Gary Kaiser's The Inner Bird: Anatomy and Evolution

Good, non-technical books on anatomy are rare; good, non-technical books on avian anatomy are just about non-existent. Gary Kaiser's The Inner Bird: Anatomy and Evolution stands out as one of a kind - it is not brand-new (having been published in 2007), but still has yet to be widely recognised as the valuable piece of scholarship that it is. I will state here at the outset that I cannot recommend it highly enough.

Containing a wealth of information that ranges from the Mesozoic ancestry of birds to neornithine phylogeny, flight dynamics, functional morphology and ecology, it should be sought out and consulted avidly by anyone seriously interested in avian anatomy and evolution. Kaiser's volume not only serves as both a wide-ranging introduction and review, but also includes new data and several interesting new hypotheses. The author also challenges widely-held ideas where appropriate and brings attention to the fact that erroneous, highly dated views on avian anatomy and function are still being promoted. The Inner Bird is also eminently readable: it is not a text-book.

The book begins with an introduction to avian anatomy (covering both bone structure in general, and the many skeletal and soft-tissue peculiarities of the different bird groups) and then compares and contrasts modern (neornithine) birds with their dinosaurian ancestors. There is a huge amount of information here that has not been distilled before. Kaiser's discussions of such areas as sesamoid distribution and function, neck mobility, the form of the lumbar-synsacrum junction, chewing behaviour and bill morphology in cuckoos and turacos, the possible role of the glycogen body, and the massive variation seen in furcular anatomy among neornithines are extremely welcome [motmot skull shown below: from Kaiser's collection of skull photos archived at http://innerbird.com].

A second section begins with a review of the history of avian classification and is then devoted to a discussion of avian phylogeny. Kaiser spends a lot of time on convergent evolution and focuses on ambiguity and disagreement more than their opposites. He also uses this as an opportunity to examine the wonderful but always annoying Hoatzin Opithocomus hoazin. The amazing anatomy and controversial phylogenetic position of this South American bird evidently fascinate the author and he returns to it many times over, also featuring it and its skeleton on the cover.

The anatomy and function of feathers, their evolutionary origins, and their distribution in dinosaurs form the subject of the third and final section. Kaiser uses his knowledge of modern bird aerodynamics and flight behaviour to look anew at Archaeopteryx, confuciusornithids, enantiornithines ('ball-shouldered birds' of his usage) and other fossil birds, and the final chapter in this section - that on the structure and function of marine birds - is one of the best in the book. A glossary and numerous graphs, tables and diagrams are included, and the text is fully referenced.

The volume takes its title from one of Kaiser's most profound intellectual proposals: the notion that neornithine birds are "puppeteers that hide behind a screen of feathers"; creatures that have evolved a highly novel body plan for vertebrates. Neornithines are, generally speaking, small, fragile, thin-limbed animals, protected and covered by an extensive 'environmental suit' formed from their integument, and with only a few of their extremities truly exposed to the elements. This is the 'inner bird' body plan, and it has enabled some birds to make a living from freezing cold oceans, to live for months on end on the wing, and to become one of the most successful vertebrate clades in a multitude of terrestrial and aquatic habitats. As Kaiser puts it, neornithines have come a long way from their ancestors.

Extensive hands-on experience of north Pacific seabirds, waders and others provides Kaiser with a practical, grounded perspective. Among the best bits of the book are those on the ecomorphology, behaviour and breeding biology of the birds that frequent the Pacific northwest, among them sea eagles and auks. Unless you're a specialist on these birds, much of what Kaiser says will be novel. He writes of Bald eagles Haliaeetus leucocephalus that have learnt to prowl about on the forest floor at night in quest of auks returning to their nests, and alludes briefly to humanity's many disgusting abuses of seabirds for artillery practise, pet food and leather. An explosion of research on the biology of the Marbled murrelet Brachyramphus marmoratus [shown here; from wikipedia] during the 1990s demonstrated - contrarily to received wisdom - that murrelets and other small auks were far from weak-flying 'proto-penguins', but extremely fast, powerful, highly mobile exploiters of the marine prey that they rely on. Kaiser makes the argument that they are in fact among the most specialised of birds and that we should marvel at their "exploitation of power and seemingly extravagant expenditures of energy" (p. 324). In similar vein, he argues that grebes and divers should not be dismissed as poor fliers (as textbooks and tradition would have it), but that they might be considered the avian equivalents of Boeing 747s and other avionic workhorses: able to carry large payloads for great distances at high speeds, despite an energetically costly takeoff.

