Download lecture 15 – visual communication

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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
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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
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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
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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.
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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.
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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