My Phoebe at the Rattray Marsh (Mississauga) Saturday was feeding from among the rocks on the shore of Lake Ontario as well. The Phoebe would perch in a branch over the rocks, swoop down, then retreat to its perch again.
When I was exploring along the shingle/rocks later, I stopped and looked down. To my surprise (and I'll admit somewhat shock) the rocks were literally seething with spiders. In 6 square inches, there would be 6 spiders. This went on all along the length of the shore, for hundreds of metres. The spiders were big, too, about the size of a small fingernail. I took lots of photos but haven't looked them up yet. Anytime anyone walked causing vibration, or moved, casting a shadow over a spider, they would dive back into their homes between the rocks and would not re-emerge for a minute or so. So anyone casually walking on the rocks would not have noticed they were there.
It did make me wonder if the Phoebe had been after the winged insects (there were a few around the rocks) or whether it was eating spiders.
