Shorebirds of North America: The Photographic Guide

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 clas-