My advice: get into the field and look at animals. Then wonder why some of them have curved bills, why they walk round in circles.. and whether a godwit is a big dowitcher or not.
A while back I made an effort to stop writing so much about birds, and to concentrate instead on other tetrapod groups (to find out why go here). Alas, if you live in the temperate zone, birds are (after hominids) the most familiar and frequently encountered tetrapods, so I can forgive myself for writing the following. Don’t worry if you were expecting more on asymmetrical ears, or on vampires: I will still return to those subjects, it’s just that I’ve been momentarily distracted. This happens a lot.
My planned excursion to the wilds of Hayling Island (Hampshire, UK) didn’t happen today. So instead I went and visited some friends and ended up at…. Hayling Island. You should get a feel of the place, its fauna and flora, and the weather, from the adjacent photos (kindly taken by Linda). Skein after skein of Brent goose flew in as the day wore on; teal, gadwall and shelduck foraged in the pools and channels; and flocks of lapwings, dunlin and Golden plover fed or rested on the mudflats.
The bird that I most enjoyed seeing, however, was the Black-tailed godwit Limosa limosa, a Eurasian wader with a tremendously elongate, straight bill. In the image at left, Bernie and I are enjoying seeing some (it’s not a staged photo, honest). As a northern hemisphere wader with a large global population, the Black-tailed godwit is a reasonably well studied species. In their intriguing paper ‘Publication bias in waders’, Thomas et al. (2003) found that these two factors (global occurrence and population size) were the most important ones in determining how many studies are devoted to any one wader species. In summer, a Black-tailed godwit is reddish brown with a mottled back, and with dark bars on its whitish belly and flanks, but in winter it is grey and uniform. Two of the four godwit species are routinely encountered here in Europe, and they’re easy to distinguish. The Bar-tailed godwit L. lapponica is smaller than the Black-tailed, and not only shorter-legged and shorter-billed than the Bar-tailed, but also (obviously) different from it in the coloration of its tail feathers. A ‘barred’ tail is one of those distinctive morphological details that you can see scattered around in various unrelated species: amongst Old World waders it’s also present in dowitchers, curlews, shanks, the Green sandpiper Tringa ochropus, the Solitary sandpiper T. solitaria, and a few others. The Pluvialis plovers also have it.
In bill anatomy, the Bar-tailed godwit not only differs from the Black-tailed in that its bill is shorter: it is also different in that its bill is gently upcurved. Why does the Bar-tailed godwit possess a slightly curved bill? Experimental work on curlews – well known for their strongly decurved bill – shows that curved bills are more manoeuvrable within cavities than are straight bills, and better suited for grasping prey within a confined space (Ferns & Siman 1994). This applies to godwits as well as curlews because the same advantage exists whether the bill curves upwards or downwards. Given that curved bills are better suited for rotating within confined spaces than are straight bills, I am somewhat frustrated by the fact that it is the straight-billed Black-tailed godwit that is often seen to stick its bill in a burrow before walking in a circle. Great… don’t these animals read the literature that’s written about them? [adjacent image of Black-tailed godwits from blueskybirds.co.uk].
Anyway, in differing in this way, the different godwit species may avoid direct competition, though as noted they also differ in other respects, with the Bar-tailed godwit being stockier, slightly smaller and shorted-legged than the Black-tailed. A consequence of these differences is that the Black-tailed godwit is able to feed in deeper water than the Bar-tailed: they wade not just ankle-deep, but with water right up to their bellies, frequently submerging the entire head. As the tide came in around them, we remained surprised by how long the Black-tailed godwits we were watching stayed in the same spot, and we left before they did.
Getting back to bills again, even within the same species we find some interesting diversity. Not all Black-tailed godwits are alike: the Icelandic race L. l. islandica is even longer-billed than Black-tailed godwits elsewhere. This is supposed (Hammond & Pearson 1994) to be a demonstration of Allen’s rule (the idea that appendages are smaller in cold-adapted animals than in warm-adapted ones), but surely it can’t be, given that – I assume – Iceland is cooler than continental Eurasia [adjacent image of Black-tailed godwit again from blueskybirds.co.uk].
Even better, bill length differs between the sexes in some godwit species: in Bar-tailed godwits the female’s bill may be as much as a third longer than that of males. Among waders this is also true of some curlews and of various sandpipers. While I’m on this subject it’s worth noting that sexual dimorphism in bill shape is far more widespread within birds than was thought until recently, and the extinct Huia Heteralocha acutirostris of New Zealand is not an unique as used to be thought. I’ll elaborate on this issue some time in the future.
