Tuesday, February 14, 2006

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.

11 Comments:

Blogger Hai~Ren said...

Extremely fascinating. That is something to take note of when we talk about reintroduction, or the recent talk about introducing lions and elephants into America to fulfil ecological roles left void by American lions, sabertooths and mammoths.

It does seem that eagle owls are among the most dominant predatory birds around - on par with sea eagles and golden eagles perhaps? It's at times like these that I'm reminded that food chains and food webs are so overly simplistic.

Oh, and right now I'm trying to wonder if an eagle owl would whoop a great horned owl's ass. Animal Face-off! ;)

4:04 PM  
Blogger Darren Naish said...

Thanks for your comments. The proposed re-introduction of megafauna to North America - so-called 're-wilding' - is pretty radical and controversial, and already there have been objections to the idea (see Nature 437: 476). As Steven Shay (a historian at WSU) pointed out, given that people inhabit and use the area that's proposed for re-wilding, objections will be many and the project could be regarded as counter-productive in terms of how these people perceive environmentalism. I also think that people will be screwing with the biosphere (more than they already have) by introducing African natives (like extant lions, cheetahs and so on) to a place where related species have since become extinct. They won't be re-creating a lost ecosystem, but creating a new one composed of alien species.

On Eurasian eagle owls vs Great horned owls, the latter is about as nasty, but in fact substantially lighter and usually smaller (450-510 mm compared to 530-730 mm, or thereabouts). So Bubo bubo would most likely win over B. virginianus, were one to try and kill the other.

11:38 AM  
Blogger Mike said...

This is a really balanced and well-considered look at a complicated question, Darren. I invite you to submit it to the next I and the Bird...our international coterie of bird bloggers and blogging birders would love it.

4:12 PM  
Blogger Tommy_Tyrberg said...

I think You shouldn't be too worried. It is true that Eagle Owl hunt raptors (I can add osprey to the list), but the only species that is known to be appreciably affected by their presence, at least here in Scandinavia, is the peregrine. In this case competition for nest sites is probably as important as predation.
Generally Eagle Owls will hunt just about anything, but rodents are usually the most important prey, except at coastal sites where larids often dominate.
As a matter of fact I think that the English countryside would be very suitable for Eagle Owls. In contrast to what most people think (even here in Sweden) Eagle Owls are not wilderness birds. They prefer a mosaic of forest, farmland and wetlands, and are often found quite close to human habitations (which is presumably also the reason they are relatively easy to exterminate). When not molested they can be amazingly unafraid of humans. I know of several pairs that breed on the outskirts or even inside towns and cities. In such cases the rats are undoubtedly the main attraction, though they undoubtedly appreciate the odd cat and pigeon too.
However should a pair settle in London I would suggest that You keep the ravens in the Tower indoors. Ravens hate Eagle Owls for good reasons.

7:45 PM  
Blogger Steve Bodio said...

Two thoughts.

The Kazakh eagle- falconers of Mongolia and Kazakhstan say that Eagle owls ("Ukhu") will kill Golden eagles at night.

And: Great horned owls were a serious predator of young Peregrines during the reintroduction program in the 1970's (I worked for the Peregine Fund on this). That they were so bad on some traditional sites in the northeastern US was one reason that urban sites became an important alternative.

4:14 PM  
Blogger Matt Mullenix said...

Darren

Your point that top-o-the-food-chain reintroductions should start from the ground up instead of the reverse is a great one---But destined to failure since slugs and locusts make such poor poster material! :-)

Steve's point about the Bubos and peregrines is important also, although I think it should be fairly easy to discover what your owls are eating. You know, I'm sure, that owl day-roosts are good pellet sampling spots and that active nests are often piled high with remains. There is little cryptic about an owl's diet.

7:08 PM  
Blogger Darren Naish said...

This is an old comment emailed to me by my good friend Carl Buell. It's been sat in my inbox for ages: I hope you don't mind me adding it here Carl. He wrote...

----

Hi Darren,

I still can't comment because I'm not a blogger user, but I wanted you to know I read the Eagle Owl post with interest. Good post. They all have been, but I know a little more about birds than turtles. That's an interesting problem you have. If it really is 500 pairs, that seems a bit much for accidental or even purposeful release. Perhaps it’s that dense secondgrowth taking over what used to be open farmland makes it easier for the diurnal raptors to roost in peace, but here in the states, (I’m in upstate New York) we’ve got a good number of Great-horned Owls (Bubo virginianus – our Eagle Owl – I hear them every night) and still tremendous numbers of crows and Red-tailed Hawks (Buteo jamaicensis) and increasing (at least locally) numbers of Cooper’s Hawks and Sharp-shinned Hawks (Accipiter cooperii and A. striatus). The Goshawks, Sparrowhawks and others you mention are also found on the European mainland, aren’t they? Why would the Eagle Owls be more destructive to those same birds in England? These aren't real questions Darren, I'm just typing out loud.

The two studies you site about prey preference may simply reflect local availability. I don’t see the owls going out of their way to kill raptors when the rabbits are plentiful and active at night, but I have nothing to base that statement on except it’s seemingly logical. The next few years are going to be interesting. Please let us know any further developments.

I can't wait for the "Naish Guide to Eocene Whales". I'm friends with Hans Thewissen and I worked with Phil Gingerich on two projects for Discover magazine back in the 1990s. I've probably drawn most of those creatures half a dozen times; Ambulocetus a dozen times...maybe I'll get it "right" soon. I've attached a jpeg of recent whale work. Paki, Rodho, and O. orca.

My best, Carl

http://www.olduvaigeorge.com

10:03 AM  
Blogger stephen said...

So in countries that have Eagle Owls there are no medium sized raptors, no songbirds and no mammals..Which country is this?

We've far too long been taught to be scared of the predator in this country by those that love to kill them.

Birds of prey will zoom back in numbers once persecution is stopped - look at buzzards, peregrines and the like. Hen harriers are scarce because of persecution, and if they are in healthy numbers then they won't suffer from extinction from top predators.

The RSPB is happy to have the Goshawk back though these may have been escapees.

7:23 PM  
Blogger Vesna said...

I understand all viewpoints but I find the whole discussion whether any species is “good” or “bad” in some ecosystem totally irrelevant. Animals are not limited to borders, they move freely, search for opportunities and find their niches regardless our “taxon archives” and our “paleo findings” and the opinion of our eco-experts whether we should “allow” their movement. By definition nature is dynamic system, which constantly changes and consists of extremely complex interactions between species, which we cannot fully comprehend. Spontaneous changes are all natural and we can just observe. We should focus on human impact, to minimize damage by our activities and we should let nature take its course without our interference.

11:15 PM  
Anonymous Anonymous said...

Steve Bodio,

There is nothing about eurasian eagle owls killing golden eagles. Golden Eagles are considered to be the most powerful birds of prey throughout their range, often displacing Bald Eagles and White Tailed Sea Eagles in direct competition. They have also been known to kill large animals including wolves that would make short work of an owl.


1. http://www.youtube.com/watch?v=Re644qgnCtw&feature=related

2:09 AM  
Anonymous Linda said...

I always enjoy reading about birds specially eagles and their characteristics. I'm a nature lover and I like to capture images of different bird species.

12:06 AM  

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