[Illustration]
The importance of co-operation, contrasted with competition, has
assumed increased importance in discussions of evolution, as it has
in discussions of human social progress. Co-operation in nature is of
various kinds; from the manner in which a forest shelters the squirrel
to the manner in which two or more individuals of one species work
together for a common object. The working together of two birds to rear
a family is so well known an affair that one forgets that it is an
example of co-operation, not only in building the nest and brooding and
feeding the young, but also in defending the nest and the young.
Sometimes more than one species will join in ousting an enemy. For
example, when a cat caught a young robin, recently out of the nest,
the parents, in their frantic effort to make the cat release the bird,
attracted the attention of another robin and a pair of cardinals
nesting nearby in a honeysuckle. All five birds dived at the cat,
screaming and pecking it so vigorously that it released the young robin
and fled.
EAGLES JOIN EFFORTS More spectacular are some of the co-operative
activities of birds in food getting. Bald eagles sometimes feed on
ducks. Frequently two eagles may combine their efforts. The two birds
may work together to force a black duck from the air onto the water,
and when they are trying to catch a diving duck, they much more quickly
exhaust their prey by swooping at it in turn. Bald eagles sometimes
take water birds too large for them to carry, and then they must flap
along dragging their prey on the surface of the water to the nearest
shore. On one occasion an eagle dragging a large cormorant ashore was
joined by two other birds, and all three took turns in dragging it.
When they got it ashore, all three shared it.
Several fish-eating birds co-operate in capturing their prey. "The
merganser is primarily a fishing duck ... very skillful and a voracious
feeder. It pursues underwater and catches successfully the swiftest
fish. Often a party of sheldrakes may be seen fishing together, driving
the panic-stricken fish into the shallows or into some small pool where
they may be more easily caught," according to A. C. Bent.
When a school of fish approached a flock of white pelicans, the birds
suddenly assumed a circular position, surrounding the school. All the
pelicans moved slowly but cautiously toward the center of the circle,
their heads near the surface of the water or partly submerged and their
necks slightly extended. The birds moved in perfect unison, making the
circle progressively smaller, ready to engulf their helpless victims at
the first opportunity. When all the pelicans were close to the fish,
the birds made rapid jabs at the fish and apparently consumed a large
number of them. It appeared that every bird got from one to several
fish.
13,000 BAND TOGETHER Avocets and, to a lesser extent, the black-necked
stilts also band together for co-operative drives on small fry and
aquatic insects. Such drives are made in water of wading depth.
Instead of forming circles the birds present compact spearhead and
wedge formations and sweep the bottom muck with the characteristic
back-and-forth side movements of their long bills. As many as 13,000
avocets have been observed taking part in such co-operative feeding
projects.
Another striking example is furnished by black vultures observed by
E. A. McIlhenny. A three-quarters-grown skunk was wandering across a
field. A vulture alighted near the skunk which immediately stopped
and raised its tail. Other nearby vultures joined the one nearby the
skunk, and when six or eight of them had gathered one suddenly attacked
it. The skunk immediately discharged its defensive scent, but without
effect, for the vultures attacked in a mass and other vultures circling
above joined in until there were probably twenty-five or more around
the skunk. With much flapping and croaking, the vultures pulled it
about until it was dead, and then devoured it.
On another occasion a black vulture came from high in the air to
alight near two full-grown opossums following a narrow cattle trail.
The first vulture was almost at once joined by many others until there
were probably between seventy-five and one hundred black vultures
following the opossums. Suddenly three or four of the vultures attacked
and the others joined in. Quickly both opossums were covered with a
swarm of hissing, flapping birds, and within fifteen minutes there was
nothing left of them but the larger bones and the hides, and these were
stripped of every vestige of flesh.