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1 Matt Johnson HSU Wildlife 365 Ornithology ORNITHOLOGY (Humboldt State Univ. WILDLIFE 365) LECTURE 15 – VISUAL COMMUNICATION I. II. III. Introduction – birds communicate with each other and other animals with displays – vocal and/or visual. These displays transmit information from the birds (sender) to some other animal (receiver), which may be a potential mate, a rival, or a predator. Today’s lecture is on visual displays. Plumage Color. A primary mode of visual display in birds is by way of plumage color…… A. Cryptic coloration…..and the principal “display” performed by coloration is to be inconspicuous. Cryptic coloration is animal camouflage. And in birds it is well developed in some species. Some examples: 1. Female Eider. 2. Others. -- common potoo (posture and plumage) 3. Disruptive coloration – Killdeer. 4. Countershading – basic plumage Dunlin. B. Other plumage coloration. 1. Reverse countershading – alternate Dunlin. 2. Uniform coloration is conspicuous to signal to conspecifics– Mt. Bluebird/Cardinal. 3. Contrast edges enhance signal patches – Hooded Merganser/Hooded Oriole. How birds ID themselves. A. You think you have it hard IDing all these birds! Think about a young Herring Gull in a winter mixed flock of California, Western, Herring, and Glaucous-winged Gulls, all ages, all sexes….determining who is who is no easy task. Actually, as you might have guessed, birds are profoundly better at distinguishing subtle differences in behavior that reveal a bird’s species, age, and sex than we can ever dream of being. They don’t need field guides. Nonetheless, there DO appear to be some adaptations in birds to make species, age, and sex very obvious in some cases. B. Sexual differences in coloration usually reflect inequalities in selection pressure based on different roles in reproduction (males attract mates, females incubate): males are selected for bright, attractive colors to maximize probability of breeding. Females 2 are selected to minimize predation risk. This pattern varies as you’d expect when reproductive roles vary: species in which both sexes incubate (Ash-thr. Flyc.) are sexually monomorphic. Species in which females attract mates and males incubate show bright females and dull males (Wilson’s Phalarope). We’ll talk more about this in a bit when I talk about sexual selection. C. Delayed plumage maturation – many birds have different plumages as young than as adult birds. These immature plumages are often more cryptic than adult plumages, but not always. These have evolved at least partly to maximize cryptic coloration. The obvious reason is to reduce risk of predation. But many immature birds are subordinate, so it also behooves them to be inconspicuous to dominant conspecifics. Also, many immature male songbirds fail to attract mates to a defendable territory. However, they still attempt some breeding by “sneaking” copulations with already paired females. Their cryptic coloration is multipurpose indeed. Redstart slides. 1. Why delayed plumage maturation? 4 ideas: using costbenefit analysis (these are not mutually exclusive). 2. Cost of course is reduced reproductive success. These birds do NOT attain adult breeding plumage which has evolved for successful mate attraction. Instead, they remain cryptic. So what might be the benefits of remaining cryptic for an extra year (these benefit must outweigh the cost or this molt sequence would be purged from the population via natural selection.) 3. First, the benefit could be enhanced survival simply because risk if predation on a cryptic bird is less than on a bright plumaged bird. This would be especially advantageous for long-lived bird species, because they could then reap the benefits of early survival for many years down the line. Thus, early survival contributes more to lifetime reproductive success for long-lived the short-lived species. We’ll talk more about LRS in a couple of weeks. 4. Second, the benefit could be a reduced energy expenditure for growing brown (energetically cheap) feathers rather than bright (energetically expensive) feathers. 5. Or, there could be more subtle benefits. For many birds, their likelihood of reproducing successfully in their first year is poor (regardless of plumage) because of their lack of experience. Thus, they are in a bad situation, and perhaps delayed plumage maturation has evolved simply to make the best of an already bad situation. Two hypotheses in this line of thinking. 6. Third, a dull plumage could reduce intra-specific competition from breeding adults (e.g., males) by sending the 3 plumage signal that the dull bird is in its first year and thus less of threat than other adult birds. The cryptic plumage could tell other adults, “I’m a pee-on, don’t pick on me.” 7. Fourth, birds (especially males) in their first year of life may be unlikely to successfully breed on their own. But as we study birds that we previously thought were largely monogamous more and more, we’re gradually learning that there is a lot more “extra-pair copulations” out there than we previously thought. A female-like plumage may increase an immature birds chances of obtaining “sneak copulations” with other females because they will both more inconspicuous and less imposing to adult males than if they were to be adorned with bright adult breeding plumages. D. Individual recognition. Redstart overhead? (not covered) IV. Evolution of displays. In text, see Pelicaniformes and Manakins. V. Rituals. A. What are rituals? – Ritualization is the evolution of stereotyped signals and displays from “normal behavior” movements. For example, a bird pecking another bird is clearly sending a signal, but it is not a ritual, it is real. A bird that cocks its head as if to peck another is sending a signal via a ritual whether or nor that behavior is actually ever followed by a peck. B. From where do they originate? Rituals usually develop from incomplete locomotor movements – called intention movements. For example the head-throw display of courting male Common Goldeneyes probably evolved from the initial movements associated with leaping out of the water. OVERHEAD C. Why do they evolve? 1. To increase efficiency of communication. Much information can be transferred without going through entire motions. 2. To increase clarity of intent. Rituals are usually very stereotyped, this is, they don’t vary much. They are often allor-nothing. This makes them clear. 