Why Birds Do What They Do

Bushtits—almost impossibly tiny gray birds that live in flocks across the western United States—are not hard to spot in the Bay Area. I usually become aware of them by noticing a chorus of peeping in part of an oak tree that seems to be jiggling. Their nests, though, are well hidden, and they’re different from what most people would expect. Made out of spiderwebs, fur, lichen, and plant material, they hang down from a couple of branches like a strange-looking sock with a side entrance near the top. A month or so ago, when my friend Joe showed me the nest he’d found, we watched the birds ferrying bits of fuzz and what we speculated were oak flowers, adding them thoughtfully to the growing blob.