Mine are thinner this time of year- they've just sort of started gaining weight again. Have you been using light in your coop this winter to keep them laying? Do they have full crops when they go to roost at night? If yes to both/either, they probably just need more dietary fat/carbs to keep the weight on. Mine get all types of leftover carbs and protein from the dinner table this time of year + animal fats, suet, dairy on the verge of sour, extra BOSS scattered, etc... whatever the people don't eat or whatever I get a great deal on and buy too much of (of the bird-safe variety, of course). I give them "treats" in the morning so that they are guaranteed to fill up with their typical diet at night.
I would guess that they will probably fill out again as it gets warmer, though... especially since they are free-range and probably have access to all sorts of spring goodies & insects.
If it seems extreme and you are worried about weather, you might check with your weather service and find out which direction the worst winds come from in your area and make that wall (or those walls) solid + ventilate the other walls better to make up for the reduced air-flow in summer. In mid-MO we use a solid Western wall and ventilate other walls, since that's the direction almost all of our bitter winds/fronts/blizzards come in from. I don't think the cold bothers them much really- but the wind can definitely take a toll.
Excessive dust in the coop and/or not enough air-flow (ammonia build-up) are the most frequent causes of respiratory issues/sniffles or generally mild sneezing, from what I have seen.
