As has been mentioned this is the first Pennsylvania record, but one that is probably overdue. They breed across central Eurasia and winter in Africa, se. Asia, and Australia, and has occurred a number of times in eastern North America primarily in mid-Atlantic states. There are several records in our immediate vicinity, including birds in Ontario, New York, Delaware, and New Jersey at least, as well as Virginia and a few other states north and south. They are much rarer west, with a handful of records in Alaska and California and a few other interior sites, suggesting that most birds that arrive here in North America are originating from the European/African side of their range. But they were formerly more frequent, and North American records have declined noticeably since the 1990s, and nearly all North American records are from about May to September. There are two records of White-winged Tern hybridizing with Black Tern in North America, at Saint-Gedeon, Quebec 1985 and 1986, and at Watertown, New York 1992. Hybrid White-winged x Black Tern pairs are also known in Europe (where a different subspecies of Black Tern is native).
As for Nessmuk Lake bird feeding, like other "marsh terns" including Black Tern, insects are a significant portion of this species' diet, and they are not known for plunge-diving, but rather when taking fish they tend to just skim small prey off the surface of the water. In nearly three hours of observation yesterday evening, we didn't see the bird attempt to take a fish, but after the rain showers stopped, it frequently sallied out to catch bugs on the wing, and could be seen looking around randomly while it was perched, presumably watching insects nearby. It did seem lethargic at times but hopefully isn't in any real trouble health wise. It preened frequently and had no trouble in flight.