Posted by The mystery skull from the other day is indeed that of a charadriiform: more specifically that of an auk and, most specifically of all, that of a Razorbill Alca torda. Well done Dartian and Kryptos18, and well done everyone else for trying. I admit that I deliberately showed the skull in ‘front view’ (rostral or anterior view) because this made things more difficult. Furthermore, I was hoping that at least some people might make the fairly obvious mistake of identifying the skull as that of a dodo or raptor. Had you not recognised the skull immediately, here’s how you might have identified it…
For starters, this is obviously a bird skull: it has long toothless jaws, rhamphothecae, and an ‘open’ skull morphology with huge orbits and no postorbital bar. You can also see the notches associated with the prokinetic hinge zone, if you know what to look for. As for what sort of bird it is, the biggest clue comes from the obvious bony hollows you can see on the skull roof, over the eyes [see below]. These are the supraorbital fossae: in life, they house the supraorbital salt glands. Seabirds use these organs to excrete excess salt. So, we’re dealing with some sort of marine bird.
With this in mind, we now have to start thinking about the rhamphothecae (the beak tissue). Which seabirds have deep bills, marked on their lateral sides by vertical grooves? Answer: auks (or alcids). Several auk genera are deep-billed, including razorbills (Alca), puffins (Fratercula and Lunda), and parakeet auklets (Cyclorrhynchus). Several extinct auks were too, most notably the Great auk Pinguinis impennis. We’ll come back to this species a few times in the following discussion.
Among these deep-billed taxa, only puffins, Razorbills and Great auks have obvious vertical grooves. And puffins are out, as their rhamphothecae extend far posteriorly (all the way back to the gape). Similar comments apply to the Great auk: its rhamphothecae were way more extensive posteriorly than what’s shown here. In the Razorbill – the only bird we’re left with – the rhamphothecae are restricted to the rostral half or so of the rostrum (the rest is feathered). So we’re left with the Razorbill, and that’s what it is.
The Razorbill is a North Atlantic auk, its largest breeding colonies being those around the shores of Britain, Scandinavia, western Greenland, Iceland and north-east Canada. It dislikes ice, avoids brackish water, and prefers rocky islands with broad ledges as nesting areas. It’s a large, stout-bodied auk, and is longer-tailed than other large auks (like guillemots*). Long tails in seabirds typically relate to improved aerial manoeuvrability: I can’t find any comments in the literature on the aerial manoeuvrability of the Razorbill relative to other auks (all of which have much shorter tails), but – as we’ll see – an adaptive reason for the long tail might have been identified.
* As a British person, I associate the name ‘guillemot’ with both Uria and Cepphus. If you’re North American, you’ll know the Uria species as ‘murres’, and will restrict the name ‘guillemot’ to Cepphus. Another thing: I pronounce guillemot ‘ghee-le-mott’, but have learnt that some Europeans say it ‘ghee-le-moh’ [image below, from wikipedia, shows Razorbill and Common guillemot or Common murre Uria aalge].
Its English common name comes from the superficial resemblance its deep, laterally compressed bill has with an old-fashioned razor (its old name was ‘Razorbilled auk’), and it uses this bill to grab and hold sand-eels, capelin and other mid-water schooling fishes. As many as 20 fish can be held in the bill at any one time. People often wonder how puffins – which manage the same trick – can hold fish while continuing to catch others, yet no-one ever asks this question of Razorbills. I presume Great auks did the same thing, but unfortunately virtually nothing is known of their diet or feeding habits (for data on possible prey species see Olson et al. 1979). Anyway, apparently, the birds use their tongue, the horny papillae on the palate, and the mobile prokinetic hinge zone to retain fish at the back of the bill while continuing to grab new ones at the front. It’s still hard to appreciate how this might work, but it obviously does.
The inside of the mouth is yellow. Incidentally, the Great auk also had a yellow mouth interior according to some reports. However, the inside of its mouth was described as red or orange by others, so we’re really not sure. I tell you, it’s shocking how little we know of the Great auk: a species that only went extinct in 1844 (or thereabouts).
What’s with the lateral grooves on the Razorbill’s bill? We don’t know, but a few suggestions have been made. One idea is that they play a role in sexual display, and this seems likely given that the bill is used extensively for signalling during courtship. Rather more interesting, however, is the hypothesis that the grooves ‘function rather like the sights on a rifle as the bird dives in pursuit of its prey’ (Freethy 1987, p. 58). This seems unlikely to me given data showing that few birds can see the sides of their own bills (e.g., Martin et al. 2004). Again, highly similar structures were present in the Great auk (and presumably in the fossil species intermediate between these two). Debate exists as to whether the grooves of the Great auk were white or not [in the specimen shown below, from wikipedia, the grooves are not obviously white].
