February 10, 2013 | 47
As a regular Tet Zoo reader (right?) you’ll be aware of the petrel series. I’m keen to finish it (hey, just as I am with all the other still-incomplete Tet Zoo series), so let’s crack on. In previous articles, we looked at gadfly-petrels, the members of Fulmarini, and also at the evolution, biology and diversity of petrels in general: see the links below. Here, we look at some (but not all) members of the next petrel clade – Puffinini, the shearwaters. These birds are well named for their stiff-winged style of gliding, their (typically) long, slender, pointed wings seeming to ‘shear’ the waves.
Puffinini includes the many shearwater species included in Puffinus and the three Calonectris species: the Mediterranean or Cory’s shearwater C. diomedea, the Streaked or White-faced shearwater C. leucomelas and the Cape Verde shearwater C. edwardsii. Shearwaters tend to have longer and more slender bills than other petrels, with the Puffinus shearwaters having the longest and most slender bills of all tubenoses.
Puffinus shearwaters are famously social, often forming large feeding rafts at sea (though some are strictly solitary when foraging) and often nesting in colonies sometimes known to number in the millions. In the Great shearwater colony on Nightingale Island, Tristan da Cunha, there’s such competition for burrow space that some birds are forced to give up and lay their eggs on the surface of the ground. Apparently, about a quarter of a million eggs end up this way each year (Nelson 1980). Incidentally, there are suitable, unoccupied islands a short flight away, but even these desperate birds choose not to nest there. The choice of nesting habitats is diverse. There are shearwaters that use burrows at sea-level and on the sides, slopes and tops of cliffs, but others that dig burrows among the trees of tropical forests and yet others that use snowy mountainous places.
Puffinus was found to be paraphyletic by Heidrich et al. (1998), Nunn & Stanley (1998) and Pyle et al. (2011): in these studies, Calonectris was found to be nested within Puffinus sensu lato, and the sister-taxon to the specific Puffinus clade that includes the so-called small shearwaters. Other studies have instead found Calonectris and Puffinus to be sister-groups (Kennedy & Page 2002, Penhallurick & Wink 2004) while others are ambiguous on this point (Austin et al. 2004).
So far as I know, the jury is still out on this issue and Calonectris is still widely treated as a taxon distinct from Puffinus. Heidrich et al. (1998) wondered whether the solution might be to lump the Calonectris species into Puffinus, or if it might be time to split Puffinus into distinct ‘genera’. Penhallurick & Wink (2004) recommended that we use Ardenna Reichenbach, 1853 for the larger shearwaters and that, within this clade, the Buller’s or New Zealand shearwater P. bulleri and Wedge-tailed shearwater P. pacificus represent a clade for which the name Thyellodroma Stejneger, 1888 should be used (see also Heidrich et al. (1998)).
A few additional names have been used for various hypothesised clades within Puffinus. Hemipuffinus has been used for the Pink-footed shearwater P. creatopus and Neonectris for the Christmas shearwater P. nativitatis and Short-tailed or Slender-billed shearwater P. tenuirostris. However, Neonectris doesn’t seem to be monophyletic (Heidrich et al. 1998, Kennedy & Page 2002, Penhallurick & Wink 2004).
The extremely distinctive Kerguelen petrel Lugensa brevirostris was found to be the sister-taxon to the other shearwaters by Nunn & Stanley (1998) and Penhallurick & Wink (2004), though it had previously been suggested to be a petrodromine or a close relative of fulmars. Olson (2000) argued that the generic name Lugensa couldn’t be used for this species since early specimens associated with that name couldn’t be identified with confidence as Kerguelen petrels. He therefore proposed the new name Aphrodroma (and used the combination A. kidderi for the species). Bourne (2001) argued that this course of action was inappropriate, since the creator of the name Lugensa did specifically state that it was intended for brevirostris (Mathews 1942).
Why is this species “extremely distinctive”? It’s slate-grey overall with unusual reflective, silvery patches on the undersides of its wings and an especially stubby bill. In view of its overall shape, you can understand why people have generally not regarded it as a close relative of shearwaters.
