"Spring" has birds flapping all over the place—and why not? It's spring, after all. But the birds in this little ditty do much more than simply make for nice, spring-y imagery. Shakespeare uses specific birds, engaged in some very specific activities, to emphasize some of the poem's key subjects, like infidelity and man's connection with the natural world.
Lines 5-9, 14-18: Did you ever see Hitchcock's movie The Birds? Well… this is nothing like that. But the married guys in the poem probably feel the emotional equivalent of what those Hitchcock bird-attack victims felt. Here's the deal: these lines make up the song's refrain and they are definitely centered on that little troublemaker, the cuckoo bird. It isn't all his fault. The problem is with his name and his song. See, "cuckoo" sounds like "cuckold" (an old-fashioned term for a guy with a wife who is, well, not really into the whole monogamy thing). So whenever these married guys see or hear one of these birds, it makes them contemplate the unsavory, embarrassing prospect of being a cuckold. And guess what—these little suckers are just about everywhere ("on every tree") in the springtime. It's the kind of thing that can really take the bloom off an otherwise beautiful spring day.
Lines 10-11: In these lines, we can see Shakespeare making the connection between man and the natural realm. Those shepherds are piping away on their reed flutes just like… yup, birds. The "merry larks" that show up in the next line are making some music of their own. These little guys are notorious for singing with the sunrise, so the "plowmen" are using them for alarm clocks to let them know it's time to get up and hit the fields. To sum up: the men are mirroring birds with the songs they are piping and the birds are mirroring the clocks of the human world. Connection, anyone?
Lines 12-13: Things get kind of explicit here. Turtles, rooks, and daws are all bird names—and they are all "tread[ing]." What's does it mean, to "tread"? Well, these birds are making some music, but not the tweet tweet, chirp chirp kind. "Tread" can be used to describe the act of birds mating. Bottom line: these birds are getting busy. When put together with the next line, we get this: when birds start mating, young women start dressing to impress, "bleach[ing] their summer smocks." It sounds like the maidens are in the market for mates just like their feathery counterparts. Who knew feathers and smocks could get so steamy? Yeah—don't answer that. On a duller note, this mirroring of the maidens with the birds gives us another example of the connection between the natural and the human realms. Just thought you'd like to know.