Eagle owls take over Britain

Popular mythology has it that the introduction of the 1976 Dangerous Wild Animals Act led to the mass release of numerous pet leopards, pumas and god knows what else into the British countryside, and it’s these former pets that today haunt our moors and wooded areas. That’s a subject for another time, but right now a very similar subject, but concerning a very different kind of animal, is getting lots of coverage in both the popular and zoological press: the apparent presence in the British countryside of numerous feral Eurasian eagle owls Bubo bubo. A formidable bird, it can reach 4 kg with a wingspan of 1.5 m and a length of over 70 cm. What’s this all about, and what’s the big deal anyway?

Officially, eagle owls are not British natives. Well, ok, actually they are on the ‘British list’, but then so are Magnificent frigate birds and Red-billed tropic birds (basically, any bird seen within the boundaries of the British Isles becomes a member of the ‘British list’). If they do occur here today, it’s as vagrants from continental Europe. So the apparent presence of numerous eagle owls here right now – as many as 500 pairs (some of them breeding) according to a recent TV programme – must result from accidental escape, or deliberate release. Unlike the case with British big cats, the existence of feral eagle owls is not doubted by officialdom. They are here: it’s official.

The question now is.. should we ‘keep’ them, or should we make efforts to get rid of them? Herein lies the debate. We know without doubt that Britain had eagle owls in the recent past as they’re known from palaeontological and archaeological samples. To be sure on this I checked the literature. My usual port of call on British Pleistocene vertebrates is Stuart (1974), but he doesn’t list bird taxa. However, specific British Bubo bubo specimens from the Pleistocene were described by Harrison (1979, 1987). Intriguingly, there’s a new spin on this, mentioned by Jim Giles (2006) in his Nature article on the British eagle owl problem. According to Giles, John Stewart (of University College London) has data indicating that eagle owls survived here for longer than had been thought before: ‘ornithologists had previously assumed that remains [from the past 10,000 years] came from tame eagle owls that had been imported for hunting’. Inevitably people have compared this with the recent discovery that lynxes, similarly, were around in Britain until just a thousand or so years ago. If Stewart has data indicating survival of British eagle owls right up to (say) the dawn of the agricultural revolution, he hasn’t (to my knowledge) published it. Altogether it strengthens the case for eagle owls being regarded as ‘rightful’ members of the modern British avifauna, and if we have them back here by accident.. well, that’s ok.

But should we have them back here? This is the problem. Ok, it’s all very nice, but arguments invoking the former presence of a taxon within a region are never particularly convincing because, after all, pretty much all of the Northern Hemisphere’s megafauna naturally inhabited Britain at some time within recent geological history. So while I welcome the idea of having eagle owls back in our fauna, I feel that it’s problematical, given that the reappearance of any arch predator is dependent on how intact the rest of the food chain is. It’s as if we’re hoping that we can reconstruct the country’s ancient ecosystem by building from the top down. There is lots of talk of reintroducing lynxes and wolves, but not so much talk going on about boosting the numbers and diversity of ground-dwelling slugs, rodents or passerines. Yet it follows that the trophic pyramid that will hypothetically support these arch predators must be reconstructed at its lower levels before things further up are going to fit in nicely. Rabbits. Yes, nowadays we have lots of rabbits that make up a lot of biomass, and they weren’t here when eagle owls and lynxes and wolves were truly native, so that might repair part of the problem (rabbits are not native to Britain and were introduced by the Normans). Indeed some studies find eagle owls to prefer rabbits to all other prey (Hume 1991). If this is right, then things maybe aren’t so bad after all. In fact we have too many rabbits as it is, so more predation of them is welcome.