The Calonectris shearwaters
Until recently, only two living species were recognised in Calonectris, both of which were well separated geographically. The Streaked or White-faced shearwater occurs in the north-west Pacific (though it disperses as far west as the Philippines and Borneo) while the Cory’s or Mediterranean shearwater occurs where you might guess it does, as well as in the Atlantic all the way down to South Africa. It used to breed further north than its present-day northern-most breeding limit (the Azores), since evidence for its presence as a breeding species is known from Great Britain.
The Mediterranean populations of Cory’s shearwater move into the Atlantic during the winter (probably due to the Mediterranean’s low productivity at this time) while those breeding further south winter off the coasts of Namibia, South Africa, Natal, and in the western Indian Ocean (Harrison 1988). As I said in one of the previous Tet Zoo petrel articles, the individuals in the Indian Ocean normally (it is thought) move back into the Atlantic before heading north, but the presence of individuals in the northern Red Sea suggests that some of these birds get back into the Mediterranean by migrating north along the east side of Africa, not the west side.
The resurrection of the Cape Verde shearwater from synonymy with Cory’s shearwater means that there are now two Atlantic Calonectris species. The two remaining subspecies of Cory’s shearwater (C. d. diomedea and C. d. borealis) are distinct enough that they might warrant distinction as species too (Heidrich et al. 1998). [Adjacent image of Streaked shearwater by Kanachoro.]
All three Calonectris species look something like a cross between a Puffinus shearwater and a Procellaria petrel; their soaring flight is “often likened to that of a mollymawk” (Harrison 1988, p. 256). Cory’s shearwater feeds on the wing a fair bit, reaching down to grab small prey from the water or performing short surface dives; some books say that they don’t dive as frequently as do the Puffinus species (Nelson 1980). However, all of this may be inaccurate, since proficient aquaflying has now been filmed in this species: more on that below. Xavier et al. (2011) recently documented a dietary shift in members of this species living around the Azores: since 1994, they seem to have switched from a diet of boarfish, trumpetfish and saury (all small) to one consisting mostly of mackerel. This shift might be due to changes in fish distribution that are occurring in response to sea temperature, or it might be linked to the increased use of mackerel as live bait in the tuna-fishing industry.
Calonectris has a fossil record that extends back to the Miocene, with the oldest specimens being known from the east coast of the USA (Olson 2008). One Pliocene species, C. krantzi, was about similar in size to a Procellaria petrel, and thus gigantic for a Calonectris. A fossil Calonectris from the Pleistocene of Bermuda – C. wingatei – seems to have become extinct after a catastrophic sea-level rise of about 21 m inundated the island about 400,000 years ago, probably removing the shearwater’s breeding areas (Olson 2008). The last resident albatrosses of the north Atlantic (Short-tailed albatrosses Phoebastria albatrus) became extinct at the same time, and Olson (2008) suggested that “this event would have had a major impact on seabirds worldwide” (p. 401). Modern and future sea level rise may, equally, prove catastrophic for many species.
Large-bodied shearwaters vs small-bodied shearwaters
Finally, we come to Puffinus. Why the name Puffinus is used for shearwaters and not for puffins is rather complicated and I’m not about to cover this issue here. So far as I can tell, it mostly relates to early confusion between these birds: the term ‘puffin’ may originally have been used for shearwaters, not for auks.
How many species should be recognised within Puffinus (sensu lato) remains the topic of debate. The group seems to be divisible into distinct small-bodied and large-bodied clades: molecular data indicates that they diverged about 10 million years ago, during the middle part of the Miocene. The small-bodied clade includes the Manx shearwater P. puffinus and Little or Dusky shearwater P. assimilis while the large-bodied clade includes such species as the Great shearwater P. gravis and Sooty shearwater P. griseus. We’ll look at the large ones first.
The large shearwaters include both mostly dark petrels as well as species with a fair bit of mottling and a lot of white on the underside. Wingspans are round about 1 to 1.2 m.
