Science Questions

Why do chickens lay all year round?

My dad used to keep chickens and I’ve always wondered why the same chickens lay eggs all the year round when most birds are seasonal? Also, why they haven’t got bloodspots in them like most eggs have? Ken

Well, it’s 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, I’m 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 you’d 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 they’ve been fertilised. But there’s 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 chicken’s blood into the egg. It’s not harmful and doesn’t mean the egg’s been fertilised, it’s just a bit of blood. So that’s why you have bloodspots.

October 2007