Friday, February 12, 2010

Egg-xactly right - Friday's fascinations, facts and folklore

What is the right number of eggs to lay?

Some birds are termed determinate egg layers because they lay only a certain number of eggs and no more.  The crow, budgie, tricolored blackbird and many shorebirds are examples of determinate egg layers.  If an egg is destroyed, the parent(s) will simply tend to whatever remains.  In experiments, it is also seen that these birds do not curtail egg laying if eggs are added to the nest.

Other birds are termed indeterminate egg layers.  Three types of indeterminate egg layers are seen:
  • removal indeterminacy - after  laying a clutch of eggs, if one is stolen/destroyed/removed, the female will lay another in its place.  Examples are goldeneye ducks, California quail, cockatiels, coots and of course - fowl like chicken! In one experimental case where an egg was removed every day from a Northern Flicker's nest, the female flicker laid 71 eggs in 73 days!
  • addition indeterminacy, - if eggs are added to the nest in experiments, the birds layer fewer eggs than normal
  • removal and addition indeterminacy - birds respond to egg removal by laying more, and respond to egg addition by layer fewer. 
Other factors obviously come into play too - for example, light levels, temperature, food supply, longevity of species, etc, etc.
 Photo by Michelle Eve
Please don't try any experiments at home! It is illegal to remove bird's eggs from nests (Migratory Bird Treaty Act 1918).

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