Stanza 2 Summary

Get out the microscope, because we’re going through this poem line-by-line.

When shepherds pipe on oaten straws,
And merry larks are plowmen's clocks,

The poem's second stanza starts off much like the first, continuing the pattern of "when X then Y." However, in stanza 2 what's happening is a little less clear than stanza 1's blooming flowers.
Stanza 2 starts off with shepherds, "pip[ing] on oaten straws." Um, what? Shepherds are common enough, but what about all this piping?
It turns out to be simply the reed pipes (think simple handmade flutes made of reeds) that shepherds played to pass the time in the fields while they tended the sheep (remember, no iPhones—things could get super-dull).
Now that we have that all cleared up, we can move on to the peculiar time-keeping habits of "plowmen."
Plowmen are exactly who you probably guessed they were: guys who plow—a.k.a., farmers.
Those "merry larks" are birds (yes, more birds). These little fellas tend to be morning singers, so in springtime the farmers could use the lark's song like an alarm clock, telling them it's time to hit the field and start plowing.
Oh yeah, it's another day of plowing.
As in stanza 1, things start off with a pretty happy, lively vibe. We've got shepherds playing their pipes and the larks singing their morning song. Seems like a pretty idyllic spring vision so far.

When turtles tread, and rooks, and daws,
And maidens bleach their summer smocks,

Line 12 is gibberish. Ignore it and move on to line 13.
No, we're just kidding.
"Turtles" is just a shortened name for (surprise, surprise) another bird, the turtledove. "Rooks"and "daws" are also bird names. So, that should about do it for line 12. Well, except for that verb—"tread."
"Tread" means, how can we put this… the birds are mating. There, we said it. It's out there. So, the birds are getting busy. As we said before, it's spring… it was bound to happen.
In addition to those birds getting their feathery swerve on, the young unmarried ladies (the "maidens") are getting spruced up. They are getting their "summer smocks" (super-sexy) all ready in the hopes of, perhaps, attracting a mate themselves.
It looks like it's true what they say about springtime: love is certainly in the air. (Get it? Birds? In the air? Yeah—sorry.)

The cuckoo then, on every tree,
Mocks married men; for thus sings he,
Cuckoo;
Cuckoo, cuckoo: Oh word of fear,
Unpleasing to a married ear!

The poem's final five lines are identical to the final five lines from stanza 1. Remember, this is a song. This repetition works as a kind of chorus or refrain.
Just like before, the cuckoo's song isn't beautiful music to the ears of married men—it's more like fingernails across a blackboard.
That whole cuckoo-cuckold (remember that a cuckold is the husband of a cheating wife) association takes all the joy out of the song and makes it seem "mock[ing]." Even with all that love in the air, the thought of an unfaithful wife can really be a mood-killer.
That rhyme scheme and meter we talked about in stanza 1 is still going strong. And when we consider the poem all together, we can see that the whole thing is one big "when X then Y" statement: when spring rolls around, then love is in the air and you'd better keep a close eye on your beloved because, well… cuckoo, cuckoo.