No, none of those actually seem logical, but I have a question. Is the six foot canary with the mentality of a child you are referring to, Big Bird? This might seem obvious, but the reason I am asking, is because I always thought Big Bird was an ostrich, but of course, I could be wrong. I am just wondering.
About the human and bird relation, it is illogical, and it brings up a very disturbing assumption that I am not going to post here, but consider these two things. A pig and frog love with each other, maybe someday bearing a child, that seems more disturbing to me than the walrus and hippopotamus from "Meet the Feebles" loving each other, maybe someday having a child, I am sorry I forgot their names. A frog and bear are brothers; this is a joke in "the Great Muppet Caper", but still it is kind of disturbing.
I guess the idea of Bert and Sam the Eagle being related is more about my annoying pestering about trying to find a muppet who is as physically, emotionally, and mentally similar to Bert, as Lew Zealand is to Ernie, than my logical reasoning. However, these are just the muppets, so I guess anything is possible.
Muppetmonster