I am not a baseball fan, but this is apparently a rarely (or commonly, depending on your age) used expression for the one who is on top, or having the upper hand, or an advantage, coined by the baseball announcer Red Barber.
And what does this have to do with Backyard Biology, you might ask?
Well, this morning I saw the first catbird in the yard — as in, first of the year, and first ever in the backyard.
The expression “the catbird seat” originally derives from the antagonistic behavior of male catbirds toward one another. As they jockey for dominance, each tries to get the upper hand, by climbing higher in the tree while singing or posturing, until one of them runs out of room — and the winner of that contest has the “catbird seat”.