Population Growth: Why So Slow?
By Tom Stehn
Whooping Crane Recovery
Photo Joan Garland, ICF
Whooping crane population needs every adult pair to do two things:
live long lives, and try to raise a chick every year. The
population has little
to increase in size rapidly. These are some reasons:
even start nesting until they are at least 3 years of age,
and they usually don’t lay eggs and hatch a chick until
about age 5.
have to experience nesting to get it right. They must gain experience
and protecting a chick from predators to ensure its safety.
Much of this parental behavior is instinctual, but the experience
cranes to do it the right way.
though Whooping crane pairs lay 2 eggs every year, normally only
the chick that hatches from the first egg survives.
The eggs hatch 1-2
days apart, so the chick from the first egg is usually stronger.
It will even peck at its sibling repeatedly to try to drive
it off. That's how
the stronger chick makes sure it gets most of the food
brought by the parent cranes.
Ability to Have Young =
Slow Growth in Numbers
all the things that can go wrong with trying to hatch an egg and
then protecting the flightless youngster from
predators for 80
it learns to fly, a Whooping crane pair usually only
brings a chick to Aransas NWR every other year. I
know one male crane that
and only once successfully brought a chick
cranes are not like mallard ducks that can lay and hatch 8 or more
With this limited ability to have young, and the
fact that Whooping cranes
normally aren’t successful parents until they
are about 5 years old, the population can only grow
Like the Tortoise Racing the Hare
make up for this
slow reproduction by living long lives. Birds
that live 30 or more years
of age are very unusual in the bird kingdom.
Parrots are another bird that
can live many years, but most kinds of birds
live less than 10 years of age. Birds with short lives start
age 1 or 2 and can
often produce several young every year. Think
of cranes like the story about
the tortoise racing the hare. The tortoise beat
the hare in a race, but only by continuing to plod forward
a long time. If you are
and reproduce slowly, you have to keep going
a long time in order for your species to survive.
This: Journal Questions
- It can be
tough being the younger sibling! Can you relate to this? Is your older
and sister allowed to do things you aren’t allowed to do? Why does
it seem unfair?
- 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?
After you list your ideas, see what Tom Stehn says: >>