Last October I was trying to get a video of the sparrows round the feeder. As if by magic  all the sparrows flew away.  

A second later this sparrowhawk flew in. I only caught it on camera for a few seconds.   Apologies for the video being short and not 100% clear, .

This  shows sparrows know when a predator is about.

I think it also shows that sparrowhawks do not only go for weak and ill birds