The 'Catcher and the Spy' is likely about the story of a catcher who gets involved in some sort of spying activities. Maybe it's about a baseball catcher who stumbles upon a spy plot in the world of sports, or perhaps it's a more general story where someone in the role of a 'catcher' (could be in a metaphorical sense) discovers the actions of a spy.
Since I don't know the exact story, I can only assume that the main characters are likely the 'Catcher' and the 'Spy'. The 'Catcher' is probably the one who will be the hero or the main focus as they are the ones who will be involved in detecting the 'Spy' and trying to stop whatever the spy is up to.
One possible theme could be survival. If the bird catcher is in a tough situation, catching birds might be a means of survival, either for food or to trade for other necessities.