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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Petrels: some form-function rules , and pattern and pigmentation (petrels part III)

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Time for more petrels. This article is another introduction, this time to generalities of behaviour and form-function. The previous petrel articles are here and here, and see the list of links below as well.

Several distinct foraging styles are employed by petrels and they’re more diverse in feeding behaviour than most accounts imply. The majority of species are flexible feeders, their diet mostly being determined by the abundance and availability of fish, crustacean and cephalopod prey species.

Aquaflying shearwaters in pursuit of fish. A still from Blue Planet, BBC (c).

Many feed by alighting on the surface to feed on plankton, fish, squid or floating carrion. Surface seizing of prey is fairly common (where the birds reach down to grab prey from the sea surface while in flight), but some species practise aerial pursuit of other seabirds (that is, they’re aerial pirates or kleptoparasites: more on this in a later article). Others dive into the water in pursuit of prey, practising either short, surface dives from a few metres up (a technique termed surface plunging), or longer, more extensive dives in which they pursue prey well beneath the surface (a technique termed pursuit diving). Indeed, some species (some shearwaters and possibly some others) even ‘fly’ down in the water to depths of 10 or even 20 metres and are capable aquaflyers (Habib 2010). I’ll be coming back to aquaflying later on as well. It’s a fairly well known bit of behaviour nowadays, having famously been featured in the 2001 BBC series Blue Planet.

Prions move in a peculiar way across the sea surface, paddling with their feet, holding their bodies lightly against the water, their wings out to the side, and their heads disappearing and reappearing into the water as they scoop up plankton with the bill (Nelson 1980). This form of feeding has been termed hydroplaning.

Ashmole's (1971) classic diagram of seabird feeding techniques (illustrator: Jon Ahlquist). Note the hydroplaning prion and 'pursuit plunging' shearwater. Today we know that a few shearwaters should be included within the 'pursuit diving with wings' (= aquaflying) category.

In one of the neatest papers ever published in ornithology (that’s just my opinion of course), Spear & Ainley (1998) showed how the morphological characters of petrels correlated with their diet and foraging style. Tropical species, they showed, tended to have longer and deeper bills, longer tails, and longer wings with greater surface areas than polar species (Spear & Ainley 1998). Presumably these differences have arisen because tropical species have to cover much larger expanses of ocean in order to find food; furthermore, tropical prey items (like flying fish and flying squid) are typically fast-moving, agile and capable of flight, whereas polar prey are less mobile and tend to be concentrated near the water surface. The presence of longer bills in tropical species might relate to thermoregulatory constraints (it’s better to have small appendages in cold climates).

A polar specialist: the all-white Snow petrel (Pagodroma nivea). Photo by Samuel Blanc, from wikipedia.

Polar species tend to have a relatively low overlap in morphological characters compared to their tropical relatives, though it’s not immediately clear why this is so given that more resources are available to polar seabirds than to tropical ones. [Adjacent photo of Snow petrel Pagodroma nivea by Samuel Blanc © http://www.sblanc.com/] Possibly, polar petrels have evolved alongside a greater diversity of other seabird groups than have tropical ones, and as a consequence the number of niches available to polar petrels may have been relatively low (Spear & Ainley 1998). On the other hand, polar environments may provide seabirds with more niches, and hence more opportunities for specialisation. In contrast, tropical regions are comparatively sparse over large areas: consequently, seabirds that evolve here may have to be generalists.

Mostly blacks, whites and greys, but browns as well: patterns and pigmentation

Tahiti petrel (Pseudobulweria rostrata), image by Aviceda, from wikipedia.

Petrels are mostly patterned in greys, white and blacks. White undersides and dark dorsal surfaces are common and dark wing-tip markings, pale rump bands and facial masks and caps are seen in various species [Tahiti petrel Pseudobulweria rostrata shown here; image by Aviceda]. Some species – gadfly-petrels and prions in particular – have complex pigmentation pattern. The Snow petrel has entirely white plumage [see image above].

Bretagnolle's (1993) diagram of plumage variation seen within tubenoses. This paper is a must-read if you're properly interested in seabird colouration and its adaptive significance.

It may be somewhat surprising to those who know little about seabirds and expect all species to be patterned in grey, white and black to learn that quite a few petrels are solidly black or dark brown. There are black-brown species of gadfly-petrels, shearwaters and Procellaria petrels, and the Bulweria and Pseudobulweria species are wholly dark as well. Some populations of Macronectes (the giant petrels) are wholly dark brown and the Northern fulmar Fulmarus glacialis – generally familiar as a mostly white bird with a grey back and wings – also includes a mostly brown form in parts of its range.

Is there any consistency to this diversity of colours and patterns? That’s hard to answer. It might be ideal, for purposes of camouflage from prey and predators, for a seabird to be pale ventrally and greyish or bluish dorsally. However, numerous sometimes incompatible selection pressures mean that organisms do not necessarily evolve in an ‘optimal’ direction when it comes to pigmentation (Endler 1978, Burtt 1981). Seabird colouration seemingly incorporates sexually selected display traits, so there are evolutionary pressures to be showy and distinctive.

