By Steve N.G. Howell
Subspecies and the Birder: Variation and Probability
Putting names to birds, especially at the level below species, is often about probability. Few subspecies are so well defined that 100% of individuals can be identified with confidence in the hand, let alone in the field. If we accept the 75% rule, this means that up to 25% of individuals – 1 in 4 birds we see – may not be diagnosable. Of course, many of these 25% may well look like the other 75%, but this doesn’t have to be the case.
In 1948, A. L. Rand discussed the problems of how one determines whether an atypical bird in an area is from another population or simply reflects variation within the regular population of that area. As far as the birder goes, it’s all a matter of probability, of educated guesswork. Examples Rand discussed include Spruce Grouse, where females of one subspecies are typically grayish toned, whereas those of another are reddish toned. The average differences are not in dispute, but if you found a reddish female far into the range of the grayish subspecies, would it be a vagrant or local variation? Given that grouse don’t move much, the probability favors local variation.
The breeding subspecies of American Robins in the Newfoundland region (nigrideus) is typically blacker above than eastern mainland breeders (nominate migratorius). But individuals resembling nigrideus have been found breeding west to Manitoba. Are these vagrant nigrideus or simply variants of migratorius? We don’t know, we can’t know – at least for now.
This Savannah Sparrow shows characters of the Sable Island race princeps, often known as the Ipswich Sparrow. You might feel pretty confident that it is certainly an Ipswich Sparrow, but do you really know how much variation other subspecies show? Dare County, North Carolina, 15 February 2010. © Steve N. G. Howell.
Another of Rand’s examples involved a Red-tailed Hawk matching the characters of the western subspecies calurus, which was nesting in eastern Canada, on Prince Edward Island. This recalls the recent report of an apparent eastern Red-tailed Hawk breeding in Alaska (Sullivan 2011). But are these really examples of the western or eastern subspecies, or might they simply represent occasional throwback variation of the local subspecies? Honest answer: we don’t know. Each case comes down to personal opinion and probability. This conundrum brings with it the Catch 22 problem: do we describe subspecies based on appearance, or on breeding range? Is it a western Red-tailed Hawk because it looks like one, or is it an eastern Red-tailed Hawk because it’s breeding in the East?
A pioneering study that examined the feasibility of identifying subspecies in the field came in 1957 from the Savannah River area, Georgia, courtesy of a banding study by Robert Norris and Gordon Hight on – appropriately – the Savannah Sparrow. In the first winter, specimens were collected and compared with museum series to determine which subspecies occurred at the study site; 3 paler subspecies and 2 darker subspecies were identified. The second winter, banding and field observations were made, often with baseline series of specimens on hand for comparison. The field studies ascribed 195 birds to subspecies, while a further 252 birds were considered to show characters intermediate between subspecies (and perhaps some came from undescribed or different populations). Thus only 44% of birds could be placed with a described subspecies – far less than the 75% rule would decree. But is it better to know something about 44% of a population rather than nothing about 100%?
These two Savannah Sparrows show the characters of, well, Savannah Sparrows. A critical review of plumage variation in mainland North American populations of Savannah Sparrow still needs to be undertaken. Neither of these individuals particularly resembles the local breeding birds of this area, but besides that their geographic origins and subspecific status are open to conjecture. Marin County, California, 13 October 2010. © Steve N. G. Howell.
Subspecies and the Birder: Relax, be Honest
It is clear that the term subspecies has evolved and is still evolving. Today it appears to mean different things to different people. To some scientists it is a rigorously defined entity satisfying the 75% rule (say, Subspecies version 3.3), but to birders it still has an older meaning related to average population differences (Subspecies version 2.1). For birders, the bottom line is whether a subspecies can be distinguished in the field, and with what degree of confidence. However, until the characters of most subspecies are examined critically, we can’t answer that question.
So is there a solution? Perhaps, at least in part. One approach, sometimes used in the field and in some birding publications, is that when referring to an individual bird or a few birds, one can say that the bird shows characters of subspecies X, or of a western interior population, rather than asserting that it is of subspecies X. This seems like a reasonable and realistic way to deal with subspecies and individual birds – assuming, of course, that you know what every subspecies looks like and thus can judge from the full range of possibilities. For example, we could say “this Wilson’s Warbler shows characters of the eastern race pusilla” meaning: it is likely of the race pusilla, but it might just be a dull individual of the race pileolata, or an intergrade between pileolata and pusilla. Is there something wrong with being honest rather than forcing false precision on something you can’t know? Moreover, in using the term race, rather than subspecies, we are subtly but semantically shifting from the scientific to the colloquial lexicon.
