When books die. And owls. I don't mean "when owls die"... I just mean: owls. As in: this article is mostly about owls.

There's something they don't tell you about freelance writing. It's about all the fails: the many, many projects that get pitched, worked on and made into proper presentations that then get sent to book fairs, interested companies and so on, but ultimately explode on the launch pad, or die a slow, lingering death. I don't know if it's that I'm especially unlucky, or if it's that I've pitched an unusually high number of books, or if it's that I've genuinely worked on a high number of projects that were never destined to succeed but, whatever, I've now worked on loads of failed book projects. It's not all bad, by the way - you still get paid for the time and work you've put in.

Anyway, the reason we're here is that one of those (so far) unsuccessful book projects is a big one on the evolutionary history and diversity of birds. To give you some idea how far down the line this project went, check out the various screen captures. I can't mention company names or whatever, but things on this one have definitely failed and I'm still interested in seeing the project through to publication (and, no, I am not putting the book out there and then trying to sell it - I have learnt from bitter, bitter, bitter experience that this idea just doesn't work out, at least not without starving and losing your home through total loss of income). If you can help, or are interested, you know where to find me.

Long-time readers will know that, thanks to various other failed book projects, I have tons of un-used text sitting around on all manner of zoological subjects. When time and inclination allows, I sometimes update and recycle bits of said text for Tet Zoo. As you read this, I'm away from my desk and out in the field, and as I didn't have time to generate much novel Tet Zoo content, I decided on a whim to recycle stuff from the bird book. Given that I wrote briefly about owls the other day, I decided to use the owl text (updated a little, and with references). Enjoy.

Introducing owls

Few groups of birds are as strongly associated with mythology, legend and superstition as the mostly nocturnal, predatory owls, properly called the strigiforms. Owls are well known for their exceptional eyesight and hearing, and for being equipped with a unique plumage that allows near-silent flight. Although owls as a group exhibit relatively little variation in shape or behaviour, the approximately 225 living species have adapted to virtually every conceivable habitat, from temperate woodlands and tropical forests to tundras, deserts, grasslands, and marshes. There are some excellent books on owl diversity: my three favourites are Wink et al.'s prohibitively expensive Owls of the World (second edition), John Burton's 1973, err, Owls of the World and Michael Everett's 1977 A Natural History of Owls.

A number of specializations seen throughout the body make owls among the most distinctive of birds. They combine a peculiar skull and facial anatomy with extraordinary eyes and ears, unusual feathers, and highly modified legs and feet. Their sensory abilities are phenomenal. Their enormous eyes are directed forward and a substantial part of their field of vision - corresponding to 50-70° - overlaps, enabling excellent depth perception. The exceptional ability to rotate the head is well known and improves an owl's ability to locate and pinpoint prey.

Owl eyeballs are tubular rather than spherical, and the ring of bones embedded within the eyeball - the sclerotic ring - is shaped like a turret and is immovably fixed to the edges of the eye socket. This tubular shape means that both the cornea and lens can be as big as possible relative to the retina, the light-sensitive region at the back of the eye. The owl retina is proportionally large and covered with a particularly high number of light-sensitive cells (as many as 56,000 per square mm in the Tawny owl Strix aluco).

Owls have a large number of feathers for their size (10,000 in the Long-eared owl Asio otus), and their feathers are also soft and downy compared to those of other birds. Soft fringes that run along the rear edges of the wing feathers enable near-silent flight: these should be visible in the photos below [the bottom one is by Kersti and is from wikipedia].

A disk of stiff feathers - known as the facial disk - is thought to help channel sounds towards the enormous, slit-shaped ear openings, in much the same way as the ear flaps of mammals help to 'collect' sound.

Asymmetrical ears

Owl ears are comparatively huge (though hidden by feathering) and unique in often being asymmetrically positioned: one is higher up on the side of the head than the other. In some owls, such as eagle owls and the Strix wood owls, it's the positions of the fleshy, external parts of the ears that are asymmetrical. In others, such as some Asio and Pseudoscops species, the external parts of the ears are asymmetrical in shape, though not in position, while in others, such as the Tawny owl, it's the skin folds around the external ears that are asymmetrical (Norberg 1977). In yet others, including the Great grey owl Strix nebulosa and Tengmalm's owl Aegolius funereus, the bony surroundings of the ears themselves are asymmetrical [see Tengmalm's owl skull shown below, from Norberg (2002)]. Some owls lack ear asymmetry and have normal, symmetrical ears (Norberg 1977). [Adjacent diagrams, from Norberg (2002), show how peculiar owl ears look when you move the feathers and both the preaural and postaural flaps. Yes, owls have fleshy flaps around their ear openings.]

Because the asymmetrical owls differ so much in their anatomy, and because they're not all close relatives, it appears that ear asymmetry has evolved on as many as seven separate occasions within owls (Norberg 2002). This is extraordinary given that asymmetrical ears are not present in any other group of birds so far as we know (though they were apparently present in troodontids).

