Ha, I like the metaphor of the title as a relationship. Free verse, so nothing to evaluate about the construction. Simple but effective.

I'm a little confused about the "waken into crabs" line. I think it means the hatchings are preyed upon by crabs, but I wasn't sure if it meant some of the hatchlings were crabs. Likewise I was confused at first about the early stanzas, since the creatures described didn't seem to be the titular birds, though I figured later that was because you had the species evolving. Mostly a message about a common enough evolutionary strategy: have enough offspring to ensure some survive predation. I can't find any fault in the rhyme/meter.