Like with chickens' combs? I haven't had any problem here in past 2 winters with my RIR hens. Back in the 1990's when I had barred rocks, a couple of them got some frostbite and lost the tips of some of their combs. Don't want to have that happen with the turkeys. To define "north" I'm in SE Michigan, right on the border of USDA Zone 6 in the "urban heat island" of metro Detroit. Generally, our coldest temps are around -10 to possibly -12. It used to get much colder back in the 1970's but things have warmed up since then, probably due to all of the development, houses, asphalt and concrete.