Bird Navigation: the solution of a mystery? By R. Robin Baker. Hodder & Stoughton, London, 1984. 256 pages. Paperback, £9.75. This book fills for the birdwatcher of the 1980s the niche which G.V.T. Matthews's pioneer Bird Navigation filled in the 1950s and 1960s. It is vastly informative and thoughtfully provocative. It is also a little daunting, for the number of jigsaw pieces to be fitted together seems to have quadrupled in the last two decades. One of the largest conceptual advances has been that of 'redundancy of information', first put forward by Professor Keeton in 1972. In short, migrating birds have at their disposal a range of back-up navigational systems. Thus, in addition to their ability to determine direction from sun and stars, we now know that some species can make use of scent patterns, of infrasound, and particulary of elements of the earth's magnetic field. Some can also detect polarised light patterns and may use these and/or a sensitivity to ultra-violet light during daylight hours. The research summarised is almost all based on experimentation, it being difficult at present to see any alternative approach. Yet to prove that a pigeon (by far the most commonly used experimental animal) can detect some subtle environmental variant is not to establish that it exploits its ability to any significant extent. Twenty-five years ago, radar workers were recording migrants completely disoriented by cloud or fog, and the author cites a similar event (p. 218). On such occasions, why cannot the birds use their back-up system? We must be grateful to the experimenters, who have made virtually all the discoveries of the last two decades.