Birds - egg production

I’m a molecular/evolutionary biologist rather than a zoologist, and I was mocked recently for my ignorance in supposing that domestic egg-laying chickens might require the presence of a rooster. Evidently they do not - they usually produce an egg every day, even if there’s no rooster around. It seems that my understanding of reproduction in actual birds and bees is incomplete.

What happens with wild birds? Making a bird egg requires a huge energy expenditure, so surely it can’t be the case that unfertilized ova in wild birds routinely develop into fully-formed unfertilized eggs? Is this just a trait that we have bred into domestic chickens?

Wild birds will sometimes lay unfertilized eggs. I wouldn’t say it was “routine.” It happens rarely, when something goes wrong with mating or the fertilization process. It can be a problem since the bird will continue to sit on them even though they will never hatch, preventing it from starting a new fertile clutch.


Thanks both of you. The article x-ray linked to is exactly what I couldn’t find to confirm this. At least I don’t feel like I was being dumb now.

I had a parakeet (budgerigar) that laid unfertilized eggs. I don’t remember exactly how many but there were at least 6 or so over the course of several months.

Quail do this, ducks do this, turkey’s and many others. What triggers them to stop laying is when they get enough eggs to form a clutch, this may vary from chicken to chicken. As long as we keep removing the eggs they will keep laying. I think there ability to cover and incubate the eggs might play into how large of a clutch they lay. In most natural settings a male would always be present so evolution had no reason to discern whether or not a male was present.

Perhaps it’s also sensitive to the availability of ample food.

Not at the rate that thousands of years of selective breeding allows for nor for the total amount of eggs hens are born with.

Interestingly, turkeys have been found to be capable of producing offspring parthenogenetically from unfertilized eggs. Through a quirk of bird genetics, all offspring produced this way are male.

Unfertilized chicken eggs can sometimes start to develop parthenogenetically, but development stops once the eggs are incubated, so no young are produced.

Here is my question: How does the bird stop laying eggs? What is the trigger? It sounds like something not under conscious control, but something biological. It’s not like they can think “Oh, this is enough eggs for now, I will stop laying”, is it?

Thank you, x-ray vision! Ignorance fought!

We have backyard hens (no rooster), but I thought I’d add that broodiness varies from hen to hen. We have one who gets very broody after a clutch of 2-3 - she sits on the eggs, refusing to leave the nest & won’t even let us remove them, so we have to do it at night. While other hens will ignore a clutch of 6-8 eggs & range all day. Not sure if this is due to genetics or just differences in personality.

I think it may have something to do with their body being able to cover all the eggs, when it feels like they have reached the limit if eggs they can cover I suspect they stop laying. As said above some of them will stop and sit at 3 or 4 eggs.

Different species of birds lay anywhere from one to more than a dozen eggs in a clutch, so that’s not it.

Most likely, there is a hormonal trigger that tells a female to stop laying after the typical clutch size is complete.

We have extended this trait by selective breeding, but it was there already. Non-domesticated birds also produce unfertilized eggs, just not as often.

And it occurs in other species, too.
Human females, for example, produce an egg every month, whether fertilized or not (discarded via menstruation).

I’m not sure what you mean by “no” - what did you imagine we were talking about other than artificial selection?

Discarding a single cell doesn’t waste the energy required to form a bird’s egg. My question arose because of that energy requirement.

I guess it depends on one’s view on what “routinely” means within the bounds of what the OP was asking (considering selectively bred domestic hens lay year round and tens of times the amount of wild fowl, which lay primarily in spring) and his mentioning an energy expenditure by the hen. Based on his response, my interpretation resulted in him getting the info he was looking for. As was mentioned in the first reply by Colibri, the majority of eggs laid by wild birds will be fertilized. Also, your responded with “we have extended this trait by selective breeding, but it was there already” doesn’t take into account my “no” response regarding the factors I just mentioned.

However, the information in my link is correct. Whether or not I should have prefaced with a “yes” or “no” is subjective.

Well, it depends on interpreting the question that I asked…

…as meaning something other than artificial selection, i.e. selective breeding for desirable traits within the range of natural genetic variation. I’m not sure what it could be taken to mean other than that.

I have some young zebra finches. One of the females laid about 7 eggs over the course of a week or so. None developed. I’m told it was because she was so young - only a few months old - and the next clutch might develop.

Seems weird to me. But other species aren’t required to function according to my lack of knowledge.