Unfortunately, I didn't get a photo of all the activity surrounding this nest. While on the far side of the ravine, I noticed a grey (black) squirrel in the ravine busily stripping a cattail of lengths of fibre. After several minutes he ran up the opposite side of the ravine and into a tree with this nest. Just after he disappeared into the nest with his booty, another squirrel emerged, a grey, grey squirrel this time, and ran down to the same spot. He, or she, collected more cattail strips and took these back to the nest also. I went around to get a better look at the nest from the north side of the ravine, and just as I got into viewing position, two black squirrels emerged, rushed down the trunk and dispersed. One back to the cattails, the other to pick up some leaves and other debris lying on the snow beneath the tree. These he took back up to the nest. Meanwhile the grey came back again and gathered some more leaves, also brought up to the nest. Then one of the black ones came back empty-handed. Instead of going up the tree, he picked up some black locust seed pods and stripped the seeds from them, having a nosh. It was too darn cold to continue watching further, although had it been a little warmer I'd have settled in for a long wait to see what else transpired with these three.