Buller’s shearwater, also called the New Zealand grey-backed shearwater, is one of the most distinctive shearwaters of the Pacific. It’s large, with a white belly and underwing, and a grey back and dark, V-shaped area extending across the dorsal surfaces of its wings. This recalls the ‘M-shaped’ pattern seen in some gadfly-petrels and prions.
The Wedge-tailed shearwater P. pacificus might be the sister-species of Buller’s shearwater (Nunn & Stanley 1998, Kennedy & Page 2002, Penhallurick & Wink 2004); it has a dark morph that doesn’t look much like Buller’s shearwater at all, but there’s also a pale morph where the underside is extensively white. The wedge-like shape of the tail is, unfortunately, not a reliable field characteristic (Harrison 1988). A Buller’s shearwater + Wedge-tailed shearwater clade has been recovered as the sister-group to the remaining large shearwaters (Nunn & Stanley 1998, Kennedy & Page 2002, Penhallurick & Wink 2004, Pyle et al. 2011).
Within that latter group, the Flesh-footed or Pale-footed shearwater P. carneipes is one of several where the plumage is mostly or wholly dark grey or brownish-grey [adjacent image of this species by Duncan]. The others are the Wedge-tailed shearwater, Sooty shearwater P. griseus and Short-tailed shearwater. Unlike those others, the Flesh-footed shearwater has a distinctly two-toned bill where the main section is yellowish and the tip is dark. It occurs throughout the Indian and eastern Pacific oceans, but individuals also wander as far as the western coast of North America. The Indian Ocean birds breed on St Paul Island (located about half-way between Madagascar and Australia) as well as on various islands off the coast of Western Australia. The Pacific ones breed on New Zealand, Lord Howe Island and surrounding islands.
The Sooty shearwater is a long-winged, mostly sooty brown species where a white region on the underwing (surrounded by dark grey feathers on the rest of the wing) is readily visible at distance. This is one of the most wide-ranging of shearwaters, occurring through the Atlantic and Pacific but having breeding bases in and around southern South America and New Zealand and south-eastern Australia.
The Great shearwater is a large, robust and distinctly patterned shearwater of the Atlantic, breeding on various southern Atlantic islands includes Tristan da Cunha, Inaccessible Island and Nightingale Island. It has a dark cap surrounded by white, a distinctive dark belly patch (also surrounded by white), and wingtips and a dorsal tail surface that are distinctly darker than the rest of the dorsal plumage. There’s a white, U-shaped band across the tail base. [Adjacent image by Patrick Coin.]
The Pink-footed shearwater of the eastern Pacific (it breeds off Chile) is also relatively easy to identify in the field, thanks to its dark-tipped but otherwise pinkish bill and mottled underwings. It’s brownish-grey dorsally and white ventrally. This species is quite similar to the Flesh-footed shearwater, both group as sister-taxa in molecular phylogenies, and there have been repeated suggestions that they should be regarded as subspecies of the same species. The Great shearwater, Sooty shearwater and Short-tailed shearwater appear to be successively more distant relatives of this Pink-footed + Flesh-footed clade (Nunn & Stanley 1998, Kennedy & Page 2002, Penhallurick & Wink 2004, Pyle et al. 2011).
The small shearwaters
The small shearwater group includes all those species close to the Little or Dusky shearwater. All are brownish, dark grey, blue-black or blackish dorsally and pale ventrally; bold demarcations between dark and white areas on their faces and proportionally short wings give some of them a superficially auk-like flight style (the overall appearance, flight style and behaviour of a bird are combined to produce the nebulous concept referred to as ‘jizz’ by birdwatchers). They occur throughout the world’s oceans but generally stay away from the poles.
By far the best known member of this group is the Manx shearwater [adjacent image by Ómar Runólfsson]. Its size is typical for a small shearwater: total length is 30-38 cm, wingspan is 76-89 cm (Harrison 1988). This is a mostly North Atlantic species and the commonest and most frequently encountered shearwater in the region. Manx shearwaters winter off the coast of South America, with some individuals travelling as far south as Cape Horn or even the southern coasts of Africa. Remember that a seabird that moves from the North Atlantic down to the far south may be quite capable of wandering as far as the eastern Indian Ocean or even Australasia, and this is exactly what a few Manx shearwaters do. It also seems that some of them travel west around Cape Horn and into the Pacific. They then end up migrating north along the western side of the Americas.
