Massing Swallows

Every morning when we ride out Mehl Creek Road and back we observe a large number of Violet-green Swallows collecting on the power lines.  Today I walked out and made an actual count.  To my amazement there were over 1,300 swallows. This is an annual event every September at the junction of Mehl Creek and Henderer Roads. It leaves me with a couple of puzzling questions. Is this a collection site where the swallows gather until some critical mass has collected or certain secret migrating date occurs and they all scurry off to Mexico together? Or is this a collecting spot where swallows collect and leave daily? In other words, am I seeing the same birds collecting daily in anticipation of their great migration date?  Or am I seeing new birds each day that collect and then fly off together, only to be replaced the next day by birds on their way south from further north?