Broody Hen Questions

[Domes Farm]

I currently have 4 hens that have been broody for about a week. The eggs they are sitting are not fertile and my plan is to get some fertile eggs. The question is, do the hens know how long they have been sitting on the eggs. If I get some eggs for them to sit on will they stay broody long enough to hatch them?

 

[ray2310]

We had a hen that we put eggs under a week after she went broody and she sat on them until they hatched.