Hi! I've personally never heard this word before, so no, it's not common. At least not in the United States.

Thank you, Dan. You wrote very useful information. I just want that my speech would be bright and vivid, I will be very carefull with idiomatic expression. Obviously I have to refer another dictionary) If you have any recommendation I would follow it)

No. I have literally never heard the word "birdbag" until you mentioned it. "Birdbrain" is common and understood, but it is slang and it's also very disrespectful. Although I do check urbandictionary occasionally, it is NOT RELIABLE. It's full of made-up words and "dysphemisms" (words that have been deliberately made into WORSE words by making them obscene). urbandictionary is NOT your secret reliable guide to the "real English." Check anything you see in urbandictionary against a real dictionary. It should show up in the real dictionary, perhaps labeled "vulgar slang." If it doesn't, then it is either made up, rare, or local. Finally, be aware that "bag" has a number of nasty overtones--"scrotum" or "testicles" and "douchebag"--and sometimes it used in slang expressions just to give them a little bit of extra nastiness.

Thank you, Jerry, for your opinion and recommendation. I think you are right absolutely, that's why I check all information asking native speakers.

Urban Dictionary is in no way a resource that anyone should take seriously. It's full of stupidity. At best, it's a gag site for people to say stupid, nonsensical things. At worst, it misleads people. A native speaker might be able to sift through it and find something accurate. But why bother? There seems to be a trend for people to use it though despite repeated warnings not to.