I read this too (sorry, can't remember where) but don't recall the part about feild vs nurses part. The study did state however that there are certain bees within the colony that are able to generate and withstand higher body temps. Evidently these "heater bees" crawl into cells in the brood areas to radiate heat through the brood. This would explain those random empty cells in an otherwise solid frame of brood.
"In the first place, we should insist that if the immigrant who comes here in good faith becomes an American and assimilates himself to us, he shall be treated on an exact equality with everyone else, for it is an outrage to discriminate against any such man because of creed, or birthplace, or origin. But this is predicated upon the person's becoming in every facet an American, and nothing but an American...There can be no divided allegiance here. Any man who says he is an American, but something else also, isn't an American at all. We have room for but one flag, the American flag...We have room for but one language here, and that is the English language...And we have room for but one sole loyalty and that is a loyalty to the American people."
Theodore Roosevelt 1907