I was advised this too. Does anyone have any idea why bats gets heavier in the cold. Also how much weight loss is there in the bat after you keep in the house I.e warmer conditions?
Basically Willow naturally takes on/loses natural moisture to match it's environment. Colder conditions tend to be more damp/have more moisture in the air, so the Willow takes some of this on.
I'm.a warmer place, such as a house, there is less moisture present I. the air than a workshop, so the bat will lose some moisture to match it's environment more closely. The opposite happens of you leave a bat in a steamy room such as a bathroom, it will take on moisture and gain some weight.
That is also why bats from warmer countries such as India or Australia tend to be "dryer" than English bats.