In recent years, the number of Manx shearwaters being seen off the eastern seaboard of North America has increased to the extent that the birds can now be considered common there: compare this with the situation in the early decades of the 20th century when the species was regarded as a rare vagrant to the western Atlantic, with but three records (from Greenland, Long Island and Maine, respectively) prior to 1931 (Lee 1995). The species was reported as a North American breeder in 1973 (at Massachusetts), with other confirmed cases being reported in 1977 (Newfoundland) and 2009 (Maine). Based on what we now know about the local extinctions that have affected petrel, albatross and other seabird populations, it’s certainly reasonable to wonder whether the Manx shearwater actually bred regularly in North America during prehistoric or historic times. While there are various probable Manx shearwater fossils from Florida, the Bahamas and elsewhere, their identification isn’t certain (so far as I know) and better remains are needed to confirm this possibility. A 1947 specimen from Newfoundland has been said to perhaps suggest that breeding was occurring there prior to the 1970s (Lee 1995). The situation with the Manx shearwater in the west may therefore strike a parallel with that of the Cahow Pterodroma cahow in the east. As we saw in the gadfly-petrel article, Cahows were long associated with the western Atlantic but overlooked as birds of the east, even though they were probably always there and probably ‘normal’ denizens of the area prior to modern times.
Within the small shearwater group, the Manx shearwater has often been hypothesised to be especially close to the Black-vented shearwater P. opisthomelas, Little shearwater and Aubudon’s shearwater P. lherminieri (Kennedy & Page 2002, Austin et al. 2004, Penhallurick & Wink 2004). Several populations once regarded as Manx shearwater subspecies and included within the ‘puffinus complex’ have recently been shown to be phylogenetically distinct and worthy of recognition as distinct species. In fact, some studies find them to be less closely related to the Manx shearwater proper than are other, long recognised species like the Little shearwater and Audubon’s shearwater (Heidrich et al. 1998). Austin et al. (2004), however, did find the Manx shearwater to group with the endemic Yelkouan, Levantine or Mediterranean shearwater P. yelkouan and the critically endangered Balearic shearwater P. mauretanicus, two taxa traditionally regarded as P. puffinus subspecies [Balearic shearwater image below by Govern de les Illes Balears].
Indeed, the number of species included in the small shearwater group has long been somewhat confused, since many are only subtly similar and might be subspecies or other populational variants or subsets of others. Furthermore, some birds that look extremely similar have different distributions and different behaviours and might not be as closely related as their appearance suggests. Harrison (1988) said of these petrels that “few groups engender such fierce arguments, even among experts, as to number of recognisable species” (p. 257). For these and other reasons I had to give up on my initial plan to discuss all of the species and subspecies within the group. [Image below of Fluttering shearwater P. gavia by JJ Harrison.]
By the way, there might still be new species to find: the tiny Bryan’s shearwater P. bryani was named as recently as 2011 (Pyle et al. 2011)… the fact that it’s known for a single specimen collected in 1963 has led some to suggest that it might now be extinct. However, several 2012 sightings made near Japan might be of this species.
Austin et al. (2004) recently examined the molecular phylogeny of small shearwaters and supported the validity of 14 taxa (an additional five were suggested to be synonyms of various of these 14) that grouped into five clades. They recommended that these 14 taxa should be regarded as subspecies, with the five clades representing the units we term species (Austin et al. 2004). A full discussion of their conclusions is beyond the scope of this article, but the take-home points are that small shearwaters mostly clustered into distinct North Atlantic, Australasian-Southern and tropical Indopacific groups, that most species and subspecies do indeed represent ‘good’, independent lineages, and that populations do not always fit where they might be expected to on the basis of morphological similarity (Austin et al. 2004).
