I'm answering your question (and edited your question to reflect this) as if you were asking about Aristophanes' play, The Birds. If you were writing about the Du Maurier story, please submit your question again and specify that.
All of Aristophanes plays are centered on a "happy idea" - something that drives the action. It's sort of a "wouldn't it be great if . . ." premise. In The Birds, the happy idea is presented by the two main characters searching for a place beyond Athens, a place where there are fewer taxes and fewer lawsuits (yes, the lawyer jokes are not a contemporary device). They go in search of the Hoopoe, a bird who used to be a man, thinking he can help them. They find him and come to the happy idea that they could build a new world - Cloud Cuckoo Land - that is in between Athens and the Gods. They could then charge a fee for requests to pass through from people on earth and the Gods. They are turned into birds (of a sort) and, after some scraps with interlopers (human and divine), come to a peaceful existence.
There are many topical references which are difficult to understand now, but the happy idea comes through loud and clear.