Antipredator Defenses in Birds and Mammals.—Tim Caro. 2005. University of Chicago Press. Chicago, Illinois. 592 pp., 15 halftones, 130 drawings. ISBN 0-226-09436-7 $38.00 (paper).
In this book, Tim Caro summarizes the massive literature on antipredator defenses of birds and mammals. He first considered writing about all animals but decided that this would be too much. He next considered limiting himself to mammals, but found that the crucial work he wanted to discuss was done with birds. The result is a fascinating book that compares and contrasts what we know about defenses in birds and mammals. The book has something to offer for almost any ornithologist. Here, I review the book's contents and major conclusions to highlight its remarkable breadth.
The book is organized along the sequence of interactions within a predator-prey encounter, starting with the ways prey avoid being found by predators and ending with defenses at the point of capture and consumption. This is a highly effective organizational scheme, though admittedly some defenses work at multiple junctures. The text starts with a careful definition of terms. The author points out that we often speak of alarm or warning signals without data demonstrating alarm or warning. The first chapter also discusses predator recognition, as recognizing predators is important to avoiding them. Predator recognition shows both strong innate tendencies and considerable learning.
The first defense against predators is to avoid being detected. This often involves morphological traits such as crypsis, countershading, or contrasting colors that disrupt the outline of the body. Female birds are likely to be cryptic when they perform nest activities alone and in species that suffer heavy nest predation. Mammals and birds also provide potential examples of countershading, disruptive coloration, and polymorphism, but no clear evidence that the observed patterns are effective in avoiding predation. The colors and patterns of bird eggs may be cryptic, but the potential for such a function is not well understood. The nests of birds show better evidence for concealment than do the eggs within them.
Animals can also change their behavior to avoid being detected by predators. These changes principally involve concentrating activities in times and places with few predators and reducing dangerous activities, notably foraging and reproduction. Birds often place their nests in relatively safe places and behave around the nest in ways that limit detection by nest predators. Risk of predation alters what, where, when, and how much birds eat. In contrast, reproductive suppression seems not to be studied in birds. Caro suggests that adult birds rely less on behavioral mechanisms to avoid being detected than do adult mammals, and that birds mainly seem to avoid detection by aerial rather than terrestrial predators.
If predators detect potential prey, the prey may detect predators at a distance before the predators can attack. Caro devotes a chapter to vigilance and group size. Safety-in-numbers drives the well-replicated pattern of declining vigilance with increasing group size. Caro discusses why individuals do not “cheat” and avoid vigilance. The best-documented suggestion is that they would give up reliable personal information and be left with unreliable information generated by others.
Vigilance is affected by many factors besides group size, including age, sex, and dominance status. In addition, animals tend to be more vigilant at the edges of groups, with fewer close neighbors, and when farther from safety or in environments that can conceal predators. Adults with young often show heightened vigilance, though this is better documented for mammals than for birds. Caro emphasizes that these factors are often studied separately but probably interact in nature and pronounces, “Rather than demonstrate that a particular factor exerts an influence, it would be more profitable to reexamine its influence, taking many confounding variables into account” (p. 174). Across species, vigilance often depends on body size, attack rates, and how prey detect and escape from predators, but we are not good at predicting species differences in vigilance a priori. Caro also reviews sentinel behavior, including a reference unknown to me after a dozen years working on the subject.
Individuals who have detected a predator often give warning signals. Caro points out that the costs of giving warning signals seem to be very low in some situations and with some predators. He takes issue with the interpretation that callers lower the fitness of receivers in those cases where callers have higher fitness than receivers. Calls could be mutually beneficial in these cases. Deceptive alarms during foraging are a possible exception to this rule. Different calls may be associated with response urgency or escape tactic. Responses to warning signals develop rapidly in mammals though at the same time youngsters call too promiscuously and only gradually limit themselves to genuine threats. I suggest that these principles also apply in birds but have not been documented in the same detail as in mammals.
