Animal hearts not rising

Hi everyone.
So I'm pretty new to Stardew valley and still have a long way to go, but I'm having a few issues with my animals. I'm on year 3 and my chickens are 4 months old. I feed them everyday let them out every morning and they have plenty of grass to play with and eat I also have heaters installed in the coop and barn for the winter yet the hearts don't seem to be showing up. Can someone tell me why that is?

 

@Kazza84 Do you pet them every day?

 

@Verne Yes every morning when I collect the products from them.

 

Okay my heart's have began to show. When I close the doors before 60 and pet them and collect the produce before I let them out the hearts begin to show.