Apparently red is a hot colour and shows up in their vision sensors better. I have also heard they target the odd ones out, so if you have one dark one in a flock of blues, apparently it is the easier target. I don't think they are that fussy though. They need to eat and any food will be their target. This time of year is about the worst it gets since they are both hunting and teaching their young how to kill, so up to four where there used to be just one.