Picture yourself as one of England’s majestic Bewick’s swans, about to take off on your annual long-distance flight to Arctic Russia, when out of nowhere a scientist grabs you and methodically gropes and measures your butt. It’s all for your own good: Researchers are hurriedly sizing up as many round rumps as they can lay their hands on, in a bid to understand what’s wiping out their population.

Smaller than the more common mute swans, which stay in Britain yearlong, Bewick’s swan has seen its population in Europe decline from 29,000 to 21,000 between 1995 and 2005, and researchers at UK’s Wildfowl and Wetlands Trust (WWT) in Slimbridge, Gloucestershire, are willing to fondle the birds to save them.

They’re sizing up swans to test whether changes in the their habitat are to blame for their decline: The size of swan keesters indicated whether they have enough fat to survive their over-2,000-mile journey. Basically, if the birds are plump, then that rules out the possibility that they aren’t getting enough food, and opens the playing field for other culprits, such as power line collisions, lead poisoning, and hunting.For those of you without ...