Seabirds are undeniably cool. They often look neat, they often have very cool names (witness such examples as Macronectes, Oceanodroma and Cerorhinca), and their biology is often amazing. They include some of the largest and longest-lived of birds, the most numerous (there might be over 50 million Wilson’s storm-petrels Oceanites oceanicus in the world), and the most wide-ranging. While crossing the English Channel recently, I kept a look out above-deck for, well, whatever.
And I was rewarded with excellent views of Northern gannet Morus bassanus [adjacent pic from wikipedia]. Together with their close kin the boobies (the six species of Sula), gannets form a bird family called Sulidae (conventionally included within Pelecaniformes, the group that also includes pelicans and cormorants). Gannets are big (their wingspans can exceed 1.7 m), and they exhibit specialisations that allow them to indulge in a pretty incredible piece of behaviour: they hurtle down to the sea like arrows, plunging deep beneath the water…
While undeniably alike, there is little question that gannets and boobies can be distinguished on the basis of overall appearance and skeletal morphology (van Tets et al. 1988), and the fossil record indicates that they diverged something like 20 million years ago. Olson (1985, p. 204) therefore stated that ‘There is no justification … for combining Morus with Sula‘. On the other hand, all the extant members of the group are pretty uniform and the differences – while obvious – are all very minor. In recent years it has become increasingly fashionable for gannets and boobies to be given separate genera, but not everyone does this [a white-phase Red-footed booby S. sula shown below, from wikipedia].
Less well known is the proposal that Abbott’s booby (originally Sula abbotti) is distinct enough for its own genus, Papasula Olson & Warheit, 1988. Abbott’s booby lacks characters present in both Sula and Morus and differs from them in details of vertebral morphology, and in having its sclerotic ossicles fused together to form a ring (a totally unique – and bizarre – character). Olson & Warheit (1988) proposed that it represented the most basal sulid lineage: in a molecular analysis, Friesen & Anderson (1997) found that it was the sister-taxon to the gannets.
Sulid anatomy is highly distinctive. They’re streamlined, with long, high-aspect wings, large, totipalmate* feet, and long, pointed bills. The eyes face forwards, presumably allowing binocular vision (I say ‘presumably’ because it’s not always possible to be sure when binocular vision is present). The long bill is approximately conical, sharply pointed, and with strongly ossified walls that are covered with numerous channels made by blood vessels [skull of Northern gannet below, from Mayr (2002)]. The tip of the upper jaw is gently curved (but strongly hooked in Abbott’s booby), and in Abbott’s booby the tomial edges are deeply serrated. External nostrils are entirely absent in all species: a vertical, slit-like aperture which persists at the posterior end of the bill (just in front of the nasofrontal hinge) is covered in life by a flap of rhamphotheca that is forced shut over the aperture when the bird plunges into the water.
* Totipalmate = all four toes are united by extensive webbing.
Sulids are predominantly (though not entirely) white (more on this in a later post). As is the norm among seabirds, the wing-tips are black. Some people think that black wingtips have evolved because melanin granules (which are insoluble) help provide strengthening and hence reduce damage to these extremities. That might be partly true, but it can’t be the whole story because there are of course some seabirds (mostly gulls) that have no black at the wingtips at all. As with so many other details of morphology, it is probable that black wingtips have several functions, but they probably serve predominantly as species discriminators and display structures.
Gannets (and apparently Abbott’s booby too) are the sister-taxon to the boobies. There are three extant gannets (Morus bassanus of the Atlantic, M. capensis of the southern African coasts and M. serrator of the Australasian coasts), but they’re so similar that some workers (most notably gannet expert Bryan Nelson) regard them as only partially differentiated and still as part of the same species. Fossils show that gannets were present in the north Pacific until late in the Pleistocene (namely M. reyannus Howard, 1936), and fossil species are also known from the Miocene of California and Peru, and the Pliocene of Florida (it’s interesting to note that the present distribution of seabird groups does not necessarily reflect their distribution in the recent past, witness the recently extinct albatross populations from the North Atlantic).
