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How Many Whooping Crane Eggs Does it Take to Make Another Whooping Crane Egg?
By Brian Johns, Canadian Wildlife Service, Ret.

Between 1977 and 1988 the Canadian Wildlife Service banded 134 young Whooping cranes. By keeping track of the survival and breeding history of these banded birds we are able to answer many questions, including: How many Whooping crane eggs does it take to make another Whooping crane egg? The answer is not as simple as it seems!


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.


Crane chick and parent
Photo: Vickie Henderson
Why Don’t The Numbers Add Up?
It doesn't work exactly like that because of these factors:

1. Maturity: It takes about five years for the birds to reach maturity and successfully breed.

2. Weather and Habitat Conditions: 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. (Cranes roost in water so they can hear approaching predators in time to escape, but good roost spots are fewer in times of drought.)

3. Accidents, Diseases, and Predators: Collisions with power lines can kill adults and subadults during migration. Each year birds can be lost to diseases, old age, and predators.
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, probably eight young will hatch.
About 1/2 of the hatched young will survive the summer . . . So of the eight young that hatch, probably only 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, probably only two will survive to breeding age and lay eggs. Since the gender 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 … and usually only one of the chicks survives . . .

So To Answer the Question…
We would have to wait five years for our 16 eggs to produce two next-generation eggs. Or, five years for eight Whooping Crane eggs to produce one next-generation 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. Some birds have lived more than 29 years of age and have raised over ten young in their lifetimes!

 

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