runny egg whites

[fluffycreatures]

Does anyone know what causes a runny egg white?

Specifically my friend who has been raising chickens for eggs for quite a while says:

I collect eggs daily (from Rhode Island Red birds) and generally they are eaten within the week. When I crack an egg for frying, sometimes the white is thick and holds its chape in the hot pan. Other times the white runs off into a liquidy puddle in the same hot pan.
Does anyone know what conditions lead to such runny egg whites?

Thanks
Dianne