Avian builders: more than meets the eye

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Bird Nests and Construction Behaviourby M. HansellCambridge University Press, 2000.£50.00/$80.00 hbk (xii + 280 pages)ISBN 0 521 46038 7Nearly all birds construct, excavate or scrape their nest, resulting in a diversity of bird-built structures that is truly staggering: some birds hardly bother and scratch only shallow scrapes, whereas others construct sophisticated nests and bowers that are several metres high, or pile up mounds reaching up to 11 m in diameter. Substantial costs are involved; for example, penduline tits spend three weeks on weaving their hanging-basket nests, whereas swiftlets take over a month to construct nests from their own saliva.Nests and nest construction are somewhat neglected topics in ornithology. Typically, only a few pages are devoted to them in textbooks, and these often refer to the same set of classic studies. Hansell makes an excellent attempt to revitalize our interests in nests and nest-building behaviour. First, he investigates how the nests are built and what materials are used. These chapters are teeming with fascinating natural history; for example, long-tailed tit nests can contain 3700 pieces of tiny lichens and mosses, and each parent spends over 20 km on the wing to gather these bits and pieces to put together such a seemingly simple structure. Hansell has developed an intricate scheme to describe the structure and form of nests and, although the magnitude and breadth of this data set is impressive, one wonders why these data have not been analyzed more extensively, for example, by using some of the phylogenetic comparative methods currently available.The second half of the book concerns the functions (or the adaptive values) of nest building. Nests are built for three reasons: (1) to attract a mate; (2) to provide a safe site for the eggs and young; or (3) to shelter the birds from adverse weather. Hansell thoroughly evaluates the various costs and benefits of these explanations. Apart from the handful of topics that has attracted much attention in recent years (e.g. bowers and mate choice, and nest materials and ectoparasite load), most evidence is nonexperimental and is therefore open to alternative explanations, a bias that reflects the current state of the field. A major gap in functional studies is the appreciation of the costs of nest building. Recent studies revealed that egg laying, incubation and brood raising often precipitate into increased adult mortality. Given the considerable effort many birds spend on constructing their nests, I suspect that nest building also has serious consequences for survival and future reproduction. One also wonders about the life-history consequences of removing such costs; for instance, it would be revealing to evaluate the life-history responses of natural cavity-breeders to the reduction of such costs, for example, by providing them with ready-made nest-boxes.The final chapter, on the phylogeny of nest building, left me a little unsatisfied. Although it is nice to know the reconstructed history of various nest types in, for example, swallows and weavers, one wonders about the selective forces that have influenced nest types. Phylogenetic reconstruction of characters, such as nest type, is an important step in revealing historical events, although these also need to be backed up by phylogenetic analyses of ecological and life-history forces that might have shaped the behaviour over evolutionary time.This fine book falls short of my expectation in three respects. First, the focus is on bird-built structures and the discussions at length of wasp nests, termite mounds and badger dens were distracting. Hansell is obviously very familiar with all kinds of animal-built structures, but I felt that the nonavian examples were not essential to the main arguments of the book. Second, a concluding paragraph, or summary, at the end of each chapter would have been very useful. These could also provide guidelines for future studies. Finally, some avian groups are better represented than others are. For example, I found relatively few references to raptors and ground-nesting birds (shorebirds, waterfowl and grouse), although this selectivity might reflect publication bias or the composition of museum collections, rather than preference exerted by the author.Overall, Bird Nests and Construction Behaviour is a scholarly written review on a marvellous subject. It is beautifully illustrated and makes fascinating reading. Clearly, a proper understanding of nest construction requires collaborative efforts by physiologists, behavioural ecologists and comparative biologists, and this book is an excellent step in this direction. I recommend it to all students, researchers and amateur ornithologists interested in avian ecology and behaviour.