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. 1. Common
and Yellow-billed chinned Loons (6) SEE: Species
and Speciation;
Sibling
Species;
Hybridization;
Great
Plains Hybrids. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.
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.
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)