Masked shrike, by Claudia Becher, from wikipedia.

Plumage colours may also reflect the need for social cohesion (species that feed in groups need to be able to see conspecifics from a long way off), they may play a role in thermoregulation, and birds may sometimes use feather colour to reflect or absorb sunlight. Dark masks, for example, may help reduce glare and enable predators to see better in strongly lit environments. That last idea is mentioned in the ornithological literature with respect to the dark masks of peregrines and other raptors, and it’s even been suggested that raccoons and other mammals might benefit from their masks in the same way. A recent experimental study on shrikes provides support for the hypothesis, since Masked shrikes Lanius nubicus with dark facial masks were more effective hunters (being far better able to hunt facing the sun) than ones with artificial pale masks (Yosef et al. 2012). [Masked shrike image by Claudia Becher.]

Differential erosion of feathers according to melanin content. At top: the unpigmented primary tips of a Herring gull (Larus argentatus) are worn away while the dark remainder persists. At bottom: brown feather segments in the Snow bunting (Plectrophenax nivalis) are worn away during the year, meaning that these feathers appear to change from brown to black. From Strauch (1991).

The role of melanin in protecting feathers from abrasion and UV damage may also mean that feathers, or their tips, are dark for reasons unrelated to ecology or behaviour. Some petrel (and other seabird) species may also be coloured to mimic others (more on this later). Feeding style and prey type may also have some control over pigmentation, and diurnal and nocturnal species are obviously going to be under different pressures when it comes to camouflage. And do the selection pressures that act on nesting (often burrow-dwelling) petrels have as much, or even more, influence on their pigmentation than the pressures they experience while foraging and avoiding predators at sea? Remember that some petrels may be visiting their nest burrows for more than seven months out of the year. Species are also sometimes patterned or coloured the way they are because that’s what their ancestors were like – that is, their patterns and colours aren’t necessarily adaptive. Some biologists argue that exaptation is a very dodgy idea since it fails to account for (often under-studied) adaptational explanations for structures and behaviours. There may be some truth in this, but a historical, phylogenetic perspective demonstrates that exaptation must have occurred in many lineages.

Some of the graphs from Bretagnolle (1993), here showing the results of correspondence analyses between colouration, diet and feeding style in petrels and other tubenoses.

Bretagnolle (1993) looked specifically at the adaptive significance of tubenose pigmentation and concluded that no one factor had a dominant effect. However, foraging style and group size seemed to be the most important controlling factors. Small species that feed in groups (like prions and some gadfly-petrels) seemingly tend to be cryptically coloured in whites and greys, but dark upperparts may also aid concealment against the sea surface when aerial predators (like larger tubenoses and skuas) need to be avoided. Prominent countershading (like that present in many shearwaters) was suggested to correlate with the underwater pursuit of fish and the avoidance of aquatic predators.

These proposed explanations are somewhat speculative and, as I said above, the incompatibility of various of the selection pressures acting on pigmentation may mean that some (or many, or most) of these birds don’t really possess optimal coloration for their lifestyle. Bretagnolle (1993) also suggested that tubenoses might differ from the majority of other seabird groups in pigmentation since they tend to practise continuous feeding (that is, they forage constantly across the whole duration of their time at sea, rather than commuting to an area of prey abundance). In other words, they might not follow the same rules as other seabird groups.

And, yes, more on petrels to come. For previous Tet Zoo articles on seabirds, see...

Refs - -

Ashmole, N. P. 1971. Avian Biology Volume 1. Academic Press, London.

Bretagnolle, V. (1993). Adaptive Significance of Seabird Coloration: The Case of Procellariiforms The American Naturalist, 142 (1) DOI: 10.1086/285532

Burtt, E. H. 1981. The adaptiveness of animal colors. BioScience 31, 81-102.

Endler, J. A. 1978. A predator’s view of animal color patterns. In Hecht, M. K., Steere, W. C. & Wallace, B. (eds) Evolutionary Biology, Volume 11. Plenum, New York, pp. 319-364.

Habib, M. 2010. The structural mechanics and evolution of aquaflying birds. Biological Journal of the Linnean Society 99, 687-698.

Nelson, B. 1980. Seabird: Their Biology and Ecology. Hamlyn, London.

Spear, L. B. & Ainley, D. G. 1998. Morphological differences relative to ecological segregation in petrels (family: Procellariidae) of the Southern Ocean and tropical Pacific. The Auk 115, 1017-1033.

Strauch, 1991. Feathers. In Brooke, M. & Birkhead, T. (eds) The Cambridge Encyclopedia of Ornithology. Cambridge University Press (Cambridge), pp. 20-26.

Yosef, R., Zduniak, P. & Tryjanowski, P. 2012. Unmasking Zorro: functional importance of the facial mask in the Masked Shrike (Lanius nubicus). Behavioral Ecology doi: 10.1093/beheco/ars005

The views expressed are those of the author and are not necessarily those of Scientific American.

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