The sweater is likely to function like snow gear that is too tight. I ride a motorcycle even when temperature is below -10 F and I have no protective shield. My attire is more like that used by riders of snow mobiles than motorcycles which means I am exceptionally well protected. I have to be careful in how the clothing is worn. The snow mobile suite is a good insulator and wind break that approximates the chicken sweaters discussed here. Under that are layers of cotton and wool as well as thermal undergarments. When outer outfit is too tight it causes two issues. First, trapped air between layers is lost reducing insulatory value which can get me cold to core. Second I can restrict blood flow to extremeties (arms and legs) making it hard to keep them warm even with best gloves and boots. Another challenge is that if my attire gets wet the only part that insulates well still is the wool and it generally is not good enough. The chicken sweaters compress the feather layers forcing out the insulatory air and could be very bad news if bird is exposed to a lot of precipitation or even heavy frost. I suggest exploring other approaches to keep the birds warm first. Wind breaks, lost of straw and some sort of heat source tested for fire risk would be first approaches. The sweater approach would be used only in conjunction with the previous three and bird would not get outdoor time. I have had game roosters that got seriously de-feathered fighting in muddy hog lot. Sweaters would have been applicable there but birds would still need to be kept in tight but dry quarter and simply given lots of food. Additionally the broken feathers are pulled to promote rapid regrowth. Doing the sweaters for the cute factor is putting your birds are risk.