Recently extinct shearwaters
Quite a few recently extinct (or supposedly recently extinct) species are included among the Puffinus shearwaters. Several of the Canary Islands were the breeding base for the apparently endemic Lava shearwater P. olsoni, radiocarbon dating of which indicates that it was still alive about 1200 years ago, meaning that it was still around in about the 9th century (Rando & Alcover 2008). The Great auk Pinguinnus impennis is thus no longer the only seabird known to have become extinct in the north-east Atlantic in historic times. DNA analysis shows that the Lava shearwater (characterised by an especially low, gracile skull) is probably the sister-taxon of the Manx shearwater (Ramirez et al. 2010). Another Canary Islands species, the Dune shearwater P. holei, became extinct 2000-3000 years ago and also seems to have been made extinct by human hunting. Other extinct shearwaters are known from the Pliocene, Pleistocene and Holocene of Europe as well.
On Bermuda, P. parvus became extinct after human arrival in the 16th century, and P. spelaeus on New Zealand became extinct following the arrival of humans and their commensals. Other recently extinct shearwater species have been described from the south Pacific.
Deep diving and aquaflying
Shearwaters use several different techniques to find and catch prey. To paraphrase Keitt et al. (2000), the general assumption that petrels and other tubenoses only exploit aquatic prey from the upper 50 cm or so of the sea ignores the various pelvic and hindlimb specialisations for proficient diving present in these birds, some of which were commented on as far back as the 1950s. Some species (like the Christmas shearwater P. nativitatis) sit on the sea surface and then swim powerfully and quickly beneath the surface, powered by strong legs and large, webbed toes.
However, thanks to the use of depth gauges and, more recently, underwater photography, we now know that several shearwater and other petrel species, including Cory’s shearwaters, Short-tailed shearwaters, Sooty shearwaters and White-chinned petrels Procellaria aequinoctialis are proficient wing-propelled divers, diving beneath the surface to ‘fly’ in pursuit of prey at depths of between 10 and 20 m (Skira 1979, Brown et al. 1981, Huin 1994). This behaviour is known as aquaflying. Some excellent sequences of aquaflying Cory’s shearwaters were featured in the BBC series The Blue Planet – this might have been the first time this behaviour was filmed.
It may be possible to distinguish between those petrels that dive to depth using their wings for propulsion and those that don’t since the wing-propelled diving species have much thicker-walled humeri (Kaiser 2007). Habib (2010) incorporated data on aquaflying shearwaters into his study of aquaflight in birds, mostly concluding that the species that do it aren’t much different in wing proportions or bone shape from species that only fly in air; in fact the two shearwaters he included in his study “are mechanically indistinct from albatrosses” (p. 695). Wing shape and proportions therefore don’t provide a reliable guide to the presence or absence of an aquaflying ability – thick bone walls seem to provide a clue, however, perhaps because the thicker bone helps to act as ballast (Habib 2010). We should look at fossil seabirds, and maybe pterosaurs too, with all of this in mind.
Here’s an idea: was aquaflying more common in ancient, extinct shearwaters than modern ones? Some fossil shearwaters from the Miocene are supposed to be bigger (on average) than post-Miocene ones; I’ve heard it informally suggested that a post-Miocene decline in body size might have been due to increasing competition and predation from marine mammals. As I discussed in a 2008 article on gannets, there’s evidence of occasional predation on diving seabirds from big fish. This makes it tempting to speculate that the predatory behaviour of evolving pinnipeds, cetaceans, scombroids, sharks and even albatrosses* had an impact on the body size, diversity and diving behaviour of shearwaters: maybe aquaflying and deep-diving ones are less common today due to interaction with these other groups. I emphasise that this is shameless speculation.
* Perhaps surprisingly, some albatrosses are routine predators of small petrels. More on this another time.
Incidentally, albatrosses belonging to all major lineages (mollymawks, sooty albatrosses and great albatrosses) are also capable of diving down to depths of a few metres at least, though with dives of over 7 m being recorded for species like the Shy albatross Thalassarche cauta (Hedd et al. 1997). The record for the group (held by a Light-mantled albatross Phoebatria palpebatra) is a ridiculous 12 m (Prince et al. 1994). These deeper dives involve swimming, and not just plunging into the water at speed: Tickell (2000) said that “sooty albatrosses appear to approach the underwater proficiency of shearwaters” (p. 30). Given the shape, size and proportions of albatrosses, I find all of this remarkable: not exactly the sort of behaviour you might predict. And therein we find a familiar theme. [Adjacent illustration created with kind help of Paulo Nicolaides of Ecospaces Ltd. Like the facebook page!]
