It seems that there are a few reason for birds to fly upside-down with their necks twisted up to 180 degrees around to keep their heads in normal position. This maneuver helps with braking (slowing flight), dropping down faster or in some cases, what it seems like, for a pure pleasure and/or exercise and still have great control of focused vision. Perhaps the best known cases are of geese and ducks (and some shorebirds) during whiffling but photos are scarce. Raptors can flip body upside-down during aerial attack or defense but they usually do not twist their necks as they keep their eyes on the opponent above (these photos are easiest to get). Other examples are hummingbirds which can fly upside-down very well or ravens (one was observed to fly upside-down for about one kilometer - article in Nature magazine, 1930).

Here, finally, I put together photos showing examples of the Least, Forster’s, Gull-billed, Black, Royal and Caspian Terns, Black Skimmer, Laughing Gull, Semipalmated Sandpiper, American Avocet, Northern Pintail and Osprey during aerial maneuvers with their bodies turned upside-down but heads held in normal position. As there are very few photos posted on the web of birds in upside-down positions with their necks twisted and none of them are showing terns (at least I could not fine one) some of you might be interested to see it.