Interesting, I didn't know they started flying south so early. We have a couple here already (Ruby Throated).
Yep, there's quite a few birds moving through. Really, after they breed they have no reason to hang around unless they molt before fall migration (as opposed to those species which moult after migration)
If you're familiar with purple martins (those swallows that breed in multi-room houses), some adults begin heading south right after breeding, which means the beginning of June in San Antonio. So southern-breeding martins heading south meet Canadian members of that same species heading north.
The same early migration applies to many warbler species, one, the Cerulean Warbler, heading out in late June from our Northeastern States beginning in late June to go back to the Andean Cloud Forests in South America.
A bunch of Arctic-breeding shorebirds start moving back south in early July, mostly adult females in those species that leave the young in care of the male. Down here in Texas, southbound migrant shorebirds are already common by July 4th.
Pretty much the norm among hummingbirds for the adult males, which play no role in raising young, to head out first, in the Mountain West these movements are concentrated along high mountain corridors as opposed to surrounding deserts. Hold a feeder at arm's length in the Sacramento Mountains of NM about now and you'll likely have thirty hummingbirds swarming around before you can even hang it up.
Some of the increase you'll be seeing at your feeders though may be young of the year (look like females), even without migration, you'll have about three times as many hummers in late summer after the young are out and about.
Unless its concentrated along mountain chains like hummers out West, fall migration in summer tends to go mostly unnoticed: Its spread out over a longer period of time and the birds themselves tend to migrate slower, with stops and layovers en route.
Birdwatcher