Tetrapod Zoology

Fish owls in reverse

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By popular request, in this and in a few later articles I’ll be reposting the rest of the Ten Bird Meme text originally posted at Tet Zoo ver 1 in 2006 (where appropriate, I’ve added corrections and updates). The ‘Ten Bird’ birds I’ve covered so far are Ifrita, Shoebill, Tibetan ground-pecker, and Flying steamer duck. And here we continue with… Blakiston’s fish owl Bubo blakistoni [images from here].

If you think evolutionary convergences are cool, then you’ll love reversals. Morphological features or aspects of behaviour that have been modified during the evolution of a lineage don’t have to ‘stay’ changed – as Louis Dollo thought they did (this is where so-called ‘Dollo’s Law’ comes from) – they can change back to the ancestral state if this is what works (of course, they’re not really ‘changing back’; rather, they’re becoming modified further, such that they only resemble the ancestral state). Of the four Asian fish owl species, Blakiston’s is the biggest and most formidable, with a wingspan approaching 2 m and a total length of c. 60 cm (could it beat a Eurasian eagle owl in a fight? Perhaps). Endemic to Siberia, eastern China, Japan, Sakhalin Island and other eastern Asian islands, it inhabits cool, remote forests and can cope with harsh winters. Some have suggested that during its history it may have suffered from competition with sea eagles (Hume 1991), which are similar in size and ecological requirements. Sadly, this remarkable bird is highly endangered.

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As is well known, owls in general have soft plumage and unusual fringes of tiny barbs along the leading margins of their flight feathers. These features – both specializations that permit silent flight in a group of birds that rely on sensitive hearing – are derived relative to the condition that owls inherited from their ancestors. But fish owls don’t need to be silent given that they’ve specialized to prey on animals that live under water, and they’ve consequently reversed back to the primitive condition. Fish owls also have longer legs than those of other owls, and their feet sport rough spiny scales that resemble those of fish-eating raptors like ospreys (Fogden 1984). B. blakistoni is unique among fish owls in having feathered legs. Interestingly, fish owls walk down to the water’s edge and will even wade into the shallows. They then sit motionless, waiting for prey to come within range. They don’t just eat aquatic prey, but also terrestrial birds and mammals. Frogs are important in the diet of B. blakistoni but crayfish – said by some authors to be a major food item – are apparently not (Slaght & Surmach 2008). Fish owls are also remarkable among owls in reportedly feeding on carrion.

How does B. blakistoni fit into owl phylogeny? Until recently, the fish owls were considered to represent a distinct genus, Ketupa. Ketupa was considered closely related to, but distinct from, the eagle owls Bubo. Recent genetic studies have found instead that the Ketupa species are nested within Bubo (as is Nyctea, the Snowy owl), and consequently both Ketupa and Nyctea have been sunk into synonomy with Bubo (Wink & Heidrich 1999, Olsen et al. 2002). These results are supported by osteological characters, but unfortunately this data has yet to be published (it’s included in N. Ford’s 1967 phd thesis, and I’ve heard that a version of this is due to be published soon [UPDATE: still no news on this in 2008/9]). The feathered legs and other characters of B. blakistoni suggest that this is the most basal of the fish owls… but I could be wrong.

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PS: an important new paper on B. blakistoni appeared last year: Slaght & Surmach (2008). It’s available free here. See also the Blakiston’s Fish Owl Project [screen-capture shown here]: download a Blakiston’s fish owl ringtone and buy fish owl merchandise – seriously!

For previous Tet Zoo efforts on Strigiformes see Why do some owls have ear tufts and (from ver 1) Eagles owls take over Britain and British eagle owls: an update.

Refs – -

Fogden, M. 1984. Fishing owls, eagle owls and the snowy owl. In Burton, J. A. (ed) Owls of the World. Eurobook (Oxford), pp. 53-85.

Hume, R. 1991. Owls of the World. Parkgate Books (London).

Olsen, J., Wink, M., Sauer-Gürth, H. & Trost, S. 2002. A new Ninox owl from Sumba, Indonesia. Emu 102, 223-231.

Slaght, J. C. & Surmach, S. G. 2008. Biology and conservation of Blakiston’s fish owls in Russia: a review of the primary literature and assessment of the secondary literature. Journal of Raptor Research 42, 29-37.

Wink, M. & Heidrich, P. 1999. Molecular evolution and systematics of the owls (Strigiformes). In König, C., Weick, F. & Becking, J.-H. Owls: a Guide to the Owls of the World. Pica Press (London), pp. 39-57.

Comments

  1. #1 Neil
    January 14, 2009

    Wow a cool bird. Im not sure Ive heard of fishing owls. How does Blakiston’s compare in size to the European Eagle Owl? Is it a contender for its ‘biggest owl species’ crown.

