I agree with Fsty. There is no *for sure* way to know with tiels' unless you DNA them. I haven't heard about the tail feathers theory before, but I have heard about the cheek colors, darker for boys, etc. I *think* mine are boys. They have bright cheeks and are about 2 1/2 now, and no eggies
But I am not for sure.
In parakeets, you can tell by the cere, girls are tan/brown, boys are blue. Snowy is a girl and Casper is a boy.
In the big birds, like tiels, the only way to know is by DNA test, and Angel, my Umbrella Cockatoo, we still haven't taken him to get it yet.(I just call Angel him
Also, it is said the males talk more than females. *I* am not a believer in this, b/c my female parakeet talks a mile a minute, where as all my other birds don't. Sunny(tiel) can say "Hi sunny bunny" and they both whistle a Wal-mart commerical we taught them.I think it's all in how you work with them. Angel is still a baby and is trying!
Everyone says "Oh parakeets don't talk, especially females" I say "Whatever you want to think! come see Snowy!" lol!