Another observation I made a few years ago was when i was sitting on my back deck a willy wagtail came and perched on the railing, he sat there for about 10 minutes just looking around, then he started doing a high pitched twittering for about 2 minutes or so then pause and start again, he was looking up at the roof as he did this so I looked up and as he was twittering i saw the spiders coming out from there hiding spots on there webs looking around thinking that they had caught something. Then he would swoop up and nab the spider. All i can surmise is that the twittering was at the right pitch to make the webs vibrate. Was amazing to watch, as this went on for about 30 mins or so, then he just flew off. It's amazing what nature can teach us, if we only stop long enough to enjoy.