It is late winter and bitterly cold outside, so why have some of my hens have just now started molting? What should I do about it?
Chickens molt annually, and some chickens do molt later than others. Often this varies by breed as well as the conditions they have.
I have some that start at the end of August or beginning of September (my EEs), while some other breeds wait until November or even the beginning of December (my speckled sussex). Your chickens begin molting when their bodies tell them it is time, and that is usually based on daylight hours. Decreasing day length is the normal trigger. Do you add light to their coop? Some people add light to try to keep their birds laying better year round, but this can throw off the natural cycle. Eventually their bodies will tell them to lose their feathers anyway, but if it is not based on the usual triggers, it can come at an odd or inconvenient time. If you add light, that might have caused them to wait until it's very cold. If you don't add light, stresses caused by a temporary shortage of water (like frozen water), temporary shortages of feed, or other things can trigger a molt if your flock hadn't molted at the regular time of year.
Late winter could certainly be problematic if you live in a very cold area, and if your girls are going through a "hard molt" rather than a "soft molt." With a hard molt, a chicken can lose most of her feathers all at once, which would leave her very cold.
If you have a saddle , apron or something similar, I use those for my late-molting girls when it is very cold outside. We sell saddles or aprons on our website.
If you are crafty, you might try making a jumper for your hens. Jumpers are sometimes used for rescue birds, who might be molting at odd times, or might simply be bald from picking and stress. You might like to try this free jumper pattern (scroll down on that page) if you think your hens will be too cold.