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Feathers and flight, section 4
The Display and Fighting Hypothesis
Either the arboreal or the cursorial hypothesis would work, and work
much more easily, if a protobird already had long, strong feathers in
the right places and already had powerful arm movements. Jere Lipps and
I suggest that display was involved in the evolution of flapping flight
as well as in the evolution of feathers. Display provided long, strong
feathers on arms and tail. Successful display was increased by
lengthening the arms, especially the hand, and by actively waving them,
perhaps flapping them vigorously. Flapping in display would have
encouraged the evolution of powerful pectoral muscles.
But a threat display must not be seen as an empty bluff. Fighting is
the last resort. Living birds often fight on the ground, even those
that fly well. Wings are no longer clawed but are still used as weapons
in forward and downward smashes (steamer ducks are particularly deadly
at this). Beaks and feet can be used as weapons too and are most
effective when used in a downward or forward strike.
A strong wing flap, directed forward and downward, is also the power
stroke that gives lift to a bird in takeoff. Lipps and I suggest that
strong wing flapping is a simple extension of display flapping,
encouraged by fighting behavior. Powerful flapping used to deliver
forearm smashes could have lifted the bird off the ground, allowing
it also to rake its opponent from above with its hind claws. The more
rapidly the wings could be lifted for another blow, the more effective
the fighting. This would rapidly encourage an effective wing-lifting
motion that minimized air resistance, so the wing action would then
be almost identical to a takeoff stroke.
A variant of our idea has also been proposed by Kevin Padian,
who prefers to think of the wing stroke evolving from the arm
strike used by a theropod in predation. ItUs not clear how this
could have led to whole-body takeoff, however.
A few living birds use their wings extensively as weapons. The steamer
ducks of the South Atlantic are large, powerful birds with heavy, bright
orange, horny knobs on the wings of both sexes. These are used by both
sexes in display and fighting. Steamer ducks (especially males) fight
a lot among themselves for mates and territory, and they often kill
other species of water birds, holding them by the neck and beating
them to death with the wing knobs. Some species of steamer duck are
flightless; in other species, the males are often too massive to fly,
even though juveniles and females can fly well. Selection has favored
fighting ability over flying ability for many steamer ducks. Flight is
perhaps less important for them than for many birds, because they live
in shoreline habitats where food is plentiful all year round.
Archaeopteryx fits our display-and-fighting hypothesis well.
It was well adapted for display. Like any small theropod, it
was well adapted for fighting with its teeth and the strong claws
on hands and feet. Archaeopteryx did not have long primary feathers
on its fingers, probably because they would have hidden the claws
in display and would most likely have broken in a fight.
Did Archaeopteryx Fly?
Archaeopteryx was at best a poor flier. If it flew at all, it could
not have sustained flapping flight for long. In small flying birds
today, the wishbone acts as a spring that repositions the shoulder
joints after the stresses of each wing stroke. It probably helps to
pump air in and out of the lungs, and to recover some of the muscular
energy put into the downstroke. In Archaeopteryx and in other theropods
with wishbones, the bone is U-shaped and strong and solid; it could
not have acted as an effective spring. Archaeopteryx did not have the
pulley system of the shoulder that gives a rapid upstroke, and it must
have had particular difficulty flying at low speed, and in takeoff or
landing. Its shoulder joint did not allow it to raise its arms very
far above the horizontal, so the downstroke could not have been
powerful. Furthermore, Archaeopteryx did not have the long primary
feathers on the wing tips, or the breastbone anchoring the chief
lifting muscles for the wing that are needed for routine takeoff
and landing on the ground. In fact, it evolved structures that were
active deterrents to flight. Its tail was long and bony, with long
feathers. Among living birds with display feathers, this sort of tail
is aerodynamically the worst of all possible tail styles, adding a lot
of drag and little lift. Even the feathers themselves are more like
those of flightless modern birds than feathers used today for flight,
as Speakman has documented.
Archaeopteryx, then, was a fierce little fast-running, displaying
bird, which probably spent its life scurrying around the Solnhofen
shore, hunting for small prey such as crustaceans, reptiles, and
mammals. In hunting style, Archaeopteryx was probably much like the
roadrunner of the dry country of the American Southwest, but its
ecological setting was closer to that of a steamer duck - a shoreline
with year-round food supply. Archaeopteryx did not compete in the air
with the pterosaurs that are also found in the Solnhofen Limestone.
>From Display to Flight
In our theory, display and fighting were simple selective agents
that encouraged the evolutionary transition from small dinosaurs
to birds. The idea fits with our current knowledge of the biology
and behavior of living birds. Display, and fighting if necessary,
is very important, even within a species. Bald eagles and frigate
birds often try to rob other birds of food instead of catching prey
themselves. Because the penalty for wing injury is high, many birds
can be intimidated by display into giving up their catch rather than
fighting to defend it.
Display and fighting in birds, whether for territory, dominance,
or food, takes a lot of energy, but only for brief periods or seasons,
and it provides an enormous payoff in survival and selection.
Sexual display in most living birds must be done correctly, or no
mating takes place. New behaviors are quick to evolve, and they are
evolutionarily cheap, because they usually do not require any important
morphological changes in their early stages. Bowerbirds, for example,
show distinct behavioral differences in display between closely
The display hypothesis suggests that a protobird gained flight
behavior, anatomy, and experience at low ground speed and low
height, ideal preflight training. The selective payoff for successful
mastery of the flight motions gave significant advantages, even before
flight itself was possible. From that point, the many advantages of
flight were added to those of social or sexual competition.
Lipps and I envisage Archaeopteryx as a small, fierce predator,
capable of liftoff but not true flight. Once liftoff was achieved,
flapping flight quickly followed. There is no need to suggest any
difficult evolutionary sequence to complete the final transition
to full powered flight. In more advanced birds than Archaeopteryx,
the pulley system of the shoulder evolved for quick wing upstrokes,
while the wishbone evolved into a spring. The breastbone evolved as
the anchor for the flight muscles. The forearms became longer,
lighter, and more fragile in bone structure, becoming specialized as
wings, and losing the finger claws. The feathers became more
aerodynamically suited to powerful swishes through air. Meanwhile,
the feet and beak became the dominant fighting weapons, as in most
living birds today.
The display hypothesis for the origin of flight is particularly
attractive because it demands few of the assumptions required by
the arboreal or cursorial hypotheses, yet it is fully compatible
with the morphology of Archaeopteryx and the biology of living birds.
NOTE: If you want to quote this hypothesis, IUd suggest that you use
the phrase Cowen and Lipps in Cowen (1994), and refer to my textbook.
I hope that Jere and I will be able to get the full-jargon version
out in a journal some day. Meanwhile, Jere and I are responsible for
the idea, and you have to blame me for the detailed wording that I
prepared for my text.
References to follow.......