The theropod ancestry of birds

Kaiser is convinced by the evidence for the dinosaurian origin of birds, and long sections of the book are devoted to discussing the similarities and differences seen between birds and their non-avian relatives*. The notion that birds cannot be dinosaurs is heavily promoted in the ornithological literature - most notably in Alan Feduccia's The Origin and Evolution of Birds (Feduccia 1996). Because Feduccia's book is one of the most visible of volumes on bird evolution, audiences can be forgiven for thinking that ornithologists as a whole reject the hypothesis of a dinosaurian ancestry for birds. This is absolutely not true, and those interested should take every opportunity to note that all of Feduccia (et al.'s) criticisms are invalid or erroneous (e.g., that non-avian theropods are too big to be ancestral to birds, that they occur too late in the fossil record, that their anatomy bars them from avian ancestry, and that other Mesozoic reptiles make better potential bird ancestors). It is also worth noting that many of Feduccia's proposals about the phylogeny of neornithines are idiosyncratic and that his volume does not accurately represent current thinking on avian evolutionary history. The Inner Bird helps provide part of the antidote, bringing home the point that the dinosaurian origin of birds is well supported and robust, and adopted by many ornithologists interested in palaeontology.

* These days I prefer to use the term Avialae for the bird branch of Maniraptora (largely because it is unambiguously inclusive compared to Aves). I should, therefore, be referring to 'non-avialians' rather than 'non-avians'.

Indeed, if anything is clear from the explosion of recently described feathered maniraptoran theropods (virtually all of which are from Liaoning Province in China), it is that the early members of several of the key groups (oviraptorosaurs, epidendrosaurs, troodontids and dromaeosaurids) were all extremely similar to the earliest birds. All were small, feathered, leggy omnivores or predators with long arm and tail feathers and delicate, toothed skulls. In fact these little maniraptorans are so similar that the members of the different lineages are sometimes wrongly allocated. The early troodontids Anchiornis and Jinfengopteryx, for example, were originally described as birds, and it has proven increasingly difficult to find reliable characters that distinguish Archaeopteryx from non-avian maniraptorans. Analyses that incorporate good taxonomic and character sampling still find Archaeopteryx to be part of the bird branch within Maniraptora (e.g., Norell et al. 2001), but it is far from inconceivable that Archaeopteryx could prove to be on the deinonychosaur branch, or that deinonychosaurs could be on the bird branch. This makes a mockery of the idea that deinonychosaurs are somehow fundamentally different from birds*. Accordingly, a reasonable interpretation of the fossil record is that birds were, originally, only one maniraptoran lineage among many: nothing special, and not obviously destined for phenomenal success.

* This is what Feduccia and colleagues argued until recently. They have since rescinded this and, like Greg Paul, now argue that bird-like maniraptorans are flightless birds. I don't think that's right, but I don't find it offensive. They continue to argue, however, that this 'expanded Avialae/Aves' is nothing to do with dinosaurs. For discussion of this area see the article (and comments) here [Berlin Archaeopteryx shown below; from wikipedia].

Worth noting is that Kaiser is a bit out of date in saying that Archaeopteryx is older than all of its closest non-avian relatives. Even prior to 2007, numerous (admittedly scrappy) remains demonstrated the presence of non-avian maniraptorans older than Archaeopteryx. Pedopenna - a Middle Jurassic bird-like maniraptoran with long hindlimb feathers - was described in 2005 (Xu & Zhang 2005). Several additional, recently described feathered maniraptorans can now be said to pre-date Archaeopteryx, including Anchiornis, Epidendrosaurus and Epidexipteryx.

While I am extremely happy that Kaiser tackled Mesozoic birds and other dinosaurs so extensively (and while his insights and hypotheses are excellent, insightful and though-provoking), his work would definitely have benefited from a check by a worker who specialises on these animals. In one section, the four-winged dromaeosaur Microraptor is repeatedly referred to as Eoraptor: the latter is an early predatory dinosaur from the Triassic of Argentina; it is not closely associated with avian ancestry. Protoavis (a highly controversial fossil from the Triassic of Texas, thought by its describer to be a bird more closely related to neornithines than is Archaeopteryx) is wrongly and consistently called 'Protavis' and quite a few other names are spelt incorrectly too. The reference to Rahonavis from Madagascar as a close relative of Archaeopteryx ignores recent evidence showing that it is more likely one of the unenlagiine dromaeosaurs. Kaiser is incorrect in suggesting that Archaeovolans (see below) might be synonymous with Arctosaurus: the former is a Chinese bird from the Cretaceous while the latter is an indeterminate archosauromorph named for a single neck vertebra from the Triassic of Canada; they have never been linked to my knowledge.

