Crane
Population Growth: Why So Slow?
Tom Stehn's Answer
If
only one chick usually survives, why do Whooping cranes lay that
second egg? What reasons can you think of why a second egg might sometimes
be important?
Photo
Richard Urbanek, ICF
The second egg is like an insurance policy:
- If
for some reason the first egg doesn't hatch, or one chick gets sick
and dies or is taken by a predator,
perhaps the second chick will be able to survive.
- When
food and weather conditions are ideal, both chicks sometimes survive.
(This happens
about 8% of the time.)
There is a
cost to laying two eggs! The
female has to use up energy to produce that second egg, and she may
spend additional
time trying to hatch it. Having two chicks hatch could result
in parents splitting the available food between both chicks so that
neither one
of the chicks gets enough food to survive.
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The
Lobstick Family
Brian Johns, CWS |
It
would not be good if neither chick gotenough food to survive.
But nature has evolved a way to prevent that from happening:
the eggs hatch two days apart, and the first chick to hatch is usually
stronger
and will actually peck vigorously at his younger sibling. Biologists
have
recorded dozens of pecks per minute against the younger chick
as it
gets pummeled by its sibling. The stronger chick begs vigorously
for food when the parents have a tasty morsel in their beaks, so
the older
chick ends up getting more to eat. Only when food is very abundant
and the stronger chick is fully stuffed and can't eat any more will
that
second chick get its share of the food. The second egg is
nature's way of taking advantage of those excellent years when food
is abundant for all.
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