A Squirrel in the Hen House

This morning when I went out to feed the hens, Amber was making such a commotion, so Betsy and I went over to see what all the fuss was about.  Amber is in a pen on her own as she has just hatched one chick and it’s only two days old.  I couldn’t see the chick anywhere and Amber was squawking like mad.  When I opened the door of the pen, there was a squirrel inside!  Betsy soon saw him off and shortly afterwards the chick came out of hiding and calm was restored.  But how did the squirrel get in?