Totally agreed about Greys. They are incredibly smart. I remember watching vids of Alex the Grey who could do all kinds of stuff, and definitely appeared to understand the concept of zero, which is a fairly abstract thing when you think of it. Unfortunately, he passed some years ago.
Alex was truly a marvel. Many things he learned because people would tell his owner “Oh, but I bet he can’t do...!” So, with teaching, he learned many concept thought unknown to birds like colors, shapes, sizes, etc. and use them correctly in sentences. He had a group of college kids teaching him all day, something most parrots can only dream of.