Dear
Journey North,
Today's question is: How many whooping crane
eggs does it take to make another whooping crane egg?
Between
1977 and 1988 the Canadian Wildlife Service banded 134 whooping
crane young. By keeping track of the survival and breeding history
of these banded birds we are able to answer this question and
many more.
How Many Eggs Would You Expect?
To make a Whooping Crane egg you need to have a mated pair of
cranes. Each mated pair produces about two eggs each year. So
you would think that:
• after two years there would be four Whooping Cranes that could
produce four eggs;
• after three years there would be six cranes
producing six eggs;
• and so on…
Why Don’t The Numbers Add Up?
Well, it doesn't work exactly like that because of maturity factors,
weather and habitat conditions, accidents, disease and predators:
• It takes about five years for the birds to reach maturity and
successfully breed.
• Too much rain at hatching can cause young to die of pneumonia
and other diseases.
• Drought can make the area more accessible to predators that
eat the young.
• Accidents such as collisions with power lines can kill adults
and subadults during migration.
What are the Real Numbers?
When we take all these factors into account we come up with the
following numbers:
About
1/2 of all the eggs laid will hatch . . . 
So
if we start with sixteen eggs laid in nests we will have
about eight young hatch. 
About
1/2 of the hatched young will survive the summer . . . 
So of the eight young that hatch, only about four will survive
the summer. 
About
1/2 of young that survive their first summer will actually
survive at least five years ( which is long enough to breed
at least once) . . . 
So
of the four that survive the summer, only about two will
survive to breeding age and nest. Since the sex ratio is
usually
50% males and 50% females, if all goes
right we will have one male and one female that hopefully find
each other, establish a pair bond and become a nesting pair. 
One
nesting pair will produce two eggs … 

So
To Answer the Question…
We would have to wait five years for our sixteen eggs to produce
two nextgeneration eggs. Or, five years for eight Whooping Crane
eggs to produce one nextgeneration egg.
It is necessary for Whooping Cranes to live a long time in order
to raise enough young to keep the population alive and growing.
At the present time we have some birds that are approaching 29
years of age and have raised over ten young.
Many of the breeding pairs are now incubating their eggs. The eggs
will hatch thirty days after laying. I am hoping that we will have
warm weather in June when these eggs begin to hatch, so the chicks
have the greatest possible chance of survival.
