding the process.

North American Superspecies
A question mark (?) in this list indicates that there is some question about the status of the species listed. The numbers in parentheses refer to the pages in the AOU checklist where the taxonomic status of the superspecies is discussed. In many cases there are taxonomists who think the species of a superspecies should be reduced to the status of subspecies within a species, and to indicate this we've added an "s" to the AOU page number. In some cases there is taxonomic opinion that the species are sufficiently distinct or too widely overlapping in range to be considered parts of a superspecies, and there we've added a "d." A question mark after the superspecies and no letter after the page number shows more general uncertainty.

1. Common and Yellow-billed chinned Loons (6)
2. Arctic and Pacific Loons (5)
3. Western and Clark's Grebes (10)
4. Double-crested and Olivaceous Cormorants? (38)
5. Glossy and White-faced Ibis?
6. Tundra and Trumpeter Swans (63)
7. Blue-winged and Cinnamon Teal? (79d)
8. Greater and Lesser Prairie-Chickens? (140s)
9. Gambel's and California Quail (147)
10. Clapper and King Rail? (152s)
11. American and American Black Oystercatchers? (173s)
12. Ruddy and Black Turnstones? (190)
13. Purple and Rock Sandpipers? (199s)
14. Short- and Long-billed Dowitchers (203)
15. California, Herring, Western, Glaucous-winged, Glaucous, and Great Black-backed Gulls? (218-223)
16. Thayer's and Iceland Gulls? (218-223)
17. Black and Pigeon Guillemots (243)
18. Atlantic and Homed Puffins (249)
19. Eastern and Western Screech-Owls (293)
20. Spotted and Barred Owls? (302)
21. Common and Antillean Nighthawks (309)
22. Chimney and Vaux's Swifts? (319)
23. Ruby-throated and Black-chinned Hummingbirds? (356d)
24. Rufous and Allen's Hummingbirds (360)
25. Gila, Golden-fronted, and Red-bellied Woodpeckers (387)
26. Yellow-bellied, Red-breasted, and Red-naped Sapsuckers (388)
27. Ladder-backed and Nuttall's Woodpeckers (390)
28. Western and Eastern Wood-Pewees? (449s)
29. Alder and Willow Flycatchers (452)
30.Great Crested and Brown-crested or Ash-throated Flycatchers? (464)
31.Steller's and Blue Jays? (500d)
32. Black-billed and Yellow-billed Magpies (508)
33. American and Northwestern Crows (509s)
34. Black-capped and Carolina Chickadees (513)
35.Mexican and Mountain Chickadees? (513d)
36.Siberian Tit and Boreal and Chestnut-backed Chickadees?(514)
37.Plain and Tufted Titmice (516)
38. Pygmy and Brown-headed Nuthatches? (519s)
39. Brown and Long-billed Thrashers? (571s)
40. California, Crissal and LeConte's Thrashers? (573)
41. Northern and Loggerhead Shrikes? (585s)
42. Red-eyed and Black-whiskered Vireos? (597d)
43. Nashville, Virginia's, and Colima Warblers (604)
44.Northern and Tropical Parulas? (606s)
45. Townsend's, Hermit, Black-throated Green, and Golden-cheeked Warblers (613)
46. Yellow-throated and Grace's Warblers (615)
47. Mourning and MacGillivray's Warblers? (627s
48. Rose-breasted and Black-headed Grosbeaks?(672s)
49. Lazuli and Indigo Buntings? (675s)
50. Snow and McKay's Buntings? (721s)
51. Eastern and Western Meadowlarks (725)
52. Great-tailed and Boat-tailed Grackles? (729s)
53. Common and Hoary Redpolls? (749)
SEE: Temperature Regulation and Behavior; Head Scratching.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.