  2. #2 Turdus
    January 14, 2009

    “(could it beat a Eurasian eagle owl in a fight? Perhaps)”

    Thank you for answering what the little boy (or girl) in all of us were wondering! Another great post.

  3. #3 Dartian
    January 14, 2009

    Of the four fish owl species, Blakiston’s is the biggest and most formidable, with a wingspan approaching 2 m and a total length of c. 60 cm (could it beat a Eurasian eagle owl in a fight? Perhaps).

    For anyone who’s seriously interested in owl-on-owl violence, I can recommend Heimo Mikkola’s book Owls of Europe (1983, T. & A.D. Poyser, Calton). This book has a detailed ‘who eats who’ chapter, where the author summarises published records* of owls as prey of other owls, of owls as prey of diurnal raptors, and of owls as predators of diurnal raptors.

    * It’s a reasonably large data set with 1,363 cases of owls killed by other owls, 752 cases of owls killed by diurnal raptors, and 748 cases of diurnal raptors killed by owls.

    These data strongly suggest a rule with remarkably few exceptions: the larger owl is always capable of killing the smaller, but almost never vice versa. This seems to hold true even when the size difference is quite small (e.g., there are records of tawny owls Strix aluco kiling long-eared owls Asio otus, but not the other way around).

    The largest Western Palearctic species, the Eurasian eagle owl Bubo bubo has on occasion been recorded as killing owls representing every other European species, starting from the only slightly smaller snowy owl Bubo scandiacus to the tiny pygmy owl Glaucidium passerinum. Conversely, in Mikkola’s data set, there are no records of the eagle owl itself having been predated on by any other European owl.*

    * It should be noted that data on cannibalism are unfortunately not included in the data set.

    Thus, as for Darren’s question about who would win in a fight, B. bubo or B. blakistoni, it probably would be whichever individual is larger. (There is some overlap in size between these two species.)

  4. #4 Dartian
    January 14, 2009

    …and slightly OT, but while speaking about the enemies of owls, one should not forget the chimpanzee.

  5. #5 Carlos
    January 14, 2009

    Strigids and indeed quite interesting, considering that they produced specialized taxa like this one. Is there any prehistoric owl post I missed?

    @Dartian: Wow, did that chimp killed the owl?

  6. #6 Adam
    January 14, 2009

    Is the African Pel’s Fishing owl (Scotopelia peli) related to Ketupa or has it evolved its fishing behaviour convergently? If the latter it would be interesting to find out if it has undergone the same reversals. Though they live here in SA, I’ve never seen one, the birders in my department say its in the ‘megatick’ category.

  7. #7 Dartian
    January 14, 2009

    Carlos:

    Wow, did that chimp killed the owl?

    I don’t know, I hope not.

  8. #8 Darren Naish
    January 14, 2009

    Response to Adam…

    The African fish owls conventionally included in Scotopelia are nested within Bubo as well, according to genetic data. However, rather than forming a clade with the Asian fish owls (those previously called Ketupa), Olsen et al. (2002: cited above) found Pel’s fish owl to be closest to the Malaysian eagle owl Bubo sumatrana. If this is correct then Asian and African fish owls have indeed evolved their fishing specialisations convergently. The hypothesis that Scotopelia and Ketupa form a clade has been proposed but seems based only on the fact that the species concerned have ‘similar specializations for feeding on fish; otherwise they are quite different in general appearance’ (Fogden 1984, p. 60).

    Like Asian fish owls, the African ones have unfeathered tarsi, spicules on their feet, poorly developed facial discs, and feathers that lack the ‘silencing’ adaptations of other owls. The reversals discussed above for Blakiston’s fish owl therefore evolved at least twice within the group. Neat.

  9. #9 Dr Vector
    January 14, 2009

    they can change back to the ancestral state if this is what works (of course, they’re not really ‘changing back’; rather, they’re becoming modified further, such that they only resemble the ancestral state)

    Sometimes, but sometimes they really are changing back, in that they are returning to ancestral genetic and developmental pathways. You could probably (given the right tools and data) distinguish between real reversals that involve reactivating ancestral pathways, and pseudo-reversals that arrive at an ancestral morphology by a different genetic or developmental route. My guess is that someone already has and I just don’t know it. Thoughts?

  10. #10 Dartian
    January 15, 2009

    Darren:

    As is well known, owls in general have soft plumage and unusual fringes of tiny barbs along the leading margins of their flight feathers. These features – both specializations that permit silent flight in a group of birds that rely on sensitive hearing – are derived relative to the condition that owls inherited from their ancestors. But fish owls don’t need to be silent given that they’ve specialized to prey on animals that live under water, and they’ve consequently reversed back to the primitive condition.