It seems that asymmetrical ears allow an improved ability to pinpoint the sources of sounds. Some owls have become such masters at locating the distance to and elevation of any sound that they can even capture prey in total darkness.

Ear tufts - nothing to do with the real ears - have evolved repeatedly among owls. Their function is uncertain. While they might help break up the owl's outline and hence contribute to camouflage, they also seem to function in communication, and an owl's moods and intentions can often be predicted by the disposition of its tufts (for previous discussion of this topic, see the article Why do some owls have ear tufts?).

Two kinds of owl

Living owls can be divided into two groups: barn owls (Tytonidae) and the typical owls, also known as the true or strigid owls (Strigidae). Various efforts to resolve owl phylogeny have been published, mostly using molecular techniques (Wink & Heidrich 1999, Wink et al. 2004, 2008) (a phylogenetic study of owls based on morphological characters - a 1967 PhD project produced by N. L. Ford - has often been mentioned in the literature but remains unpublished).

Barn owls have longer, narrower skulls and longer legs than strigid owls; the claw on their third toe is comblike and functions in grooming and their facial disk is heart-shaped. There are approximately 15 barn owl species, though several others from the Caribbean and Mediterranean (some of which were much larger than the living kinds) became extinct in recent centuries. The best-known barn owl - the Common barn owl Tyto alba [shown here, image by Luc Viatour, from wikipedia] - is the most widespread owl species and one of the most widely distributed of all birds: it inhabits Europe, Africa, tropical Asia, Australasia, and the Americas.

Strigids have mostly rounded facial disks (nicely shown here in a captive Great grey owl) and broader skulls than barn owls. This is the most diverse owl clade: it includes the Australasian boobooks, hawk-owls, and laughing owls, the little owls, pygmy owls and their relatives, the small, cryptic scops and screech owls, the tropical and temperate spectacled owls, eared owls and wood owls, and the large eagle owls and their relatives.

The smallest strigids (there are several contenders for this title, with the best known being the Elf owl Micrathene whitneyi of the United States and Mexico) weigh less than 50 g and are about 12 cm long. In contrast, the largest (the Eurasian eagle owl Bubo bubo) reaches 70 cm in length, 4 kg in mass, and has a wingspan of 1.5 m.

Death by owl

Unlike hawks and falcons, owls do not kill with their hooked bills but typically rely on their very powerful, large-clawed feet. Owl toes are shorter and more robust than those of most other predatory birds, and their talons are all similar in length (Shufeldt 1900, Fowler et al. 2009) [adjacent image, from Shufeldt (1900), shows pedes of Tyto (l) and Bubo (r)]. The fourth toe can be directed backward so that both digits I and IV can oppose II and III: this type of foot is known as facultatively zygodactyl or semizygodactyl. Owls also have a relatively short, stout tarsometatarsus, and special bones called sesamoids help improve their ability to exert force through their toes and to resist stresses in the bones and muscles. These features mean that owls have a greater grip strength compared to other birds. After grabbing a prey animal, they squeeze it to death (Fowler et al. 2009). The prey animal is then usually swallowed whole, and the less digestible parts - such as fur, feathers, wing cases and bones - are later ejected from the mouth as sausage-shaped pellets. One or two pellets are usually produced within each 24-hour period.

The majority of owls prey on small mammals, but small birds, large insects, frogs, and earthworms are also common prey. Large species (like some of the eagle owls) may kill hares, hawks, falcons, and other owls, and eagle owls may even have a significant impact on populations of other predatory birds. Some owl species prey on bats, frogs, crabs, and fish. Owls are not limited to nocturnal hunting. Many species hunt either during the day or night, while others - a good example is the Hawk owl Surnia ulula of the Northern Hemisphere - mostly hunt in daylight.

Diverse lifestyles

In keeping with their favored woodland or forest habitat, owls frequently have barred or spotted plumage and most are patterned in various shades of brown. Nocturnal owls usually spend the daytime roosting out of sight, hidden close to tree trunks and in shade. They need to be cryptically coloured to avoid predators, but also to hide from small birds; on finding roosting owls, these will mob them and thereby reveal the predator's location to other animals. Some owl species have eyelike markings on the backs of their heads: these may help intimidate would-be attackers or mobbers.

Owls are not all woodland birds. Members of several lineages have adapted to life in open habitats. Deserts are inhabited by some eagle owls and by Elf owls. The widespread Short-eared owl Asio flammeus - found across Eurasia and the Americas - is a well known denizen of treeless moors and grasslands. The Burrowing owl Athene cunicularia of North, Central, and South America is a strongly terrestrial species that frequents grasslands and both roosts and nests in holes in the ground. One of the most distinctive open-habitat owls is the striking Snowy owl Bubo scandiacus [shown here] of the Northern Hemisphere. Genetic studies have shown that this large predator of lemmings and other mammals is a specialized member of the eagle owl lineage. Not only has it evolved a distinctive white plumage (flecked with black spots in the female), it has also strongly reduced the size of its ear tufts.

