Hmmmmmmmm ... apparently it's HUGE in the US. Not so huge in Europe but people 'have heard of it' at least. It's a fable and it's philosophical, trying to bring the meaning of live to the reader.
Maybe I just didn't get it, but I thought it's a weird novel.
Jonathan is a seagull, bored with being ordinary. While the rest is only concerned about flying from a to b to get some food, he practices flying everyday. In all possible constellations, really fast, really slow, really high, really low. Of course the others don't like that and say bye bye to the weirdo. He doesn't really care and continues his flying.
He then (I think after he died) comes into a form of personal heaven and learns more and better flying. He also starts to learn a lesson about freedom.
In the last part of the he tries to get this freedom into other seagulls...partly successful.
So even though the other seagulls were mean to him and didn't care about him, he goes back to them (from his heaven) to bring a meaning to their lives.
I'd have rather read the message in a shorter short story (the book I had was 74 pages, 30 of those were pictures of seagulls but it could have been shorter), and I honestly think it would have had more effect on me. But than, maybe I'm just not a big fan of those 'find and love yourself and you'll repair your live' stuff ...
or maybe it was the translation, as I read it in German?!