Beav, You are correct for fall beaver, but changing light conditions that trigger melatonin production (a hormone secreted by the pineal gland) in the skin and start the priming process also trigger a beaver to begin producing castoreum, and by spring the older beaver castor glands can be quite plump, as I'm sure you've observed. After the vernal equinox (mid-march melatonin production falls off and castors become flat again. Castors are at the heaviest between Christmas and mid March with some exceptions in the far north and south. I wonder if both fur and castor is affected by a cloudy overcast (or sunny) fall? Much like a mink or fox rancher darkens a shed to make the fur better.