A birder is a lad who laps up attention off the ladies and thinks of themselves as pretty much a big deal. Probably quite good looking, but more importantly, thinks that they're the fittest thing to ever walk the earth.
Birders will always have several girls on the go, and probably have different names for all of them. E.g 'wifey', 'baby girl' or 'boo' - this keeps all the women feeling special and thinking that they're the only 1
Girl 1: "Hey, u heard of that josh evans?he seems pretty cool"
Girl 2: "Yeah- a big time birder, don't go there babe"