"Fieldnotes," November 24th & 25th, 2013: "Gall Foragers," by Charles Miller / Caroline Kurtz.
http://www.montananaturalist.org/
"I observed the downy woodpecker foraging for more than ten minutes. It moved from gall to gall quickly - apparently not all of them contained living larvae to munch. How did the bird know? Some researchers claim woodpeckers can hear bugs and larvae inside plants; some believe they can feel the small vibrations made by crawling insects. Once the bird located a gall with a tasty morsel inside, it spent up to two minutes completely peeling the round gall to extract its meal."