3. Graded displays. But some are in fact, graded. But these still may communicate information clearly. Steller’s Jay crests OVERHEAD. VI. What do displays mean? Deciphering the information transferred in behaviors is a huge challenge in ornithology. Often, a single display can vary in subtle ways with dramatic changes in purpose. Crouch in female redstarts (submission posture for breeding) -vs. wing droop (aggressive posture between males). VII. Agonistic behavior. A. When two birds interact, it is important to remember that each individual has selfish purposes that can foster either hostility or cooperation. A male bringing food to a female looks “nice” but 4 we all know why he does it. Other examples abound in nature of cooperative behaviors that clearly confer advantage on both participants. Thus these behaviors are, at their core, selfish behaviors. B. In this light, the nature of interactions between rivals and partners are especially interesting. 1. Rivals. Competitive encounters are complex mixtures of aggressive (threat and attack) and submissive (submit, flee) displays. These are called agonistic interactions. a. Threat displays usually mimic initial attack movements, and they serve to transmit intent while diminishing risk of injury to both parties. b. Threatening birds usually sleek their postures and look directly at the other bird, submissive birds often fluff-up their feathers and turn away. 2. Partners. Interactions between sexes often start agonistically. Males are aggressive toward female – if she does not flee, she sends the signal that this interaction is not a rival interaction, and her/his displays usually shift from hostility to appeasement, to mutual subordination, and to solicitation. Then they form their bond. C. For years biologists assumed rituals evolved for clarity – meaning they were “true.” But it is becoming increasingly clear that many times, birds are being deceitful. They bluff, they even act like females when they are males. Escape signals are usually more reliable. It’s the aggressors that are often fibbing. 1. It’s been demonstrated that a purely “honest” signaling bird society is not a so-called “evolutionarily stable strategy.” That is, if one bird began to cheat by bluffing (e.g., acting like it was the strongest bird in the group but was in fact one of the weakest) it would obtain a disproportionate amount of resources because the other birds would react subordinately to its displays, even though many of them may be stronger. Thus, this “cheater” or “bluffer” strategy would be more evolutionarily successful than the honest strategy and it would proliferate in the population. 2. Ah, but a society of all cheaters is not stable either. 3. In the end, what IS stable is a bird society made up primarily of honest signalers, with a equilibrium reached between a smaller number of “cheaters or bluffers” (those that display dishonestly) and “checkers or callers” (those that will test a display regardless of its intensity). 4. How surprising is that? Not very. Our own societies (and in fact even in ourselves) work the same way. Most people are relatively honest, a few cheat, and a few are paranoid (thinking everyone is cheating). It’s stable. 5 VIII. Sexual selection A. Dramatic differences between male and female plumage and size are commonplace in birds. A few of these may result from real differences in their ecologies, but most result from what Darwin called “sexual selection.” B. Sexual selection is evolutionary pressure stemming from contests among males (usually) for mates and among females (usually) for preferences for particular traits in mates. Easiest to see in polygynous birds. C. Females, generally, are much more “choosy” than males. This is because, ultimately, they have more invested per reproductive event because female sex cells (eggs) are larger, more energetically costly, and therefore more rare and valuable than male sex cells (sperm). Thus, females have more at risk in a particular reproductive event -- their evolution favors quality over quantity; males favor quantity over quality. D. The result is that males usually compete for mates, whereas females do less so; and therefore males’ reproductive success varies more than does females (especially for non-monogamous mating systems). E. Thus, traits that enhance a male’s ability to attract a mate are quickly and efficiently selected – often to extreme ends. F. Examples. 1. Red-wing Blackbirds. a. Females choose mates based on the quality of the territories they defend. b. Males that had their epaulettes experimentally blackened get attacked more often than control males, and usually lost their territories (and reproductive potential). Those that did manage to retain their territories still bred. Thus, the epaulettes were important signals to rival males, but were unimportant to females (they were interested in the territories). This is still sexual selection because the epaulettes serve no purpose other than to maximize reproductive output by indirectly procuring mates. 2. Long-tailed Widowbird. a. They are marsh birds like the red-wings. b. But here, evolution has favored the extension of the tail. Experimental manipulations have demonstrated that birds with short tails attract fewer females. OVERHEAD 3. Bowerbirds. a. The most elaborate sexual selection of all is perhaps in the Bowerbirds of Australia/New Guinea. OVERHEAD b. Males construct elaborate displays of sticks, shells, parrot feathers, etc. to attract females. 6 c. The duller the male (by species) the more elaborate the bower. d. Females clearly choose the most well-maintained decorative bowers. e. This probably serves as an accurate indicator of a male’s vigor, because it takes considerable effort to collect, defend, and/or pilfer the items. Only a male adept at gaining food and avoiding predation can manage the time and energy for the most impressive bower. G. Good genes vs. fashion icons. -- This brings up the idea of what sexual selection really signals to females. Two hypotheses exist (not mutually exclusive) 1. Good Genes. a. Some displays are best performed by superior (strong etc.) males. Thus, the best performer is probably very fit. b. Other sexual characteristics may signal fitness by serving as “handicaps.” Having an extremely long tail or very conspicuously bright plumage but still being alive may signal that the male is adept at avoiding predation or obtaining enough food to enable rapid feather growth. 2. Arbitrary choice (fashion icon). a. These may often start as signals for good genes. But if for some reason…any reason…females just prefer longer and longer tails, then only males with long tails will reproduce and the pattern will be perpetuated. b. This may be the case for extreme sexual plumages/displays such as those found in the birds of paradise. SLIDE