Seabirds are fascinating (as I often say: hey, just like all the other tetrapods). To me, they’ve always seemed to be one of those groups that requires a lifetime of immersion and specialisation should you hope to become properly acquainted with them. I just don’t feel that you can get to know them via books and published articles – you have to actually be out there, at sea, in diverse latitudes, armed perpetually with binoculars and a telescope. My chances to become immersed in the ‘seabird experience’ have been few and far between, but I have done my fair share of watching gulls, terns, auks, gannets and shearwaters as and when possible. [Awesome photo above from USGS.]

Seabirds of many species are phenomenally abundant, and data shows that they play major roles as predators, planktivores and scavengers in various marine ecosystems. But as everyone interested in the natural world should know, seabird populations today are beleaguered by such problems as plastic and oil pollution, climate change, human disturbance, alien predators that exploit them on their nesting grounds, declines in sea ice, and depletion of fish and plankton stocks. There’s therefore a rush to better understand their ecology in an effort to minimise the population losses and the general degradation of the marine ecosystems that these birds are part of.

Introducing petrels

Here I want to talk about one of my favourite seabird groups, the petrels, also known as true petrels or procellariids. [Adjacent photos by Patrick Coin, BrockenInaGlory*, Rosemary Tully and Mark Jobling, all from wikipedia.]

* Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

For people who like the same sort of thing that I do – recently discovered species, species known from just a few (or even single) specimens, species about which just about nothing is known, species with remarkable life histories, and species capable of really surprising bits of behaviour – petrels are where it’s at. Numerous short articles have been published reporting sightings of petrel species outside of their normal ranges and some species have been reported on so few occasions that any additional sightings are worthy of publication. Other species have disappeared and reappeared throughout history. And major contributions have been made by people able to identify and differentiate species at sea and hence report new, distinguishing characters. There are some excellent field guides to the petrels of the world. I especially recommend Tuck & Heinzel (1978) and Harrison (1983).

The main aim here – and, by “here” I mean “over the next several articles” – is to cover petrel diversity (there are about 75 living species, grouped into five main clades), to overview their biology, ecology and behaviour, and also to discuss how certain aspects of petrel behaviour and ecology match with morphology. I tried hard not to get bogged down in tedious discussions of phylogeny and systematics. As usual, however, you need to understand at least a bit about the phylogeny and systematics of the group before you can hope to realistically understand the evolutionary patterns within the group. In the end I’ve tried to combine everything in the hope that it presents a coherent picture of petrel diversity, ecology and form-function correlation. Completing these petrel-themed articles has not been easy – I started writing them in 2009 or earlier but they’ve been on the backburner.

Petrels (Procellariidae) are tubenosed seabirds, or tubenoses: part of the same neornithine clade as albatrosses (Diomedeidae), storm-petrels (Hydrobatidae) and diving-petrels (Pelecanoididae). The way I’ve just listed those groups – I mean, with all four being recognised as distinct taxonomic ‘families’ – represents the way tubenoses have been classified in mainstream 20th (and 21st) century literature. However, this ‘four family system’ does not match the phylogeny of the group as recovered by recent studies. More on this later.

I should note that ‘petrel’ is supposed to be pronounced in similar fashion to the name Peter, since it apparently originated as a diminutive form of that name (and hence started out as ‘peter-el’). The books say that petrels are named after St. Peter since he “walked (somewhat fearfully) on the stormy sea of Galilee at Christ’s invitation” (Lockley 1983, p. 8). The irony here is that the ‘water walking’ behaviour referred to here is practised by storm-petrels, not by petrels proper (storm-petrels don’t really walk on the water; rather, they patter across the surface with their large webbed feet while flying) [Adjacent image, showing this behaviour, by Patrick Coin]. So, petrels – most of which are speedy soarers – are named after the idiosyncratic behaviour practised by storm-petrels.