Greg Paul's proposal that deinonychosaurs and other non-avian maniraptorans may have been secondarily flightless is not, as Kaiser implies, the same as the 'birds come first' hypothesis. The latter notion, developed by writer-researcher George Olshevsky, proposes that small size and a scansorial lifestyle evolved early on in archosaurs, that all dinosaurs are the direct descendants of such ancestors, and that one dinosaur lineage - Aves - is unique in retaining these ancestral features (Olshevsky's 'Birds Come First' hypothesis was covered at length here and here).

It is also rather misleading to imply or state that 'Archaeoraptor' (shown here: a faked composite specimen, showcased in 1999 by National Geographic as a new intermediate in Mesozoic bird evolution) "took the professional community by surprise" (p. 221). The authenticity of this fossil was in fact doubted from the start: I mentioned these doubts to the fossil's describer (Stephen Czerkas of The Dinosaur Museum in Utah) on the very day of its debut, but was reassured that any concerns were unwarranted! Czerkas later described one half of 'Archaeoraptor' (as Archaeovolans repatriatus) in the 2002 volume Feathered Dinosaurs and the Origin of Flight, ostensibly volume 1 of a peer-reviewed publication emanating from The Dinosaur Museum (Archaeovolans is now thought to be synonymous with Yanornis martini, named in 2001). Kaiser is highly critical of this volume, noting that its contents "read somewhat like the technical discussions posted on the Internet, and they lack a tone of detached objectivity characteristic of publications with a longer history" (pp. 221-222). These views very much echo those made elsewhere in the technical research community and, in my opinion at least, it is good to have them on record.

Why 'dinosaur' does not = 'non-avian dinosaur'

These minor quibbles aside, one aspect of Kaiser's approach to the dinosaurian ancestry of birds is problematic: namely, his implication that 'birds' and 'dinosaurs' can and should be clearly differentiated. He notes at the start (p. 1) that he opts to restrict the term 'bird' to those feathered animals that fly, and the term 'dinosaur' to those non-flying forms that may or may not have feathers. I can understand that this convention seems to make life simpler if you're only worrying about literary style, but it's misleading [Torgos tracheliotos shown below in characteristic maniraptoran posture :) ].

For one, it implies that birds are fundamentally different from other dinosaurs. Those many little feathered theropods, and the anatomy of birds and other theropods in general, show that birds are dinosaurs no matter how offensive this might seem to traditional usage of those terms. As noted above, early birds - archaeopterygids and other archaic, long-tailed forms in particular - can no longer be rightfully regarded as unusual compared to other maniraptorans.

Birds are different from other dinosaurs, but the peculiarities that we have in mind when we say such things are almost entirely unique to neornithines: it's wiser to say, then, that it isn't that birds are different from other dinosaurs, it's that neornithines are different from other dinosaurs.

Secondly, the idea of differentiating 'birds' and 'dinosaurs' based on flight ability doesn't work given that some of the animals on the 'dinosaur side' of the divide were almost certainly capable of flight (examples include the dromaeosaurs Microraptor and Rahonavis). 'Non-avian dinosaur' and/or 'non-avian theropod' are, no doubt, annoying, verbose terms to some, but - if we are interested in precision - they are necessary and inoffensive. Perhaps those objecting to these terms should see how frequent the terms 'non-human animal' and 'non-human primate' are used in the behavioural and primatological literature.

Tackling neornithine phylogeny

Long sections of The Inner Bird are devoted to phylogeny. Given the uncertainty and disagreement that has surrounded certain sections of the neornithine tree, it might be predictable that any book published in 2007 will be woefully out of date on this subject. That's not obviously true (though read on): Kaiser promotes the view throughout that palaeognaths are outside a clade that includes all neognaths, and that neognaths consist of Galloanserae (waterfowl and gamebirds) and everything else (properly termed Neoaves). This is the 'consensus view' on neornithine phylogeny, and it's well supported [a rough consensus cladogram for Neognathae is shown below; from a recently submitted large manuscript that I'll be discussing here in due time].

