LOL Adam I go with the tried and true (though horribly unscientific).."well that looks about right" system of rat picking. After enough rats and snakes it seems I'm getting fairly good on that....well except for picky arse Rionnach. With her I can easily go through three or four trots back to the feeder bin until I get what in her estimation is the exact right, this is worth my regal time to eat it rat.
How sad is it that I'm a grown, reasonably intelligent female going "okay Ri Ri here's a LOVELY rat...oh not one you like...*back to the feeder bin*....now here we have a really nice one, see the lovely coat on it, just the right....ahhh crapola *back to dig through the recently weaned feeding bin*...now this one Ri, absolutely....*sigh, back to the bin again*....how about...WHUMP!!!....*sigh of relief as Miss Picky eats her dinner*