The Secret Perfume of Birds Uncovering the Science of Avian Scent

To successfully navigate their world, organisms rely on numerous senses. Birds are no exception to this; and yet, for a long time, people have been convinced that birds cannot smell. This came as a surprise to evolutionary biologist Danielle J. Whittaker. Given that smell is effectively chemoreception (the sensing of chemical gradients in your environment) and was one of the first senses to evolve, why would birds have no use for it? The Secret Perfume of Birds tells the story of 15 years spent investigating the olfactory capabilities of birds and provides an insider's account of scientific research.

The thing about The Secret Perfume of Birds, and something that might test the patience of some readers, is this: it is effectively a research memoir. Rather than a textbook-style review of what we know about bird smell, Whittaker takes you chronologically through her research, not sparing the reader from the dead ends and sometimes confusing results that come out of biological fieldwork. But, before delving in, she first sets the stage with some history and some information on her study system.

Whittaker traces the notion of avian anosmia (i.e. the absence of smell) to that famed US naturalist Audubon. In 1826, he published his results on experiments with what he thought were turkey vultures who proved unable to locate rotting prey. Despite criticism in 1837 that he had actually used black vultures that rely on eyesight to find, the myth took hold in both the scientific and popular imagination. It has stayed firmly lodged there until very recently. Whittaker came to this topic in 2007 and started working on the dark-eyed junco. This small and unobtrusive New World sparrow is widespread in North America and makes for an ideal model species. "Have you ever tried to catch a bald eagle?" (p. 28), she quips when asked why she does not study something more exciting such as birds of prey.

The first half of the book focuses on fieldwork and behavioural experiments and introduces one very special gland. To keep their feathers free from parasites and generally in tip-top condition, many birds rely on the uropygial gland (also known as the oil or preen gland). Located at the base of the tail, this gland secretes preen oil that birds daub on their bill and then spread all over their body during a good preening session. It also provides a source of body odour. In experiments, juncos responded differently to their own preen oil versus that of a stranger. Is odour just an incidental byproduct of preen oil? Chemical analysis revealed that preen oil composition changes during the breeding season. However, where other ground-nesting birds seem to chemically camouflage their nest, juncos produce smellier preen oil with more lightweight, volatile compounds. Could this be olfactory communication?

Thus armed with the first indications that juncos are capable of smell, the subsequent two chapters delve into a range of choice experiments to establish whether juncos actually care about smell and use it as a source of information to e.g. recognize each other, advertise health and nutritional status, respond to territorial intrusions, and choose mates. Especially that last question, mate choice, led to further research to see if there are correlations between preen oil composition and quality and (genetic) compatibility of mates. Along the way, she discusses other research, often on mammals, that shows the many functions of pheromones and other chemical signals. Her results are always interesting, but frequently tantalizing and not quite conclusive. This is in no way intended as a slight towards Whittaker, but represents the typically slow and incremental increase of our knowledge.

Midway through, the book takes a decisive turn when Whittaker meets Kevin Theis who asks her whether she had noticed that the volatile compounds in junco preen oil are byproducts of microbial metabolism. This leads to a fruitful collaboration that starts by showing that bacteria in the uropygial gland are the source of a bird's odour. From there, her questions inevitably turn to the role of the bird's microbiome. Then there are the major histocompatibility complex (MHC) genes that code for immune system proteins. Research in humans and other animals has suggested they play a role in odour and mate choice, so what about birds? A final chapter looks at the understudied role of odours in female behaviour, in particular in the context of sexual selection, such as mate choice by males and competition amongst females.

Only occasionally does the memoir aspect of this book discuss Whittaker's career: from a lost English major to the research director of the BEACON Center for the Study of Evolution in Action in Michigan. Nevertheless, she is refreshingly frank about the hardships of navigating academia and finding employment. I think her choice to keep the focus squarely on the research is a good one: the book already meanders a bit by narrating her research chronologically. Readers will benefit from a background in biology or an interest in how science is done. To her credit, Whittaker does everything she can to attend to her audience. The book is well organised with section headings providing a clear structure. She provides basic background information on genetics when discussing the microbiome, but avoids going into the jargon-filled weeds when discussing MHC and the complexities of the immune system. A glossary with clear entries will demystify whatever terminology remains. And, where needed, illustrations are included (though some of the stock images contain extra labels that are neither explained nor relevant). The only criticism I had in this regard was her reference list: rather than the usual alphabetical listing, references are organised chapter-wise in the order in which they are discussed. However, because there is no connection to the main text via e.g. superscripts, this is neither a proper notes section nor an easily searchable bibliography.

It is encouraging to see that the efforts of her work are slowly rippling outwards. The 2016 Handbook of Bird Biology mentions her work though still erred on the side of caution, considering well-developed smell in birds rare. Graham Martin, both in his books for a professional and a popular audience mentions Whittaker's research and fully acknowledges how the last 20 years of research have clarified that smell in birds is taxonomically widespread and has many uses, though it remains understudied. Overall then, Whittaker convincingly busts the myth of avian anosmia, even if many of the details are still being worked out. The Secret Perfume of Birds might require some patience but rewards the reader with fascinating insights into the fields of animal behaviour and chemical communication.