Brian Reports: How Many Eggs Does it Take?
May 11, 2007
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As you read Brian's report this week . . .

1. How are biologists able to study the survival and breeding history of the wild flock?

2. What factors slow the growth of the Whooping Crane population?

3. What's the answer to Brian's question?

Thank you, Brian, for your great reports and photos!

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 …