Having mentioned albatrosses, another interesting idea is that the diving abilities of shearwaters and other petrels may make albatross species more susceptible to death at the hands of the longline fishing industry. The petrels dive and thus bring bait fish (and their attached hooks) to the surface; the surface-frequenting albatrosses displace the smaller shearwaters from their catch; the albatrosses then get caught on the hooks, and die (Jiménez et al. 2012). The albatrosses do retrieve the hooked bait fish on their own, but their stealing from the petrels does increase their susceptibility to getting hooked. As I hope is well known, longline fishing is a major caught of albatross mortality, and – if you eat fish, and if you care – you should ensure that the fish you buy are not caught via this method. As you can see from the photo here (and from many articles online), the numbers of albatrosses, petrels and other marine birds killed by longline fishing are astonishing and totally unsustainable: c. 300,000 birds are estimated to be killed annually by this practice.
More petrels still to do – the series is not finished yet. For previous articles on petrels and other tubenosed seabirds at Tet Zoo, see…
And for articles about other kinds of seabirds, see…
Refs – -
Austin, J. J., Bretagnolle, V. & Pasquet, E. 2004. A global molecular phylogeny of the small Puffinus shearwaters and implications for systematics of the Little-Audubon’s shearwater complex. The Auk 121, 847-864.
Bourne, W. R. P. 2001. The status of the genus Lugensa Matthews and the birds collected by Carmichael on Tristan da Cunha in 1816-1817. Bulletin British Ornithologists’ Club 121, 215-216.
Brown, R. G. B., Barker, S. P., Gaskin, D. E., & Sandeman, M. R. 1981. The foods of Great and Sooty shearwaters Puffinus gravis and P. griseus in eastern Canadian waters. Ibis 123, 19-30.
Habib, M. 2010. The structural mechanics and evolution of aquaflying birds. Biological Journal of the Linnean Society 99, 687-698.
Harrison, P. 1988. Seabirds: an Identification Guide. Houghton Mifflin Company, Boston.
Hedd, A., Gales, R., Brothers, N. & Robertson, G. 1997. Diving behaviour of the Shy Albatross Diomedea cauta in Tasmania: initial findings and dive recorder assessment. Ibis 139, 452-460.
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Huin, N. 1994. Diving depths of white-chinned petrels. The Condor 96, 1111-1113.
Jiménez, S., Domingo, A., Abreu, M. & Brazeiro, A. 2012. Bycatch susceptibility in pelagic longline fisheries: are albatrosses affected by the diving behaviour of medium-sized petrels? Aquatic Conservation: Marine and Freshwater Ecosystems 22, 436-445.
Kaiser, G. W. 2007. The Inner Bird: Anatomy and Evolution. University of British Columbia, Vancouver.
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Ramirez, O., Illera, J. C., Rando, J. C., Gonzalez-Solis, J., Alcover, J. A. & Lalueza-Fox, C. 2010. Ancient DNA of the extinct Lava Shearwater (Puffinus olsoni) from the Canary Islands reveals incipient differentiation within the P. puffinus complex. PLoS ONE 5(12): e16072. doi:10.1371/journal.pone.0016072
Rando, J. C. & Alcover, J. A. 2008. Evidence for a second western Palaearctic seabird extinction during the last Millennium: the Lava shearwater Puffinus olsoni. Ibis 150, 188-192.
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Xavier, J. C., Magalhaes, M. C., Mendonca, A. S., Antunes, M., Carvalho, N., Machete, M., Santos, R. S., Paiva, V. & Hamer, K. C. 2011. Changes in diet of Cory’s Shearwaters Calonectris diomedea breeding in the Azores. Marine Ornithology 39, 129-134.