Exactly what sort of waders are godwits? This is a question that has long vexed ornithologists. Godwits look something like particularly big, long-billed versions of some of the Calidris waders, such as the Knot C. canutus and Curlew sandpiper C. ferruginea, but they also share some features with the similarly large curlews. Then again, they also look quite similar to the dowitchers (Limnodromus). In his large, seminal investigation of wader relationships, Lowe (1931) regarded godwits as close kin of curlews. Strauch (1978) found godwits to be closest to the Terek sandpiper Xenus cinereus, an unusual small wader with a long, upturned bill. Bjorkland (1994) found godwits to be closer to scolopacines (woodcocks and snipes) than to curlews or to Calidris waders (stints, sandpipers and dunlin) based on their sharing characters of the interorbital septum and sternum, while Chu (1995) found godwits to form a clade with dowitchers. Szekely et al. (2000) concluded that godwits were outside of a clade that included phalaropes, shanks, turnstones and sandpipers, and closer to this clade than were curlews [adjacent photos of a Short-billed dowitcher L. griseus from here].
Most recently, Thomas et al. (2004) produced a supertree* where godwits were found to be the sister-taxon to a large clade that included all waders except curlews and their putative sister-taxon, the Upland sandpiper Bartramia longicauda. This might fit with the fact that a supposed fossil godwit from the Eocene, Limosa gypsorum, indicates a surprisingly long history for the group. However, that fossil almost certainly isn’t a godwit. Some other alleged fossil godwit species have been named by the way: Miocene L. vanrossemi Miller, 1925 from California, and Pliocene L. ossivalis Brodkorb, 1967 from Florida. The former is preserved only as an impression of a skeleton, while the latter is just a part of a leg bone, and I’m not sure if it’s still regarded as a godwit.
* A ‘best fit’ phylogeny that results from combining the results of all previous studies.
As always, trying to put these animals into a phylogeny does make a difference if you’re interested in knowing about their evolutionary history. If godwits share an ancestry with dowitchers, for example, then the ancestral godwit was probably mid-sized (viz, c. 30 cm long) and straight-billed. Conversely, if the Terek sandpiper is really the closest relative of the godwits, then its small size (c. 22 cm long) and upturned bill at least raises the possibility that ancestral godwits were like this, in which case the straight bill of some godwits would be derived from an upturned bill. Until the relationships among waders are better understood, we remain uncertain about the character changes that preceded the evolution of modern godwits. This sort of research has been done on some other wader groups, incidentally (Pereira & Baker 2005) [adjacent image of Terek sandpiper from here].
I don’t know how many people go and watch birds and come away thinking about character acquisition and phylogenetic relationships, but someone has to I’ll finish by mentioning the recent discovery that Black-tailed godwits are apparently monogamous, yet with males and females following totally different migration paths. The pair have to synchronise their return and meet back up at the breeding grounds.
Coming next: the asymmetrical ears of owls (a follow-on from the troodontid post). Dammit, more birds. But lots more other stuff to follow also..
Refs – –
Bjorklund, M. 1994. Phylogenetic relationships among Charadriiformes: reanalysis of previous data. The Auk 111, 825-832.
Chu, P. C. 1995. Phylogenetic reanalysis of Strauch’s osteological data set for the Charadriiformes. The Condor 97, 174-196.
Ferns, P. N. & Siman, H. Y. 1994. Utility of the curved bill of the Curlew Numenius arquata as a foraging tool. Bird Study 41, 102-109.
Hammond, N. & Pearson, B. 1994. Waders. Hamlyn, London.
Lowe, P. R. 1931. An anatomical review of the ‘waders’ (Telmatomorphae) with special reference to the families, subfamilies and genera within the suborders Limicolae, Grui-Limicolae, and Lari-Limicolae. Ibis, 13th ser., 1, 712-771.
Pereira, S. L. & Baker, A. J. 2005. Multiple gene evidence for parallel evolution and retention of ancestral morphological states in the shanks (Charadriiformes: Scolopacidae). Condor 107, 514-526.
Strauch, J. G. 1978. The phylogeny of the Charadriiformes (Aves): a new estimate using the method of character compatibility analysis. Transactions of the Zoological Society of London B 34, 263-345.
Szekely, T., Reynolds, J. D. & Figuerola, J. 2000. Sexual size dimorphism in shorebirds, gulls, and alcids: the influence of sexual and natural selection. Evolution 54, 1404-1413.
Thomas, G. H., Szekely, T. & Sutherland, W. J. 2003. Publication bias in waders. Wader Study Group Bulletin 100, 216-223.
– ., Wills, M. A. & Szekely, T. 2004. A supertree approach to shorebird phylogeny. BMC Evolutionary Biology 2004, 4: 28.