The forked tail contributes to lift and turning, but the benefit does not increase indefinitely with length. Experimental work that lengthened or shortened outer tail feathers found that males with artificially extended streamers gained mating advantages while showing reduced foraging performance, indicating the natural male length sits past the aerodynamic optimum.
That gap is the signal. A male carrying streamers longer than efficient flight requires is advertising that he can bear the cost, and females across studied populations respond to streamer length, though the strength of the preference varies between European and North American populations along with other traits such as throat colour.
Barn swallow migration is remarkable independently of any of this. European breeders winter as far south as southern Africa, and the same individuals return to the same building year after year, a navigational feat achieved by a bird weighing under twenty grams.