That's a cute little chicken - even when she's molting. I was wondering, do the bantam breeds have a rougher time in winter? What about the really little ones like Seramas? We were tempted to hatch some, but decided to stick with the larger breeds.
[COLOR=900000]She definitely is a cutie.
As for your question, I will first say the only bird I have ever lost to the cold was a large fowl Barred Rock that was even more naked than Rosie is in that picture. Chickens of any size can handle the cold pretty well as long as they are properly equipped--and by properly equipped, I mean fully feathered in!
I won't go into too much detail, but the average feather worn by most chicken breeds is perfectly suited to insulate from the cold, and the bird itself will produce enough heat (which those feathers hold close to the bird's body) that the bird shouldn't have any problems with very cold temperatures. Breeding, however, has brought on different kinds of feathers, namely frizzled, silkied, and hard feathers. In theory, those different feather structures make the birds less insulated against the cold. My experience with a silkied feathered bird has shown me that they can handle the cold fine, but frizzled and hard feathers are less insulating as compared. In essence, if the bantam breed you are investing in has the right kind of feathers, they should handle the cold just fine.
Another thing to remember is that good genetics plays a part as well. If a breed is bred for hardiness, it's more likely to be hardy. Sometimes, within the same breed, some individuals just handle the cold better than others. My understanding is that Seramas are not particularly cold-hardy in general. I have read a few reports of them doing alright in the cold, but most say they can't handle it. That kind of tells me that Seramas would not be a good choice if you have particularly cold winters. Similarly, my Sebrights (Rose included, when she has all her feathers) have done fine in the winter, while others have reported their Sebrights struggling. It depends a lot on the individual.
If you're looking for just any bantam breed, you could go for a more cold-hardy breed in general, such as bantam Cochins or Brahmas. They aren't particularly small bantams like Sebrights or Seramas, but are small compared to most LF breeds. (And, have I mentioned I love Cochins?

) Otherwise, if you get teeny bantams, my suggestion would be to just keep a good eye on them as it gets colder and only intervene if they are clearly not thriving. It's pretty easy to tell when a bird is not doing well in the cold just by their behavior. Most, however, will do fine as long as they have adequate shelter and plenty of food and water.

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