Science Questions

Why do chickens lay all year round?

Sun, 14th Oct 2007

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Ken asked:

My dad used to keep chickens and Iíve always wondered why the same chickens lay eggs all the year round when most birds are seasonal? Also, why they havenít got bloodspots in them like most eggs have?


Well, itís actually all to do with selective breeding over thousands of years. Originally, if you get very old breeds of chicken, they are seasonal.  

Over each generation people have been picking the chickens which lay the most eggs and trying to breed those chickens. And so after generation after generation, the period which they lay eggs for is longer and longer and longer until, Iím not sure, probably the last 200 years they got to the point where they had chickens which would lay eggs all year round. So now, chickens will lay eggs all year round.

Naked Scientist Dave Ansell used to keep chickens when he was growing up , and noticed that they would lay a lot more in the Spring than in the rest of the year. In the Spring youíd get 2 or 3 times more eggs than you would, say, in the Winter.

With regards bloodspots, lots of people discard eggs that have blood spots in them because they think theyíve been fertilised. But thereís more than one reason why you can have a bloodspot in an egg.  As the egg is being formed in the oviduct, which is the part of the chicken which makes the egg, sometimes you can get a tear in the tissue or a leak from a blood vessel. This squirts a little bit of the chickenís blood into the egg. Itís not harmful and doesnít mean the eggís been fertilised, itís just a bit of blood. So thatís why you have bloodspots.


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