Prey also signal to predators that they are unprofitable to attack. Aposematic coloration is a classic example. Some birds, such as pitohuis, are known to be poisonous and many birds are strikingly colored. One possibility is that many birds signal their unprofitability through their plumage, with poisons being one aspect of unprofitability. Unfortunately, experimental evidence is decidedly mixed and we know almost nothing of the natural encounters from the predator's point of view. Animals may also behaviorally signal that they will be difficult to capture. Such signals are widespread in mammals and may be common in birds. Examples range from tail flicking by moorhens to singing by skylarks while being pursued by Merlins. In my opinion, many warning signals given to terrestrial predators could also function as pursuit deterrent signals.
When an attack happens, potential prey may benefit from being in a group because the attacker can only kill one group member, because group members detect the attack sooner, or because the predator fails to kill any group member. These benefits are unequally distributed across groups, including colonially nesting birds. Across birds and mammals, group living is more common in open-country species that rely on vigilance and escape to survive.
Prey may have morphological or physiological defenses against attack. The simplest defense is size. Escape flight in birds depends on both size and wing shape. Spines and quills are most common in medium-sized mammals but are apparently lacking in birds. Birds may defend themselves with beaks, talons, and spurs, though these primarily function in feeding and sexual selection. Some birds may employ malodorous defenses and eiders apparently produce especially repellent feces only during seasons when they defend nests. Questions abound for someone with a keen sense of smell and a strong stomach.
Caro devotes a substantial chapter to nest defense. Birds both harass and distract potential nest predators. Even though the costs of nest defense are rarely measured directly, nest defense shows many features that appear sensible from cost-benefit analysis in a life history context. Intensity of nest defense is greater for more valuable broods, whether these are larger or older. Differences between the sexes in nest defense are less clearly understood. Caro emphasizes that offspring may play a part by keeping silent while their parents are noisy.
Prey may mob a predator to encourage it to hunt elsewhere. Mobbing also conveys information to conspecifics and can lead to the cultural transmission of fearful responses. Although the definition of mobbing allows for solo efforts, a mob of one is not the same as a mob of many: mobbing behavior works best in larger groups. Mobbing is often important in colonial species and often birds of one species benefit by nesting with members of more pugnacious species. This association may be commensal, mutualistic, or even parasitic.
Even when an attack is well underway, prey can employ flight and “behaviors of last resort.” Their flights can be to vegetation or into the air and may involve aerial maneuvering. Such maneuvering is affected by body weight and fat reserves. Even when contacted by a predator, prey may scream or play dead. The evidence about fear screams does not definitively support any of the current hypotheses and perhaps these screams serve several functions.
Caro ends the book with a chapter on “Framing questions about antipredator defenses.” He emphasizes that morphology and behavior act synergistically in defensive complexes. Caro believes that predators and prey species are rarely linked intimately enough to drive coevolutionary changes in each other, and this may help explain why antipredator defenses are imperfect. Caro asks ten pressing questions about antipredator defenses. Of these, five are questions about the actions of predators. The other five involve predator recognition, multifunctional defenses, and the interactions of morphology, behavior, and coloration. These questions flow from the detailed review in the first twelve chapters of the book.
In this book, Tim Caro acts as an authoritative guide to the literature. The tour shows many places that merit revisiting and many places where further work is needed. Commentary is limited. For the most part, the data speak for themselves and the author does not interrupt. He has, of course, done a huge job in lining up what studies speak, and in what order. Tables and figures often come directly from the original studies. Other tables summarize across studies. For me, these took some study because abbreviations were not always obvious and citations were given in numbered footnotes. This book is a marvel of care, organization, and production. The most serious error I noted was confusing which vigilance experiments used Dark-eyed and which used Yellow-eyed Juncos.
Who should read this book? Anyone with a research area involving birds or mammals could benefit from the context provided by this book. Anyone looking for a research area will find a gold mine of carefully weighed studies and fascinating natural comparisons, e.g., alarm calls of Australian passerines are not the high-frequency single tones we know from the Palearctic (p. 184). Furthermore, many will want their own copy to reference repeatedly. I am sure I will use this book week-in and week-out for years to come. I applaud Tim Caro for pulling together this massive synthesis and recommend the book wholeheartedly.