However… other studies find eagle owls to predate mostly on birds (Bocheński et al. 1993) with some studies finding 83% of eagle owl diet (by weight) being made up by avian prey (Everett 1977). And even if this weren’t the case (and the owl population was still mostly concentrating on rabbits), there is still the fact that eagle owls are opportunistic predators that will still kill animals we don’t want them to. So while - back when eagles owls were healthily distributed natives - they could take their pick of whatever, without this being a problem, things today aren’t so rosy, and those potential prey species themselves are depleted in numbers, or even endangered or requiring protection. Indeed the RSPB has voiced concerns that eagle owls may start to impact significantly on Black grouse Tetrao tetrix, Hen harriers Circus cyaneus and other endangered species.

You see, perhaps the most interesting thing about eagle owls is that they are experts at intraguild predation: in other words, very very good at killing other raptors, and in fact at virtually eliminating them from an area. They routinely take Long-eared owl Asio otus, Goshawk Accipiter gentilis, Sparrowhawk A. nisus, Peregrine Falco peregrinus, Gyrfalcon G. rusticolus, Merlin F. columbarius and Rough-legged buzzard Buteo lagopus. More exceptional are cases of predation on Snowy owls Bubo scandiaca (note: no longer in its own genus), young White-tailed eagles Haliaeetus albicilla, and other eagle owls. Everett (1977) wrote that ‘up to 5% of [eagle owl] total prey may consist of other birds of prey and … these may make up as much as 36% of all the bird food consumed in some regions’ (p. 93). It seems that eagle owls take these birds while they are roosting, mostly (I assume) by sneaking up on them from behind. In fact, so significant are eagle owls on other raptors that some populations of Long-eared owls appear to migrate specifically because of eagle owl predation (Erritzoe & Fuller 1998).

I don’t need to tell you that, right now, Britain does not have raptor populations capable of dealing with this sort of predation. Like it or not, the presence of this species is bad news for the raptors we have. So the bottom line is that, while having eagle owls back in Britain is very nice, it is not good news for our native fauna, given the state it’s currently in, and in an ideal world, we would need to have the rest of the ecosystem restored before we could bring in the arch predators. However, all of this is academic given that it’s already too late. The eagle owls are here, and the native raptors will suffer.

The photo used above was swiped one of the digit-sight pages.

I’ll admit that today I didn’t spend much time thinking about owls. Mostly it was devoted to work on the British dinosaurs manuscript, but there is also the story of Mark Witton’s giant Thalassodromeus head and some late news on elephant skulls and the Cyclops myth. Details on these will be posted here in the near future, and while going through old files on disk I found much text that will be recycled for posts at some stage soon: Homotherium and the Piltdown cats, the lost cassowaries, Naish’s guide to Eocene whales, hybridogenesis and the necromonger frogs, and the world of worm lizards. I’d like to say a huge thank you to those of you who have made positive and supportive comments about my blog posts – it’s really appreciated.

Refs - -

Bocheński, Z., Tomek, T., Boev, Z. & Mitev, I. 1993. Patterns of bird bone fragmentary in pellets of the Tawny owl (Strix aluco) and the Eagle owl (Bubo bubo) and their taphonomic implications. Acta zool. cracov. 36, 313-328.

Erritzoe, J. & Fuller, R. 1998. Sex differences in winter distribution of Long-eared owls (Asio otus) in Denmark and neighbouring countries. Vogelwarte 40, 80-87.

Everett, M. 1977. A Natural History of Owls. Hamlyn, London.

Giles, J. 2006. Bird lovers keep sharp eye on owls. Nature 439, 127.

Harrison, C. J. O. 1979. Birds of the Cromer Forest Bed series of the East Anglian Pleistocene. Transactions of the Norfolk and Norwich Naturalists’ Society 24, 277-286.

Harrison, C. J. O. 1987. Pleistocene and prehistoric birds of south-west Britain. Proceedings of the University of Bristol Spelaeology Society 18, 81-104.

Hume, R. 1997. Owls of the World. Parkgate Books, London.

Stuart, A. J. 1974. Pleistocene history of the British vertebrate fauna. Biological Reviews 49, 225-266.