Birders know spring migration as a kind of paradise, when the weather is finally warm and the birds are once again plentiful. But even paradise isn’t perfect. The bugs are back, for one thing. And unlike birds sitting in bare winter trees, spring migrants always seem to be hidden behind newly emerged leaves. It’s even harder to see the bright and beautiful patterns of migrant warblers, who spend most of their time flitting about treetops and only showing us their butts. I’ve spent so much time looking up at bird butts that I should get an honorary degree in avian proctology. I’ve got enough underside photographs to start a smutty bird magazine called Rump Fancy or Tailfeathers. What I’m trying to say is, you should learn to identify warblers from below. It’s possible to quickly identify some species of warbler just by glancing at their underside, but most others require some detective work. The trick for identifying warblers giving you a bad look is to not get...