    This is what I find perhaps the most interesting thing about fishing owls. One would think that the specialized fringes of owl feathers, once evolved, would be favoured and maintained by natural selection. After all, wouldn’t silent flight be generally beneficial to any nocturnal bird, regardless of its dietary habits? However, this repeated loss of ‘silent’ flight in both Asian and African fishing owls strongly suggests that maintaining such specialized feathers is evolutionarily costly.

  11. #11 Tommy Tyrberg
    January 15, 2009

    As for whether blakistoni is the biggest extant owl, “I think so”. There isn’t a lot of data on the species (it is quite rare and threatened), but what little there is suggests that it is 5-10 % larger than the Eagle Owl. However the difference seems much larger in the field because blakistoni is almost incredibly stocky.
    As for who would win in a fight, it is impossible to say without actual field data. A Mute Swan is significantly larger than a Whooper Swan, and also seems a lot more aggressive, but in inter-species fights the whoopers almost invariably win.
    Adam: If you get a chance to visit northern Botswana Pel’s Fishing Owl is reasonably easy to see along the lower Okavango River (which is very much worth visiting in any case).

  12. #12 shiva
    January 15, 2009

    So the biggest and baddest of living owls shares a range and a primary diet with the biggest and baddest of living eagles. I wonder if there’s a connection?

  13. #13 Blind Squirrel FCD
    January 16, 2009

    Dartian said:

    This is what I find perhaps the most interesting thing about fishing owls. One would think that the specialized fringes of owl feathers, once evolved, would be favoured (sic) and maintained by natural selection.

    You bet it is costly. It increases drag on the wings and makes flight more costly in terms of energy.

  14. #14 Dartian
    January 16, 2009

    Tommy:

    A Mute Swan is significantly larger than a Whooper Swan, and also seems a lot more aggressive, but in inter-species fights the whoopers almost invariably win.

    The whoopers sometimes don’t play fair, though. I know of one case where a pair of whooper swans attacked a pair of mute swans (at the latters’ breeding area). The female mute swan fled but the male was cornered by the two whoopers, who proceeded with viciously attacking it. This 2 against 1-fight ended with the mute swan being killed.

    I know of other instances where whoopers have violently displaced mutes, so you’re right in that the whooper swan usually has the upper hand in interspecific conflicts. But not always. I’ve myself witnessed only once an interspecific swan fight*, but that time it was the mute swan who was both the aggressor and the winner. Some zookeepers are also of the opinion that the mute swan tends to be dominant over the whooper in waterfowl collections (but this, of course, need not hold true in nature).

    * And I mean a real fight, not some slight move-out-of-my-way pecking.

    Finally, to further show that swan vs. swan conflicts are not straightforward situations where the bigger always wins, there have been instances (admittedly under unnatural conditions) where the much smaller black swan has been more than a match for the larger mute swan.

    Blind Squirrel:

    You bet it is costly. It increases drag on the wings and makes flight more costly in terms of energy.

    I wasn’t suggesting otherwise; my comment was rhetorical (hence “One would think”, not “I think”).

  15. #15 JuliaM
    January 16, 2009

    “(could it beat a Eurasian eagle owl in a fight? Perhaps)”

    Well, thanks to the remarkable photos in the newspapers this morning, we certainly know the outcome of any buzzard v out-of-place phalarope contest… ;)

  16. #16 shiva
    January 16, 2009

    OMG. If i didn’t know that the Daily Telegraph was a real (and generally quite conservative/right-wing) paper, i would have thought that swan story was an entirely made up, slightly-subtler-than-the-Onion parody of British Tory racism…

    Those nasty, vicious, “dirty fighting”, immigrant BLACK swans, threatening our native British WHITE swans… and if they win, the CATHEDRAL will FALL DOWN!!!!1!!!

    And we don’t even know how they got into the country! They could be ILLEGALS! And, even worse, we’re “prevented by law from intervening”! It’s Political Correctness… gone MAD!!!!11!!!

    /sarcasm

    (Incidentally, i’m sure the “prevented by law from intervening” bit is false – Australian black swans, as a non-native species, would not be protected under any UK wildlife law, and i’m sure it would be perfectly legal to capture them and take them into captivity, say at a local zoo or bird sanctuary. And i’d be willing to bet that the “cathedral will fall down” legend is a load of old cobblers…)

  17. #17 Tommy Tyrberg
    January 19, 2009

    As for Mute/Whooper conflicts we are seeing them on a large scale here in Sweden where the formerly rare Whooper is increasing strongly and gradually driving Mutes away from their breeding haunts.

    Shiva:
    Blakiston’s Fish Owl and Steller’s Sea Eagle indeed occur in the same general area, but they don’t occur together. The Sea Eagle is coastal while the fish owl lives along forested rivers. At least that is the case in Hokkaido which is the area I have experience from.