Chicken or Beef – it’s an indicator!

I took my wife to the Hospital today for an MRI Scan and whilst she was in the machine (a bit like a big tumbler dryer) I read the paper in the waiting room and found an interesting fact whilst doing so:
Sales of chicken sandwiches increase in warm or hot weather whilst sales of beef sandwiches decline.
Now why is that? As usual I don’t know the answer, but what else changes in our world as we cycle through the seasons that we really could take advantage of knowing?
Any thoughts you can share on that?