Totally tubular (external) nostrils

Tubenoses are united by such distinctive morphological features as their tubular nostrils and strongly reduced hallux (it consists just of a single phalanx, or is absent altogether). They also share a terminal hook on the bill. The tubenose bill is composed of a series of distinct plates rather than a continuous rhamphothecal covering and hence is referred to as a ‘compound rhamphotheca’. These different bill plates all have names (Coues 1866). The naricorn, latericorn, culminicorn and maxillary or premaxillary unguis (or nail) are all on the upper jaw; the ramicorn and mandibular unguis (or nail) are on the lower.

The persistence of prominent grooves between these plates might be due to their role in helping to drain unwanted saline fluid from the salt glands. As is typical for seabirds, the glands are located in bony depression over the eye sockets, but the fluid that drains from them is discharged through the nostrils and drips away from the end of the bill (Schmidt-Nielsen 1960). If you’re wondering, in the seabirds that have sealed external nostrils (gannets, cormorants etc.), the fluid is discharged via the internal, palatal nostrils and then runs along the dorsal surface of the palate before being released at the bill tip.

In petrels, the nostrils are united in a single, dorsally positioned tube, and this is also the case in diving-petrels and storm-petrels [see adjacent photo of Short-tailed shearwater, photo by J. J. Harrison]. Albatrosses, however, differ in having two tubes, one on either side of the bill. Given that (virtually all) phylogenies nest albatrosses somewhere within the clade that includes all other tubenoses, you might like to wonder what sort of transformation occurred during the evolution of these birds. Did albatrosses have a single, dorsally placed tube ancestrally, or have albatrosses retained a primitive condition while members of the other lineages convergently evolved the single, dorsally-placed condition?

Incidentally, there are no clear indications from the tubenose skull that tubular nostrils – let alone a single, dorsally located tube – would be present in life. In a tubenose skull, the bar located between the long, oval bony nostrils is raised relative to the sides and more anterior parts of the rostrum, but I can’t see that anyone would link this with the presence of a united tube in the absence of other information. As I write this, I have Northern fulmar Fulmarus glacialis skulls right in front of me.

The nostril tube may help in channelling scent and it’s well known that storm-petrels and true petrels at least are attracted to dimethyl sulphide (DMS) (Nevitt et al. 1995), a compound released by phytoplankton when it’s subjected to grazing by zooplankton. The tubenoses that have so far exhibited high sensitivity to DMS are nocturnal foragers, so it seems logical that they might respond to olfactory cues more than visual ones when foraging (Nevitt et al. 1995). However, in view of their nocturnal habits, it’s not surprising that many petrels have highly acute night-time vision, with a flattish cornea and a lens that does most of the work in focusing light being among several specialisations for improved vision in the dark (Martin 1990).

I just mentioned that the nostril tube might help to channel scent. It has also been suggested that the tubular (laterally placed) nostrils of albatrosses contain pressure receptors that help them negotiate and exploit shifting air masses over waves (Kaiser 2007).

Next: Oil, squirting oil, using oil, and why contain oil in the first place?

For previous Tet Zoo articles on seabirds, see...

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Coues, E. 1866. Critical review of the family Procellariidae: Part V; embracing the Diomedeinae and the Halodrominae. With a general supplement. Proceedings of the Academy of Natural Sciences of Philadelphia 18, 172-197.

Harrison, P. 1988. Seabirds: an Identification Guide. Houghton Mifflin Company, Boston.

Hieronymus, T., & Witmer, L. (2010). Homology and Evolution of Avian Compound Rhamphothecae The Auk, 127 (3), 590-604 DOI: 10.1525/auk.2010.09122

Lockley, R. M. 1983. Flight of the Storm Petrel. David & Charles, Newton Abbott & London.

Kaiser, G. W. 2007. The Inner Bird: Anatomy and Evolution. University of British Columbia, Vancouver.

Martin, G. 1990. Designer eyes for seabirds of the night. New Scientist 128 (1741), 46-48.

Nevitt, G. A., Veit, R. R. & Kareiva, P. 1995. Dimethyl sulphide as a foraging cue for Antarctic procellariiform seabirds. Nature 376, 680-682.

Schmidt-Nielsen, K. 1960. The salt-secreting gland of marine birds. Circulation 21, 955-996.

Tuck, G. & Heinzel, H. 1978. A Field Guide to the Seabirds of Britain and the World. Collins, London.