Another interesting feature is the narrow channel that extends backwards from the eye. This is better known in guillemots, where – in so-called bridled individuals of Uria aalge – it is picked out by white feathers. Gaston & Jones (1998) hypothesised that this channel might help ‘aid the flow of water over the eye while the birds are swimming rapidly’ (p. 71), though this intriguing function remains untested so far as I know. Furthermore, if this were true I would expect the Great auk to have such a channel: there’s no mention of it in the Great auk literature (Fuller 1999), but – as just mentioned – our knowledge of the Great auk as a live animal is pitiful and it’s not possible to be confident about such a subtle, easily overlooked feature. The ‘lazy’ hypothesis – that the channel might function in communication – was deemed unlikely as the channel is usually all but invisible (Gaston & Jones 1998). It’s been claimed that prominent supra-orbital ridges in the Razorbill might help resist deformation of the eyeball caused by deep-diving: well, maybe. However, some deep-diving seabirds lack prominent supra-orbital ridges, and I’m not sure that Razorbills – as compared with other seabirds – have prominent supra-orbital ridges anyway.
I was surprised to learn that as much as 42% of the Razorbill diet might be made up of crustaceans and annelids, and 10% by molluscs (W. E. Collinge, cited in Freethy 1987). This is surprising because the Razorbill is almost exclusively a pursuit-diver, swimming underwater with feet and half-open wings, and its short dive time (always less than one minute) and maximum dive depth indicate that it doesn’t spend time foraging at or near the sea-floor. Maybe these benthic prey come from the stomachs of the fish it eats. These data might not be accurate, however, as some sources only discuss fishes (and predominantly mid-water fishes) as forming Razorbill diet (Gaston & Jones 1998). As with so many other seabirds, it turns out that Razorbills dive much more deeply than used to be thought. Older references say that dives are typically of 2-3 m, and that 10 m might be exceptional. More recent work indicates that average dive depth is 25 m, with the range being 11-38 m (Wanless et al. 1988, Barrett & Furness 1990). However, Piatt & Nettleship (1985) reported a maximum dive depth of 120 m for the species and Jury (1986) then reported 140 m, thereby making a mockery of all previous estimates [Razorbill image below from here].
Equally surprising is the fact that the Razorbill is adept at kleptoparatisism, and it frequently steals from Atlantic puffins Fratercula arctica. Razorbills attack puffins in flight, but more frequently ‘swim beneath the puffin and torpedo it from below or actually pursue the puffin under the water’ (Freethy 1987, p. 71). One might predict that species which indulge in aerial piracy need to be particularly manoeuvrable: is this why the Razorbill has such a long tail?
I was hoping to get away with just a paragraph or two on the Razorbill, but I guess I found a lot of interesting stuff to cover. I’ll stop there. Incidentally, I was playing with the Razorbill skull because of my work on azhdarchoid pterosaurs. I’ll say no more.
Refs – -
Barrett, R. T. & Furness, R. W. 1990. The prey and diving depths of seabirds on Hornoy, North Norway after a decrease in the Barents Sea capelin stocks. Ornis Scandinavica 21, 179-186.
Freethy, R. 1987. Auks: An Ornithologist’s Guide. Facts on File, New York.
Fuller, E. 1999. The Great Auk. Harry Abrams, New York.
Gaston, A. J. & Jones, I. L. 1998. The Auks. Oxford University Press, Oxford.
Jury, J. A. 1986. Razorbill swimming at depth of 140 m. British Birds 79, 339.
Martin, G. R. & Coetzee, H. C. 2004. Visual fields in hornbills: precision-grasping and sunshades. Ibis 146, 18-26.
Olson, S. L., Swift, C. C. & Mokhiber, C. 1979. An attempt to determine the prey of the Great auk (Pinguinus impennis). The Auk 96, 790-792.
Piatt, J. F. & Nettleship, D. N. 1985. Diving depths of four alcids. Auk 102, 293-297.
Wanless, S., Morris, J. A. & Harris, M. P. 1988. Diving behaviour of guillemot Uria aalge, puffin Fratercula arctica and razorbill Alca torda as shown by radio-telemetry. Journal of Zoology 216, 73-81.
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