This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In the previous article we looked very briefly at a few basic aspects of petrel diversity, focusing in particular on their distinctive, tube-nosed bills (though, note: not unique to petrels but present across Procellariiformes). Here, I want to look at these birds in a bit more depth. We focus here on oil production and storage and also on reproduction, longevity, mortality and some related aspects of natural history. [Adjacent photo by JJ Harrison. Annoying fuzziness of these images due to scrunching - I have to reduce them as much as possible via the ‘save for web’ function.]
Oil, squirting oil, using oil, and why contain oil in the first place?
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It used to be thought that the petrel nasal tube acted as a nozzle allowing the birds to squirt stomach oil as a defensive measure. In fact, the birds squirt oil defensively through the mouth (though, when handled, they do sometimes drip oil from the nasal tube). This oil-squirting behaviour is most famously practised by fulmars (Fulmarus) but other members of the petrel group Fulmarini (all of which nest out in the open, rather than in burrows like many other petrels) do it as well. [Fulmar image below by T. Müller.]
Fulmars in fact are able to squirt with quite some accuracy up to a distance of a metre or two, and young fulmar chicks seem to squirt at just about anything that comes within reach, including their parents (the chicks later learn not to do this). The oil mats feathers together and destroys their water-proofing abilities, so soiled birds generally die from chilling and/or drowning (fulmars seem ok when soiled by other fulmars, however, being able to preen and wash the oil out). There are cases where sea-eagles have died after being squirted by fulmars (Dennis 1970) and about another 20 species are known to have been killed as well, including herons, gulls, owls, falcons, crows and a few unfortunate small passerines (Broad 1974, Booth 1976, Warham 1977). A fulmar kept in captivity with gulls and auks managed to kill five of them by soaking them with oil. There aren’t many images of oil-spitting petrels out there, but I managed to find the one below. It’s from anneleeuw’s flickr site.
Where does this oil come from? A peculiarity of tubenose biology is that adults and young contain large quantities of oil in the proventriculus (e.g., Weimerskirch & Cherel 1998, Cherel et al. 2002).
This oil varies from clear to deep reddish-brown and solidifies to form a wax when cool. It has been used medicinally by humans as well as in the lubrication of machinery and of course in providing illumination. Oil from Short-tailed shearwaters Puffinus tenuirostris has been used in the manufacture of sun-tan lotion and as a coat shine for horses (Warham 1977).
It used to be thought that the birds manufactured the oil themselves. In fact, its chemical composition shows that they obtain it by rupturing their animal prey, differentially digesting the proteins and lipids, and concentrating huge quantities of energy in oil form (Warham 1977). This serves as an energy reserve for adults on long foraging trips but also allows them to provision their chicks with a condensed, lightweight (specific gravity 0.88) and highly nutritious substance in between periods of absence. Adult petrels tend to ‘overstock’ their chicks with oil, perhaps as a precaution against unexpectedly long trips away from the nest, and the oil-squirting abilities of chicks show that they contain a seemingly large amount of oil at any one time. They couldn’t do this if they were only being given the bare minimum to stay alive. Adults actually need to store the energy they gain on their long foraging trips in order that they can afford the many short trips required to feed the chick (Weimerskirch et al. 2003). In other words, the long foraging trips seem to allow parents to recoup their energetic losses.
Pelagic life
As a generalisation, petrels are stiff-winged soaring birds with low wing loadings. Wingspans range from about 60 cm in some of the prions (Pachyptila) to 2 m in the awesome, vulturine giant petrels (Macronectes). Petrels are pelagic, covering substantial distances across the open ocean. Some species are known to travel distances of 15,000 km on single foraging trips. This particular factoid concerns the Short-tailed shearwaters Puffinus tenuirostris that breed on the southern coast of Australia yet were shown by satellite tracking to forage at the limits of Antarctic shelf-ice (Klomp & Schultz 2000).
