Neither . . . the nest came first. Just joking about a little. A great question that has been debated since who knows when. It is like which comes first the horse or the cart. The horse passes by first when arriving. One may build a cart before buying or raising a horse. Or, a person may have a horse then buys or builds a cart. No, matter someone has to reign the horse and that is the driver of the cart and the horse.
It is like many a hot rodder will say of a car sitting in a field . . . that car is waiting for an owner, but once it did have one.
A resolve is they both at the same time. The bird arrived with an egg inside waiting to be laid. Pretty cool . . .eh?