When referring to a species overall we could say: populations of wetter northern areas tend to be darker, populations of drier southern areas tend to be paler (getting as specific as we like with respect to geography). We could even say something like: northern interior races (pallescens, pallidus, albus) average larger and paler overall than southern coastal races (obscurus, nigrescens), but in the field few if any individuals can be realistically ascribed to a given subspecies.
With its richly colored lores, this Wilson’s Warbler shows characters of the relatively bright western race chryseola, in which both sexes have a glossy black cap. Nayarit, Mexico, 3 January 2011. © Steve N. G. Howell.
The concept of subspecies groups can also be useful, as advocated by Dean Amadon and Lester Short (1992) and employed, for example, by Howell and Webb (1995) and Pyle (1997, 2008), and by eBird with the Identifiable Subspecific Form (ISSF). That is, a group of described subspecies (say, northern interior birds) can usually be distinguished from another group of subspecies (southern coastal birds), but in the field any individual subspecies within either group might not be safely identified. The first-named subspecies in the group gives its name to the group. Thus we might say, these birds show characters of the pallescens (or northern interior) subspecies group. This approach, without the subspecies names, is basically what is employed in the Sibley Guide. Of course, we still have the same problem of what constitutes “distinguishable” – but here it can be gut feeling rather than putative science, and perhaps this is the best we can hope for in the foreseeable future.
The subspecies concept undeniably has utility, and many described subspecies are likely valid. It’s not all a house of cards. But the arbitrary nature with which subspecies have been defined and described, or even evaluated, makes for a very uneven playing field, one where birders might easily trip and fall over their unlaced ingenuousness. Maybe a hundred years hence we will have sufficient samples of local variation throughout the ranges of every bird species that breeds in North America, and thus be able to make more “statistically meaningful” (is that an oxymoron?) statements about geographic variation. Until then, it might be best if we as birders show the characters of humility, and acknowledge that there are things we don’t know we don’t know.
I thank Catherine Hamilton, Peter Pyle, and Ted Floyd for comments on an earlier draft of this essay.
Amadon, D. 1949. The seventy-five percent rule for subspecies. Condor 51:250-258.
Amadon, D., and L. L. Short. 1992. Taxonomy of lower categories – suggested guidelines. Systematic Zoology 25:161-167.
AOU. 1957. Checklist of North American Birds, 5th edition. AOU. Washington, D.C. AOU. 1983. Checklist of North American Birds, 6th edition. AOU, Washington, D.C.
Ball, R. M. Jr., and J. C. Avise. 1993. Mitochondrial DNA phylogenetic differentiation among avian populations and the evolutionary significance of subspecies. Auk 109:626-636.
Banks, R. C. 1988. Geographic variation in the Yellow-billed Cuckoo. Condor 90:473-477.
Banks, R. C. 2011. Taxonomy of Greater White-fronted Geese (Aves: Anatidae). Proceedings of the Biological Society of Washington 124(3):226-233.
Benkman, C. W., J. W. Smith, P. C. Keenan, T. L. Parchman, and L. Santisteban. 2011. A new species of the Red Crossbill (Fringillidae: Loxia) from Idaho. Condor 111:169-176.
Byrkjedal, I., and D. Thompson. 1998. The Tundra Plovers. Poyser, London.
Cicero, C., and N. K. Johnson. 2006. Diagnosability of subspecies: lessons from Sage Sparrows (Amphispiza belli) for an analysis of geographic variation in birds. Auk 123:266-274.
Collar, N. J. 1999. New species, high standards, and the case of Laniarius liberatus. Ibis 141:358-367.
Cramp, S., and K. E. L. Simmons (eds.). 1983. The Birds of the Western Palearctic, vol. 3. Oxford University Press.
Darwin, C. 1859. On the Origin of Species by Means of Natural Selection, or, the preservation of favoured races in the struggle for life. London.