After breeding, adults disperse. Some species (like prions, some gadfly-petrels and shearwaters like the Wedge-tailed shearwater Pu. pacificus [shown here] in the tropic Pacific and Indian oceans and Townsend’s shearwater Pu. auricularis in the eastern Pacific) are sedentary and only move to adjacent waters, while other range far and wide, even travelling to other oceans. Most Manx shearwaters Pu. puffinus, for example, breed in the north-east Atlantic (some breed off Cape Cod in the west as well*) and then disperse to the south-west before returning north, but it seems that some travel round Cape Horn and end up going north on the ‘wrong’ side of the Americas (Harrison 1988).
* This was only discovered in 1973 (Bierregaard et al. 1975). There are indications that Manx shearwaters bred off the east coast of North America in prehistoric and historic times but became extirpated relatively recently.
Similarly, Cory’s shearwater Calonectris diomedea breeds in the Mediterranean and then disperses right across the Atlantic with many ending up around South Africa and Natal. However, some go round Africa into the western Indian Ocean. These 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 the Africa, not the west side.
Petrel predators
We saw a moment ago that petrel chicks on nests need to defend themselves from raptors and also from predatory gulls and skuas. Asio owls, caracaras and Buteo hawks are also documented predators of petrels. On Codfish Island, New Zealand, Weka Gallirallus australis were taking such a major toll on Mottled petrels Pterodroma inexpectata and Cook’s petrel Pt. cooki that the decision was eventually made to remove them (Imber et al. 2003).
More unusual predators includes crabs, snakes, tuatara and skinks, all of which are on record as eating petrel eggs and/or nestlings. Foxes predate on some petrel populations and stoats introduced to New Zealand are known to have a significant impact on breeding Hutton’s shearwaters Pu. huttoni (Cuthbert & Davis 2002). Red deer Cervus elephus on the Scottish island of Rhum famously took to biting the heads, wings and legs off Manx shearwater chicks, apparently because of calcium deficiency (Furness 1988). Petrels are not great movers on the ground – their legs are weak, they mostly move with a shuffling gait, and they generally need to climb inclined surfaces (slopes, cliffs or tree trunks) for a takeoff. Needless to say, the altricial chicks are even more helpless, so they can fall prey to just about any animal capable of finding and overpowering or dismembering them.
Domestic cats – introduced to various islands used by nesting petrels – represent a serious threat and seem to have caused massive declines or even the local extinction of some species. To give some idea of how serious cat predation can be, note that about 5000 Cook’s petrels seem to have been killed by cats on a single New Zealand island in a single breeding season (Imber et al. 2003). A colony of Bulwer’s petrel Bulweria bulweria that previously nested on the southwest coast of Gran Canaria was seemingly eradicated by cats (Luzardo et al. 2008). On Marion Island in the southern Indian Ocean, Soft-plumaged petrels Pt. mollis had extremely poor rates of breeding success due to cat predation (just 7.9% of pairs succeeded in raising chicks) while the Common diving-petrels Pelecanoides urinatrix there became extinct some time round about 1952, apparently as a direct result of cat predation. Eradication of cats on Marion Island in 1991 resulted in improved breeding success in some (but not all) of the affected species (Cooper et al. 1995) and cat eradication elsewhere has resulted in petrel recovery (Keitt & Tershy 2003).
However, while eradicating cats might seem like a great move, this isn’t necessarily always the case. People haven’t just introduced cats to islands – they’ve also introduced rats and mice. Freed from cat predation, rats on some islands have experienced so-called ‘mesopredator release’, increasing in number and becoming more significant predators of petrels (Rayner et al. 2007) [Laelaps blogged about this research soon after it was published]. A factor that may complicate attempts to assess the impact of rat predation is that adult petrels may persist for a long time (due to their longevity: read on) even when they are totally failing to raise chicks due to increased rat predation. Conversely, while cat predation may result in greater losses of adults, it may not necessarily have the same devastating impact on nestlings that rat predation does (though... it might). Accordingly, some workers urge that we need to be careful when deciding how to deal with alien predators (Le Corre 2008).
Breeding, brooding, ageing and claims of immortality
Petrels are long-lived, slow-breeding birds that put a massive amount of effort into raising their chicks. Individuals of some species (the Manx shearwater being the classic example) might be at their nesting grounds from February all the way to September, October or even November. They produce a single, proportionally large egg – somewhere round about 21-25% of the adult’s mass (and thus within the upper range limit for birds)* – and have an incubation period that is round about, and sometimes exceeds, 50 days (Warham 1983). Cases where abandoned, second chicks have been adopted by breeding pairs suggest that parents aren’t able to provision two chick, and furthermore that chicks exhibit high levels of aggression towards one other (Archuby et al. 2010).