The relationships within Neoaves have proved more problematic. Kaiser is partial to the (new at the time of his writing) proposal that Neoaves might consist of a paucispecific, mostly Southern Hemisphere Metaves and a much larger Coronaves (Fain & Houde 2004). Some additional support for a group something like Metaves was provided by Hackett et al. (2008). However, Morgan-Richards et al. (2008) argued that the data used to support the monophyly of Metaves was suspect and that its constituents (frogmouths, pigeons, tropicbirds, flamingos, swifts, hummingbirds, mesites, kagus, grebes and sandgrouse) were scattered about the neoavian tree.

Interestingly, Kaiser is slightly critical of Livezey and Zusi's enormous morphological analysis (Livezey & Zusi 2007), in part because this study supported the monophyly of alleged groupings that seem hard to accept. Ironically, these are mostly 'traditional' and Livezey & Zusi's results are conservative compared to several other recently published phylogenies. Divers and grebes are perhaps the most notorious of the alleged groupings they supported: it is difficult not to assume from the outset (as Kaiser does) that any similarities seen between these two are the result of convergence, and that some of the more profound differences between the two reveal distinct origins. Some other ornithologists have expressed a similar dissatisfaction with Livezey & Zusi (2007): Mayr (2008) argued that, for all its size, the conclusions of this analysis were misled by a low 'signal to noise' ratio.

Kaiser proclaims early on in the book that molecular techniques have failed to produce a good phylogeny for modern birds, and he also notes that the results of such studies have proved contradictory. These assertions are arguable, though one might say that 'convincing' molecular phylogenies for Neornithes have only appeared since Kaiser's book was published (e.g., Hackett et al. 2008). Anyway, given this strong criticism, it is somewhat surprising that Kaiser leans heavily in places on Sibley & Ahlquist's phylogenetic proposals of the 1980s and 90s (e.g., Sibley & Ahlquist 1990), even sometimes treating their 'tapestry' as if it were the one true phylogeny [the 'tapestry' is shown below: for more discussion see the article here]. While it is widely recognised that Sibley & Ahlquist got a lot of stuff right, the overall shape of their 'tapestry' has not stood the test of time, and it has also been argued that the phylogeny they depicted was not supported by the genetic data they compiled.

Kaiser's frequent reference to 'Paleognathae' is slightly irksome and he frequently refers to Galloanserae as Galloanserinae and even as Galloanseriformes. And because the same topic is visited on a few different occasions (see below), I was left somewhat confused by his position on the affinities of New World vultures (cathartids) and storks. In places he does seem to consider them close kin. Recent work establishes that cathartids are close relatives of accipitrids and that the once mooted cathartid-stork link is erroneous, but one still sees it trotted out as a well supported 'counter-intuitive' proposal (ironically, often by people hoping to show how up-to-date they are on avian phylogeny!).

Hoatzins and enantiornithines

A few interesting similarities shared by the Hoatzin and some enantiornithines (the anterior part of the sternum lacks a keel and the hypocleidium extends far posteriorly on the sternum's ventral surface) give Kaiser pause for thought and he suggests that they had similar lifestyles. Previous authors have hinted at the possibility of hoatzin-like specialisation for a folivorous diet in some enantiornithines (Feduccia 1996), though this has been contested as none of the other features seen in the enantiornithine skeleton are consistent with it (Chiappe 2007). Note, however, that Kaiser specifically does not say that the hoatzin is a living enantiornithine* (he actually says "I am itching to suggest that the Hoatzin might be a surviving member of the ball-shouldered birds but, sadly, it is not so" (p. 263)) [adjacent photo by Linda De Volder, from wikipedia].

* As intimated in one review of the book.

Incidentally, Kaiser has avian tarsometatarsal fusion the wrong way round: he notes that neornithines fuse the tarsometatarsus at its proximal end and that enantiornithines fused the distal end first. In fact, neornithines begin their fusion distally, enantiornithines proximally.