Dickinson, J. C. 1952. Geographical variation in the red-eyed towhee of the eastern United States. Bulletin of the Museum of Comparative Zoology 107:272-352.
Engelmoer, M., and C. S. Roselaar. 1998. Geographic variation in waders. Kluwer Academic Publishers.
Franzreb, K. E., and S. A. Laymon. 1993. A reassessment of the taxonomic status of the Yellow-billed Cuckoo. Western Birds 24:17-28.
Griscom, L. 1934. The ornithology of Guerrero, Mexico. Bulletin of the Museum of Comparative Zoology 75:367-422.
Howell, S. N. G., and S. Webb. 1995. A Guide to the Birds of Mexico and Northern Central America. Oxford University Press.
Huntington, C. E. 1952. Hybridization in the purple grackle. Systematic Zoology 1:149-170.
Mayr, E. 1942. Systematics and the origin of species. Columbia University Press.
Mayr, E. 1982. Of what use are subspecies? Auk 99:593-595.
Murphy, R. C. 1952. The Manx Shearwater, Puffinus puffinus, as a species of world-wide distribution. Novitates 1586.
National Geographic Society (NGS). 2011. Field Guide to the Birds of North America, 6th edition. NGS.
Norris, R. A., and G. L. Hight Jr. 1957. Subspecific variation in winter populations of Savannah Sparrows: a study in field taxonomy. Condor 59:40-59.
Patten, M. A., and P. Unitt. 2002. Diagnosability versus mean differences of Sage Sparrow subspecies. Auk 119:26-35.
Phillips, A. R. 1986. The Known Birds of North and Middle America, part 1. Allan R. Phillips, Denver, CO.
Pyle, P. 1997. Identification Guide to North American Birds, part 1. Slate Creek Press, Bolinas, CA.
Pyle, P. 2008. Identification Guide to North American Birds, part 2. Slate Creek Press, Bolinas, CA.
Rand, A. L. 1948. Probability in subspecific identification of single specimens. Auk 65:416-432.
Rising, J. D. 2001. Geographic variation in size and shape of Savannah Sparrows (Passerculus sandwichensis). Studies in Avian Biology 23.
Rising, J. D. 2011. Genus Passerculus, pp 550-551 in Handbook of the Birds of the World, volume 16 (J. del Hoyo, A. Elliott, and D. Christie, eds.). Lynx Edicions, Barcelona.
Rohwer, S., V. G. Rohwer, A. T. Peterson, A. G. Navarro S., and P. English. 2012. Assessing migratory double breeding through complementary specimen densities and breeding records. Condor 114:1-14.
Sibley, D. A. 2000. The Sibley Guide to Birds. Knopf, New York.
Sullivan, B. L. 2012. Apparent Eastern Red-tailed Hawk (Buteo jamaicensis borealis) nesting in Alaska. North American Birds 65:390-392.
Unitt, P., and A. Rea. 1997. Taxonomy of the Brown Creeper in California. Pp. 177-185 in R. W. Dickerman (compiler), The Era of Allan R. Phillips: a Festschrift. Horizon Communications, Albuquerque, New Mexico.
Wilson, E. O., and W. L. Brown Jr. 1953. The subspecies concept and its taxonomic application. Systematic Zoology 2(3):97-111.
Winker, K. 1997. A new form of Anabacerthia variegaticeps (Furnariidae) from western Mexico. Pp. 203-208 in R. W. Dickerman (compiler), The Era of Allan R. Phllips: a Festschrift. Horizon Communications, Albuquerque, New Mexico.
Zink, R. M., and 6 co-authors. 2005. Mitochondrial DNA variation, species limits, and rapid evolution of plumage coloration and size in the Savannah Sparrow. Condor 107:21-28.
Latest posts by ABA (see all)
- Join the ABA in India, or Closer to Home! - November 24, 2015 8:00
- Open Mic: On Being the Youngest to 4000 Species - November 10, 2015 8:00
- Open Mic: Meet the Willettes! - October 24, 2015 8:00
- Day of the Dead Tribute: Remembering Paul Green - October 21, 2015 8:00
- Day of the Dead Tribute: A Visit to the Eighth Continent - October 14, 2015 8:00