Owls are, quite rightly, generally assumed to be predators of terrestrial prey. However, owls belonging to two lineages have become specialized aquatic feeders. As suggested by their name, these fishing owls - the Asian Ketupa species and African Scotopelia species - either gaff prey from the water surface while in flight or wade into shallow water to seize their prey.

Unlike other strigid owls, fishing owls have unfeathered feet. Sharp spicules on the undersides of their toes help them to hold fish. Their facial disks are poorly developed. The need to have silent flight is no longer a concern, so their feathers lack the special fringes that provide other owls with such quiet wings. Unlike most other owls, fishing owls will sometimes feed on carrion.

All fishing owls are closely related to eagle owls (the Bubo species), and debate continues as to whether they should be included within Bubo (Wink & Heidrich 1999, Wink et al. 2004, 2008) or kept separate (at his bird taxonomy site, John Boyd suggests that it might prove most useful to recognise four clades within the eagle owl-fishing owl clade: Bubo sensu stricto, Nyctaetus, Scotopelia and Ketupa) [Adjacent image shows Greyish eagle owl B. cinerascens]. The Asian fishing owls look very Bubo-like while Scotopelia has a characteristic shaggy plumage, lacks ear tufts, and hence looks rather different. These differences imply that the similarities present between the two fishing owl evolved independently.

The fossil record

Owls have a good fossil record: as shown by the graph below - from Kurochkin & Dyke (2011) - the rate of discovery of new taxa increased markedly during the 1970s and 80s but slowed down round about 2000. This unusually good fossil record is partly due to the fact that owls often use caves as roost sites, but it's also explained by the fact that owl bones are particularly distinctive when compared with those of other birds. The oldest fossil owls are from the Paleocene, and a large number of archaic owls are known from the Eocene and Oligocene of the Northern Hemisphere (Mayr 2009). Some of these fossil owls are superficially similar to barn owls and have sometimes been regarded as ancient members of this group. However, it's possible that the features resulting in these suggestions are merely primitive characteristics that were widespread across all early owls but became lost or modified by strigids.

Modern owl lineages were definitely in existence by the Miocene. A number of fossil owls were particularly remarkable. During the Pleistocene, the Caribbean was home to several very large owls that had relatively stout legs. The largest of these - Ornimegalonyx - apparently had a standing height of more than a metre and may have had reduced flight abilities [sadly, there weren't 6-foot-tall Cretaceous super-owls, like the one shown here. See Myth of the six-foot super-owl]. Until recently, Hawaii was inhabited by the peculiar Grallistrix species, sometimes called the stilt-owls. These had particularly long legs and seem to have been dedicated predators of small birds.

For previous articles on owls, please see...

• Titan-hawks and other super-raptors (discusses giant Gargano barn owls)
• Why do some owls have ear tufts?
• From Morocco, with larks, babblers, gazelles, owls and GIANT DINOSAUR BONES
• Fish owls in reverse
• Chock-full of rodent bones
• Myth of the six-foot super-owl
• Why can't my readers be dumber? Or: replica owls
• Giant owls vs solenodons

Refs - -

Fowler, D. W., Freedman, E. A. & Scannella, J. B. 2009. Predatory functional morphology in raptors: interdigital variation in talon size is related to prey restraint and immobilisation technique. PLoS ONE 4(11): e7999. doi:10.1371/journal.pone.0007999

Kurochkin, E. N. & Dyke, G. J. 2011. The first fossil owls (Aves: Strigiformes) from the Paleogene of Asia and a review of the fossil record of Strigiformes. Paleontological Journal 45, 445-458.

Mayr, G. 2009. Paleogene Fossil Birds. Berlin: Springer.

Norberg, R. (1977). Occurrence and Independent Evolution of Bilateral Ear Asymmetry in Owls and Implications on Owl Taxonomy Philosophical Transactions of the Royal Society B: Biological Sciences, 280 (973), 375-408 DOI: 10.1098/rstb.1977.0116

- . 2002. Independent evolution of outer ear asymmetry among give owl lineages; morphology, function and selection. In Newton, I., Kavanagh, R., Olsen, J. & Taylor, I (eds.) Ecology and Conservation of Owls: Proceedings of the Owls 2000 Conference. CSIRO Publishing (Collingwood, Victoria, Aus.), pp. 329-342.

Shufeldt, R. W. 1900. On the osteology of the Striges. Proceedings of the American Philosophical Society 39, 665-722.

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

- ., Heidrich, P., Sauer-Gürth, H., Elsayed, A.-A. & Gonzalez, J. 2008. Molecular phylogeny and systematics of owls (Strigiformes). In König, C. & Weick, F. (eds) Owls of the World (second edition). Christopher Helm (London), pp 42-63.

- ., Sauer-Gürth, H. & Fuchs, M. 2004. Phylogenetic relationships in owls based on nucleotide sequences of mitochondrial and nuclear marker genes. In Chancellor, R. D. & Meyburg, B.-U. (eds) Raptors Worldwide. WWGB/MME, pp. 517-526.