10,000 Birds

Last month (Feb 08) I was “lucky” enough to arrive for a day’s birding with the rest of the 10,000 Birds team at the same time as the worst storm of the winter thumped into New York. Our exploits are here, here, and here, as is a photo-gallery of a fluffed-up and admirably stoic Horned Lark which made light of the atrocious conditions and burrowed through the snow to reach grass seeds. When I posted the photos of this richly-coloured bird I noted that the only part of the supercilium that was yellow was just above the bill (the rear part being white) and wondered whether it was possible to assign this individual to a form or subspecies based on this particular characteristic. I couldn’t: there didn’t seem to be a form that can be described by this one feature alone. Besides, as numerous correspondents pointed out, colour variation in Horned Larks is said to be ‘clinal’ - ie a gradual change in a particular characteristic from one end of the geographical range of the population to the other - therefore using colour alone would be fraught with problems. Researching further from what I could see in the field-guides I own (such as David Sibley’s iconic Sibley’s Guide to Birds) the dark-mantled birds in California looked pretty much like birds in the east, while Horned Larks in the mid-west occupied a literal middle ground and were pale-mantled with off-white throats and supercilia. From my (limited) understanding of what a cline is, this seemed paradoxical. To make matters more complicated taxonomic authorities (eg Howard and Moore, 1991) list up to twenty subspecies of Horned Lark occurring in North America alone (with many more across Eurasia and northern Africa): they seemed very sure that Horned Larks are separable despite being part of a “cline”. I obviously had more to learn! The note below is a summary of the musings that have followed, and I’d be very interested to hear what 10,000 Birds readers make of it.

Horned Lark, Jones Beach, New York, Feb 2008

Just three weeks after my New York trip I found myself in Los Angeles, and decided to go to the Carrizo Plain National Monument, a huge area of grassland that is a haven for many winter visitors to California - including huge numbers of Horned Larks (the CBC for the area notes almost 5000!). By the time I got there in early March many had apparently already departed (I saw about a hundred, though that could perhaps also be related to area coverage or change in flocking behaviour making birds harder to see), but there was still what appeared to be a mix of resident breeding birds and what were perhaps migrants from further north.

What made me think that? Being a typical birder (at least I hope that’s what I am) I set about trying to assign the larks to subspecies or even just ‘types’. Most of what I took to be resident birds (ie they were singing and/or were paired up) looked very much like diluted versions of what Sibley calls “Central California” birds (which are the ones I expected to see), while the few birds I saw on their own were to my eyes often paler mantled with either very pale yellow or almost white supercilia, ie were more like what are usually described as ‘prairie’ or “interior west” birds (or even reminiscent of Sibley’s “Western Arctic” birds) and were therefore presumably migrants.

Of course I’ve no way of knowing - based on one day’s observation - whether these pale birds were actually residents rather than visitors, and they may have even been paired up and I didn’t realise it. If they were resident too - which is perhaps possible in March? - then there surely is no such thing as a “typical” western Horned Lark. Bearing in mind how similar the Jones Beach lark had looked to many of the birds in California I also wondered again whether it was possible for to say with any certainty which forms or subspecies these birds belonged to based purely on field observation (which I already suspected was impossible) and whether the variation in colour of the Horned Lark can truly be described as clinal…

Horned Lark, Carrizo Plain, California, March 2008
Typical of the larks I saw on my visit

Clines, subspecies, populations? These are not terms that crop up in everyday conversation unless you’re an ecologist or a researcher (which I hasten to add I’m not), but let me try to clarify things a little.

Firstly, what is a cline? “…a gradual change in a particular characteristic from one end of the geographical range of the population to the other“. The first time I came across a cline in the field (rather than in a book) was in relation to Great Tits Parus major - a very familiar garden bird to me here in the UK, where their underparts are as yellow as a Magnolia Warbler with a thick black streak going from their throat to their belly. On one of my first ever trips abroad (to India many years ago) I was suddenly faced with Great Tits that were identical to our birds at home except that the bright yellow underparts were now completely off-white. They looked very different - but the Indian birds sounded the same as ours, behaved the same, and were to all intents and purposes the same - and the guide-books called (and still do call) them Great Tits, though they belong to a different subspecies eg P. m. stupae.

Great Tits: UK (left) and northern India (right)

The explanation I received at the time was that what I was seeing was a bird at a different point in a cline to the birds in the UK - which because of its geographic location was at one end of a cline stretching eastwards to India and beyond. At the westerly end of the cline (ie in the UK) are large Great Tits with greenish mantles and yellow underparts P. m. newtoni, at the other - in eg Korea and Japan - are smaller Great Tits with greyish mantles and white underparts, the superbly-named P. m. minor (the ’small great tit’). If I were able to travel the entire length of the cline I should - in theory - find populations of Great Tits that made a more or less gradual progression from large, greenish and yellow to smaller, greyish and white (though clines can be ’stepped’ as well as gradual, so there isn’t always an absolutely smooth change). Taxonomy marches ever on, and it’s worth pointing out that minor may well soon be separated as a full species, Japanese Tit, and that the rest of the Great Tits probably fall into three related groups rather than one long cline - which becomes relevant when thinking about what may happen to the taxonomy of Horned Larks in the future.

