LOL!! One case was a bird. And you just made the classis error where you admitted that you were wrong by shifting the burden of proof. If you want to claim something crazy then the burden of proof is upon you. I can give you reasons that aliens is highly unlikely.
You should learn Hitchen's razor:
"What can be asserted without evidence can also be dismissed without evidence."
Hitchens's razor - Wikipedia.
If you cannot support your "aliens" claim then it can simply be ignored. It is no different from the shouting of a loon that "Bigfoot did it". When the size, altitude, velocity, and even pulsing beat of probable wing flapping match that of a bird then the answer is that the observed object was probably a bird. We know that birds exist. We do not know that "aliens" exist. So where is
your evidence?