After four Cattle Egrets showed up nearby I became interested in the background story of Cattle Egrets in Pennsylvania. I remember seeing small flocks of Cattle Egrets when I was younger but they have been much harder to come by since I have been birding more seriously.
Cattle Egrets were first reported in the U.S. in the  1940’s and it wasn’t until May 13, 1956 that the first Cattle Egret was reported in Pennsylvania, at Tinicum. They rapidly spread across southeastern PA and by the 1960’s had been observed in all the geographic regions of PA, including the Lake Erie shore.
It is not quite clear when Cattle Egrets first bred in Pennsylvania but the first documented breeding was on Rookery Island in the Susquehanna River when a census in 1975 found 772 Cattle Egret nests. This was the only area that Cattle Egrets bred in the state. Apparently the flight of Cattle Egrets returning to their roost in the evening was quite spectacular. In the Atlas of Breeding Birds of Pennsylvania, Bob Schutsky described it as, “one of Pennsylvania’s great avian spectacles, rival to the passage of birds of prey along the Kittatinny Ridge or the massing of gulls at Presque Isle.”
The Lancaster County Bird Club conducted an annual Cattle Egret Count in the 70’s and 80’s. The colony reached a peak of more than 7,500 egrets in 1981 and then by 1987-88 their numbers had drastically declined. In 1988, Cattle Egrets built nests on Rookery Island but abandoned them before the Cattle Egret Count when only 23 birds were counted. None nested in 1989. Rookery Island was also deserted at this same time by the other colonial nesters that the Cattle Egret had shared the island with- Black-crowned Night-Herons, Great Egrets and Snowy Egrets.
Cattle Egrets are now considered uncommon to rare visitors and irregular summer visitors in southeastern PA. They are irregular to casual visitors elsewhere in the state. Generally they show up in late April-early May or during the post-breeding dispersal period in mid-July thru September. An interesting late bird was found in State College on December 11, 1979.
Sources
McWilliams, Gerald M. and Daniel W. Brauning. The Birds of Pennsylvania. Cornell University Press. 2000.
Schutsky, Robert M. Cattle Egret in Daniel W Brauning (Ed.) Atlas of Breeding Birds in Pennsylvania.
Schutsky, Robert M. 1976. Glossy Ibis, Cattle Egret and Snowy Egret: three new Pennsylvania breeding birds. Cassinia 56: 3-5. http://www.dvoc.org/CassiniaOnLine/Cassinia56/C56_3_5.pdf