How do hens know which eggs to sit on?

[BorregoMike]

i have 25 hens and one busy rooster, all 6 months old. The hens started laying a couple weeks back and I now get 15 or so eggs each day. With all these hens laying, and with no idea on my part which eggs might be fertilized, how does a hen know? And if I get chicks will they be from one hen or a combination of eggs from the bunch of them?

 

[Kiki]

When a hen goes broody she doesn't care if she is sitting on fertile eggs or rocks.

 

[rosemarythyme]

When a hen goes broody she doesn't care if she is sitting on fertile eggs or rocks.

Or golf balls, or nesting material with nothing on it at all...

 

[Percheron chick]

As you get familiar with your pullets laying, you will start putting eggs based on shape, size, color and preferred laying spot with individual girls. The only way to assess fertility is to crack them open or incubate. Very high probability that her subsequent eggs are the same. With that many pullets and only one cockerel, chances are that many of the eggs are infertile .
Most clutches of eggs are a combination of multiple hens. Again shape size and color will help you choose which ones to go with.

 

[aart]

i have 25 hens and one busy rooster, all 6 months old. The hens started laying a couple weeks back and I now get 15 or so eggs each day. With all these hens laying, and with no idea on my part which eggs might be fertilized, how does a hen know? And if I get chicks will they be from one hen or a combination of eggs from the bunch of them?

If you want to hatch chicks you need to wait until a bird goes broody,
don't just leave all the eggs in the nests, gather them all daily.

To check fertility, look at yolks when you break egg to eat:

This thread shows a plethora of examples of fertile and non-fertile yolks:
http://www.backyardchickens.com/t/16008/how-to-tell-a-fertile-vs-infertile-egg-pictures


Here's what I look for to tell if a bird is broody:
Is she on nest most the day and all night?
When you pull her out of nest and put her on the ground, doesn't she flatten right back out into a fluffy screeching pancake?
Does she walk around making a low cluckcluckcluckcluckcluck(ticking bomb) sound on her way back to the nest?
If so, then she is probably broody and you'll have to decide how to manage it.

 

[FlyWheel]

Another way to tell if an egg is fertile or not (this is simple to the point of being lame) Is note which hens your rooster is servicing; her eggs probably will be fertile.