My main complaint about the book is that it doesn't seem that well organised. The reader finds themselves covering what seems like the same ground on more than one occasion (albeit from different angles), and some of the different sections overlap substantially. Those brief introductory sections on bone and tendon anatomy make very familiar reading, but I can understand that they were needed for the more in-depth anatomical discussions that follow. Illustrations and black-and-white photos appear throughout: they are mostly very good, but it is unfortunate that there are not enough of them.

The Inner Bird: a big deal

As is hopefully made clear in this review, The Inner Bird is a significant volume that will hopefully inspire many people - palaeontologists, ornithologists and even interested amateur birders and biology students - to look at avian anatomy with new, or renewed, interest and appreciation. I reiterate the point that Kaiser's book is not just an invaluable review, but also that it includes a great deal of original thought, data collation and hypothesis formation. I am certainly impressed by his novel discussions of topics that have not previously been discussed outside of the technical literature, if at all: the role moisture droplets might have on the flight abilities of various birds, the influence bone cross-section has on flight style, the evolutionary correlation between egg size and body shape [see diagram below; courtesy of G. Kaiser], the role long tails may play in reducing turbulence, and so on. Kaiser has also made efforts to make his data available: some of it is tabulated in the book, but much more is made available in online appendices (http://innerbird.com).

Uniquely, The Inner Bird is both an excellent popular, readable account and a major contribution to the scientific literature. It, and its author, deserves great accolade.

This review is due to appear in Historical Biology and appears here with permission. Thanks to Gary Kaiser for supplying images; thanks also to Gareth Dyke.

The Inner Bird: Anatomy and Evolution, by Gary W. Kaiser, 2007. University of British Columbia, 386 pp., ISBN 978-0-7748-1344-0

For previous relevant articles on evolution, diversity and functional morphology in birds and other maniraptorans, see...

• The godwits' many bills
• Vampire finches and the path to parasitism
• Feathers and filaments of non-avian dinosaurs, part I
• Feathers and filaments of dinosaurs, part II
• 2007: a good year for terror birds and mega-ducks
• A stunning new Mesozoic bird... well, new-ish
• Long and Schouten's Feathered Dinosaurs, a review
• The giant green fragrant parrot
• Are parrots actually pigeons?
• The 'Birds Come First' hypothesis of dinosaur evolution
• Birds Come First - oh no they don't!
• Sibley and Ahlquist's 'Tapestry'

Refs - -

Chiappe, L. M. 2007. Glorified Dinosaurs: the Origin and Early Evolution of Birds. London: John Wiley & Sons.

Fain, M. G. & Houde, P. 2004. Parallel radiations in the primary clades of birds. Evolution 58, 2558-2573.

Feduccia, A. 1996. The Origin and Evolution of Birds. New Haven & London: Yale University Press.

Hackett, S. J., Kimball, R. T., Reddy, S., Bowie, R. C. K., Braun, E. L., Braun, M. J., Cjojnowski, J. L., Cox, W. A., Han, K.-L., Harshman, J., Huddleston, C. J., Marks, B., Miglia, K. J., Moore, W. S., Sheldon, F. H., Steadman, D. W., Witt, C. C. & Yuri, T. 2008. A phylogenomic study of birds reveals their evolutionary history. Science 320, 1763-1768.

Livezey, B. C. & Zusi, R. L. 2007. Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion. Zoological Journal of the Linnean Society 149, 1-95.

Mayr, G. 2008. Avian higher- level phylogeny: well-supported clades and what we can learn from a phylogenetic analysis of 2954 morphological characters. Journal of Zoological Systematics and Evolutionary Research 46, 63-72.

Morgan-Richards, M., Trewick, S. A., Bartosch-Härlid, A., Kardailsky, O., Phillips, M. J., McLenachan, P. A. & Penny, D. 2008. Bird evolution: testing the Metaves clade with six new mitochondrial genomes. BMC Evolutionary Biology 2008, 8:20 doi:10.1186/1471-2148-8-20

Norell, M. A., Clark, J. M. & Makovicky, P. J. 2001. Phylogenetic relationships among coelurosaurian theropods. In Gauthier, J. & Gall, L. F. (eds) New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom. Peabody Museum of Natural History, Yale University (New Haven), pp. 49-67.

Sibley, C. G. & Ahlquist, J. A. 1990. Phylogeny and Classification of Birds. New Haven: Yale University Press.

Xu, X. & Zhang, F. 2005. A new maniraptoran dinosaur from China with long feathers on the metatarsus. Naturwissenschaften 92, 173-177.