How
Many Whooping Crane Eggs Does it Take to Make Another Whooping Crane
Egg?
By Brian Johns, Canadian Wildlife Service
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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 this question and many more. How
many Whooping crane eggs does it take to make another Whooping
crane egg? You'll see that the answer is not as simple
as it seems!
How Many Eggs Would You Expect?
To make a Whooping cCrane 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.
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Photo: V ickie Henderson |
It doesn't work exactly like that because of these factors:
Maturity: It takes about five years for the birds to reach maturity and successfully breed.
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.)
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.
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. We have
some birds that lived more than 29 years of age and have raised over ten young
in their lifetimes!
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