Another well-known example of a cline (that as far as I know is still recognised by taxonomists) is the variation in belly colour of the beautiful Slate-throated Redstart Myioborus miniatus. Populations of these lovely birds display a change, along a cline from Mexico south into the Andes, from deep red to orange and on to bright yellow. There is apparently little difference in behavior, vocalizations, or ecological requirements of any of the populations. There seems little doubt that there is only one species involved, though it’s also obvious - as in the case of Great Tits - that populations of Slate-throated Redstarts are distinctly different from each other. This means that it’s possible to describe these distinct populations taxonomically - they are part of the whole species but belong to a subset, the subspecies.

Birders and taxonomists/scientists use the term “subspecies” frequently, but I’m not sure that us birders use it properly. We tend to use it interchangeably with ‘race’ and ‘population’ - at least, I now realise, I have done in the past. Personally speaking I stopped using ‘race’ a while ago. It’s a word that in its general usage seems to me to have lost any positive meaning, and is now associated with conflict and separation. I’m certainly not happy putting people into ‘races’, and I don’t think it means too much to describe a bird with a red belly as a different ‘race’ to one with a yellow one when everything else about the species is the same. ‘Population’ seems an altogether less loaded term. A population implies little more than a group that breeds in a particular area (whether that be a country or a habitat like prairie or tundra). They may be separated geographically from other breeding populations, but they’re very much part of the species as a whole. Populations, however, can also be subspecies. My own feeling is that ’subspecies’ is best used when it’s used as a scientific term, one that in 1963 Ernst Mayr defined as “an aggregate of local populations of a species inhabiting a geographic subdivision of the range of the species, and differing taxonomically from other populations of the species…”. The important word in that last sentence is “taxonomically” - ie a subspecies does not imply that a bird is in the process of evolving into a new species, just that it has populations that are recognisably different to each other but that clearly belong to the same species.

How does all this relate to Horned Larks you might reasonably be asking? The references I have on North America’s birds and the mails I’ve had on Horned Larks all mention “clines”. Going from yellow to white along a recognisably geographic line, eg Great Tits, or north to south as in the Slate-throated Redstart makes sense in terms of a cline and, using Mayr’s definition, subspecies. Neither of these two species are especially migratory (so, for example, it’s unlikely that there’ll be a flood of yellow Great Tits into India anytime soon to mess up the clinal progression) which helps keeps the argument neat and tidy, but the same doesn’t seem to be true of North American Horned Larks. Not only are they highly migratory (which means that populations are constantly mixing when away from the breeding grounds), but the bird I saw in New York was not at one end of a cline in either supercilium OR mantle colour but looked pretty much like the larks in California except that there was only yellow over the bill rather than yellow throughout the supercilia.

Theorising from an observation of just one individual is highly unscientific of course and our NY bird could remotely have been an exceptionally lost vagrant from the western end of the cline, but from what I can find out it looked pretty similar to the Horned Larks that are supposed to be found in the east. No-one suggested otherwise to me on the blog or by email either, except one correspondent who thought that the lack of yellow in the supercilia perhaps suggested the bird could have been allied to one of the prairie races which is apparently expanding eastwards.

Looking closely at the photographs of the larks I saw in California (including one taken in 2006 about two hundred miles further north, but still in California and roughly the same time of year) there are plenty of variation between the individuals - which are all adult males: variations in the richness of mantle colour, depth and area of yellow in tone of the throat and the supercilia, the extent of chestnut/brown extending across the breast, and the strength or lack of the dark eye-line (the area and shape of the black bib changes with posture so perhaps is not a feature worth looking at - though I’d be happy to be told otherwise). (None, incidentally, seem to show the breast streaking of Sibley’s “Pacific Northwest” birds, which presumably are strigata, a form that apparently shows enough genetic divergence from “non-streaked” forms to merit specific conservation measures according to a study I found here. This must raise the possibility that more than one species is involved?)

Horned Lark, Panoche Valley, California, Jan 2006
A very yellow-toned individual

Horned Lark, Carrizo Plain, California, March 2008
More typical of the individuals present here in March

Horned Lark, Carrizo Plain, California, March 2008

Horned Lark, Carrizo Plain, California, March 2008

Horned Lark, Carrizo Plain, California, March 2008
Three different individuals, but all of them tonally very pale on the throat and in the supercilia

I don’t know anything about the songs of Horned Larks so have no idea whether vocalisations differ between populations too (if they do then surely we’d be looking at an even more complex situation than we have now). Before anyone else points it out, I don’t know much about Horned Larks at all - which is the point of this post. Having a chance to actually look properly at Horned Larks has made me realise just how little I know. It’s all too easy to repeat the mantra that “variation in Horned Larks is clinal”, but my observations have got me asking whether that’s actually correct, and whether birders are using “cline” correctly anyway. Individual Horned Larks clearly differ, researchers tell us that populations differ (and can therefore be aggregated into subspecies), but can variation in Horned Larks truly be described as clinal? And is it feasible, therefore, for record committees to look at individual larks and correctly decide whether they’re a vagrant form or not - something that is often asked of the UK’s BOU in relation to supposed North American vagrants to our shores? Is it even possible that several species are involved (as with - potentially - Great Tits)?

I may well be re-hashing an old argument - my library of North American ornithological literature is extremely limited - and this apparent “problem” was long ago solved. If so I’d be very grateful to anyone who can point me towards papers or posts that explain just what’s going on! Over to you folks…

UPDATE: some every handy Californian birders found this post and chatted about it briefly on a local yahoo group, giving details of which subspecies are found at Carrizo - go to http://www.sialia.com/s/calists.pl?rm=message;id=184120.