I hatched my own eggs in January of this year and have been happily raising my 9 chicks, they were around six weeks of age and getting a bit to large to keep in my house. Temperatures in the UK are early spring so around 50F during the day and just above freezing at night. As the chicks were fully feathered or just about - I decided to put them out in my polytunnel during the day as the temps in there is around 70F during the daytime. They seemed really happy in there so I began leaving them in the tunnel during the night with a warm box to go in and to be on the safe side I placed a small thermostatically controlled fan heater near the entrance to the box, the heater was set to keep the tunnel at 50F, this was working very well for the past week and the chicks seemed very happy and would be in their 7th week of age . Well disaster struck last night as the heater's thermostat must have broken, I came in to find the heater appeared to have been on all night and the box was very warm with all my chicks dead inside. I am puzzled as I left the heater on today to see what it is doing and it seems to be working fine, although I am sure it over heated my chicks last night....either that or they all died of cold ? Although there was not even a frost last night and the tunnel was around 50F inside this morning, I am very upset that I did this to my chickens and quite shocked that this happened so suddenly, I must have overheated them, I cannot imagine they died of cold under the conditions, they were all so healthy and full of beans the day before, my brain is struggling to process what happened I just feel terrible about it, I am guessing chickens must be very sensitive to overheating as I really did not think the heater was making it that warm in there, but possibly combined with all there body heat it was too much... I really don't know what happened. One thing I did notice was their beaks looked bluish as if they had been suffocated which is odd as I cut loads of holes into their sleeping box..as I said I really don't know all I can think is I made them too hot.