Nelson (1980) thought that gannets evolved in the tropics, and became bigger and heavier as they moved north, the end result of this being M. bassanus [awesome adjacent photo from wikipedia]. This is the heaviest extant sulid (up to 3.1 kg) and the most cold-tolerant, its thick fat layer and reduced amount of naked gular skin helping to minimise heat loss. Its size and weight mean that it can dive deeper than other sulids, getting down to as deep as an astonishing 34 m (Brierley & Fernandes 2001), and it’s also able to take relatively large prey (predominantly mackerel and herring). From a starting point 30 m or so up in the air, gannets launch themselves at the water at about 24 m/s (that’s 86 km/h or 53 mp/h, I think). Using tail, wings and feet, they adjust their trajectory and angle before beginning the entire process: Nelson (1980) wrote that ‘Gannets may hustle down in one straight air-slide, corkscrew or even tip backwards of vertical before shooting their wings behind them and entering the water like an arrow’ (p. 45). The chest is well protected with an intricate network of large air sacs (all the books repeat this fact this but, frustratingly, it’s never illustrated), and the birds use their wings and large, robust feet to force themselves further beneath the surface.
Because the dive is so incredibly brief (lasting for less than two seconds), it’s difficult to study. Ropert-Coudert et al. (2004) found that gannets underwent no – or virtually no – deceleration when entering the water during a dive (a testament to extraordinary streamlining), and that they then continued under their own momentum at an average of 2.87 m/s. The bottom of the dive was determined – not by loss of momentum – but by the bird actively decelerating once the desired depth had been reached [Australian gannet shown below, from wikipedia].
Incidentally, none of the sulids are obligate plunge-divers: they can also make short dives from the surface, though on these occasions they cannot hunt at depth and can only take advantage of prey that are already near the surface. Ropert-Coudert et al. (2004) noted that this shallow-diving technique probably mostly occurs in situations where other marine predators, like cetaceans, seals and big scombroids (tuna and kin), have already pushed fish shoals to the surface. This brings us to another interesting issue: if their diving habits bring them into close contact with such large marine predators, are the sulids themselves in danger of predation? They probably are: Feare (1989) discussed evidence showing that Masked boobies S. dactylatra were sometimes attacked and/or eaten by big fish like groupers. In one case, a booby ringed at Boudeuse in the western Indian Ocean turned up, eleven years later and 100 km away, in the belly of a grouper. In another, a booby returned from a fishing trip with a severely bitten wing that rendered it flightless.
Much more on sulids later: incidentally, the autocorrect function in Word thinks that ‘sulid’ should be ‘solid’, how annoying.
Finally – - if you don’t get the whole ‘gannets are awesome’ thing, watch this video (sorry if it takes a while to load: it is DEFINITELY worth it though). The music is from the Stereophonics.
And happy new year!
Refs – -
Brierley, A. S. & Fernandes, P. J. 2001. Diving depths of Northern gannets: acoustic observations of Sula bassana from an autonomous underwater vehicle. Auk 118, 529-534.
Feare, C. 1989. Underwater booby-trap. BBC Wildlife 7 (3), 142.
Friesen, V. L. & Anderson, D. J. 1997. Phylogeny and evolution of the Sulidae (Aves: Pelecaniformes): a test of alternative modes of speciation. Molecular Phylogentics and Evolution 7, 252-260.
Mayr, G. 2002. A skull of a new pelecaniform bird from the Middle Eocene of Messel, Germany. Acta Palaeontologica Polonica 47, 507-512.
Nelson, B. 1980. Seabirds: Their Biology and Ecology. Hamlyn, London.
Olson, S. L. 1985. The fossil record of birds. In Avian Biology, Volume III, pp. 79-238.
- . & Warheit, K. I. 1988. A new genus for Sula abbotti. Bulletin of the British Ornithologists’ Club 108, 9-12.
Ropert-Coudert, Y., Grémillet, D., Ryan, P., Kato, A., Naito, Y. & Le Maho, Y. 2004. Between air and water: the plunge dive of the Cape gannet Morus capensis. Ibis 146, 281-290.
Van Tets, G. F., Meredith, C. W., Fullagar, P. J. & Davidson, P. M. 1988. Osteological differences between Sula and Morus, and a description of an extinct new species of Sula from Lord Howe and Norfolk Islands, Tasman Sea. Notornis 35, 35-57.