Unusual Crossbill Foraging Behavior

On 2/5/09 I was X-Country Skiing with my Dad at Pigeon Creek Park in Ottawa County. As we were preparing to leave a flock of 7 White-winged Crossbills landed in a deciduous tree and began, interestingly, to forage! I got a few photos and a video too. Turns out the tree is called a Tree of Heaven (Ailanthus) and it was devoid of it's fruits. So "what were they feeding on?" is the question. Chip Franke visited Pigeon Creek Park the following day and wrote this:
"I had 3 WWC today at the same location (Pigeon Creek Park). I went to the park to look at the tree of heaven. The birds landed in the tree briefly while I was there. I was able to watch them with a scope at close-distance and they did appear to be feeding on something. I was able to collect a branch from the tree and I carefully examined it for what they might be feeding on. I could not find anything living on the twig nor were there any tree seeds. The only think I found was some sort of fuzzy material that either was part of old seed pods that remained on the tree or some kind of insect egg case. I couldn't tell for sure. Chip"
The video I took is posted at the bottom.

Note the birds in the top of the tree.