* Caveat # 1: after I said on twitter that some tubenoses exceed kiwi in relative egg mass (in the Little spotted kiwi Apteryx owenii, egg mass can be 22% of body mass), the brilliant Mike Dickison reminded me that allometry needs to be considered. To paraphrase Mike, avian egg mass scales to body mass at about the two-thirds power, so a 200 g bird that produces an egg 25% of body mass is much less remarkable than a 1000 g bird that does likewise. Caveat # 2: while it’s relatively easy to find data on egg mass as a percentage of adult female body mass (Rahn et al. 1975), most birds produce clutches of several or many eggs, and finding clutch size expressed as a percentage of body mass is less easy. Anyway, viewed within the context of both allometry and of total clutch mass, the single eggs of tubenoses and kiwi are not necessarily that remarkable in terms of parental investment, scaling linearly for body size with those of their relatives. It’s the fact that just a single enormous egg is produced that’s so unusual: a risky strategy that relies on low egg/juvenile mortality.
Chicks grow slowly. Cory’s shearwater, for example, spend about 90 days in the nest prior to fledging. There are indications that this slow development is related to previously unappreciated peculiarities of tubenose biology. Maternal antibodies, for example, persist for an unusual length of time (20 days or so) in Cory’s shearwater (Garnier et al. 2011).
I said that petrels are generally ‘long-lived’. By this I mean that individuals of some species (like the giant petrels Macronectes) can live for over 50 years (and there are suggestions that some albatrosses can exceed 70 years). [Image above by TheBrockenInaGlory, use licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license. I apologise profusely for not correctly attributing information earlier: this was an unfortunate oversight]. Longevity (though not necessarily of exactly this scale) is not unique to giant species. An individual of Bulwer’s petrel Bulweria bulwerii, for example, was at least 24 years old when last recorded in 1992 on Johnston Atoll in the Pacific.
Some tubenoses (including giant petrels and also storm-petrels) have been at the centre of argument and interest over the role and function of their telomeres. Telomeres are DNA fragments that ‘cap’ the ends of chromosomes, shortening at each cell division event, and also shortening as oxidative stress takes its toll on an animal over time. Older individuals thus have shorter telomeres than younger ones. Sexual differences in telomere length have been reported in some insect, squamate and mammal species (including Homo sapiens).
While the database we have on telomere length is not (so I understand) all that comprehensive, tubenoses are especially interesting because telomere length does not change all that much in adults (Foote et al. 2010). Furthermore, giant petrels were the first birds in which a sexual difference in telomere length was reported, with males have shorter telomeres (Foote et al. 2010). It may or may not be coincidental that giant petrels are unusual among petrels in exhibiting strong sexual dimorphism – perhaps the strongest dimorphism reported in any seabird. What, if any, impact this has on the biology and behaviour of these birds, or whether it skews survival rates between the sexes, is not entirely clear... or, it’s not clear to me, anyway. Do say if you know otherwise.
Haussmann & Mauck (2008) reported that telomeres appeared to lengthen during growth in Leach’s storm-petrel Oceanodroma leucorhoa. This probably doesn’t mean that storm-petrels are immortal (as is intimated in some popular re-tellings of this research) but it is probably related to the fact that these birds live about four times longer than expected for their size. One explanation for this pattern is that hatchlings are variable in telomere length and that individuals with long telomeres are the ones that make it to old age. In other words, it isn’t that telomere length is really increasing with age; rather, old individuals are the ones that made it to old age because they’re the ones with the long telomeres!
MUCH more on petrels still to come. For previous Tet Zoo articles on seabirds, see...
To the Sahara in quest of dinosaurs (living and extinct) (includes discussion of gulls and terns)
When bivalves attack (or: bivalves vs birds, the battle continues)
A symbiotic relationship between sunfish and… albatrosses? Say what?
Refs - -
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