They used to send canaries (which are yellow. And birds.) down the mines to find out if there was a lack of oxygen or any other dangerous gases - if the canary died, then they knew that something was wrong, and so they didn't send the humans down.
So, "did you forget that yellow bird?" - did you forget to send the bird down first, to check if the coast was clear? Did you just plunge ahead and find out too late that there was danger awaiting you?
As for the 'yellow bird' reference:
They used to send canaries (which are yellow. And birds.) down the mines to find out if there was a lack of oxygen or any other dangerous gases - if the canary died, then they knew that something was wrong, and so they didn't send the humans down.
So, "did you forget that yellow bird?" - did you forget to send the bird down first, to check if the coast was clear? Did you just plunge ahead and find out too late that there was danger awaiting you?
Just a theory :)