[Illustration]
Our northern shrike is a songbird which has developed feeding habits
along the lines of those of a hawk. Whereas most birds its size are
content with fruits, seeds, or insects of a size it can beat or bite
and then swallow whole, our northern shrike takes not only small
insects but prefers large ones, and mice and birds too big to be
swallowed whole. It is an opportunist and takes what is most abundant
and easily accessible. The shrike's strong hooked bill is a powerful
weapon, used with a nipping motion that is directed at the back of the
head or neck of mouse or bird.
Now with the dead sparrow or mouse the shrike is at a disadvantage.
With a powerful bill hooked at the tip its feet are still those of a
songbird and are not strong enough to hold its large prey while pulling
it to pieces. Only small insects are held in one foot and pulled to
pieces. To meet this need for holding large dead prey the impaling
habit was evolved. The result of this is the so-called larders, which
form a fancied resemblance to meat hanging in a butcher shop, and have
given the birds their name of butcherbird. A thorn tree, a splintered
end of a branch, or even the barbs of wire fencing may serve. The
shrike flies to one of these, carrying the prey in its bill (rarely in
its feet), and with a pulling motion fixes the prey on a projection
point. Sometimes instead of impaling the mouse or bird it pulls it into
the fork of a branch, and so wedges it there. Now the food is firmly
held, and the shrike can use its bill effectively to pull off pieces
of flesh and swallow them. When the bird has fed, it leaves the rest
of the animal hanging where it was. It may return to this food and
make repeated meals of it if not spoiled, or dried up, until the whole
is devoured. But often parts of meals are left hanging and discarded.
If suitable thorn bushes are scarce the shrike may return time after
time to the same tree with its prey, and in time this tree may come
to be decked with many partly devoured carcasses. Such trees are the
so-called "larders." There is another aspect of shrike behavior that
adds to these larders. The shrike, even when replete, may seize any
prey that appears and impale it. The bird's organization is such that
the sight of a small moving animal may start the actions that end with
impalement even when the bird is not hungry. This food usually is not
eaten later.
Thus the shrike's "butcher shop" is not primarily a store of food,
even though it sometimes serves as such when in times of scarcity
remains of old meals are eaten. It is not a gathering of food in time
of plenty and saving it for a later use. Rather the placing of many
items in one tree is the result of its being a favorable impaling
place. And the impaling is behavior developed to overcome the weakness
of the claws in a bird whose disposition and strong beak enable it to
prey habitually on larger animals which otherwise it could not tear to
pieces and eat.