Chickens and in general, all birds are capable (but not always successful) of the same potential energy intake during the summer as in the extreme cold in winter. Take metabolism for example. There are two types known as existence and productive energy.
Existence energy is the composite of energy requirements involved in maintaining standard or basal metabolism, in chemical heat regulation, in securing food and drink (economic maintenance) and in the heat increment or "specific dynamic action" of digestion and assimilation.
Productive energy (economic production) is defined as that portion of the metabolizable food intake over and above the energy requirements of existence at a minimum level of activity. Productive energy is not to be confused with net energy which is defined as the metabolizable energy of food minus the heat increment of feeding, and which is available both for other factors in existence metabolism and for productive activities. Productive energy is of importance to the bird as this is the energy utilized for such activities as growth, locomotion, reproduction, molt, and fat deposition.
The type of food consumed, its protein content, and its balance of essential elements and vitamins may affect the efficiency of its utilization as energy.
The productive energy for any given month is simply the difference between the potential intake and the amount required for existence. So how fast your chicks grow is dependent on how much food they get over what they need to exsist and thrive. Summer months provide more food hence a faster growth rate. Winter is slim pickings for free ranging hence a slower growth pattern unless supplimentation is increased. Basically, are they going to be a marathon runner or a couch potato.
