8 mysterious flying creatures seen over Philadelphia

On August 30, 2017, an inhabitant of Philadelphia (Pennsylvania) saw eight mysterious flying creatures. They looked like a mantaray or butterfly. They came from East and moved very fast.

Witness report:

On 8/30/2017 I looked out of my 3rd floor window and saw about 8 objects fly over my city from west to east. At first from a distance maybe 1-2mi I thought they were soaring birds vultures/ redtails are common but not that many at once so that drew my attention.

I went back to my work and ended up looking again a few minutes later and the objects were much closer ~1/2 mi. This much closer I noticed that they were not birds but were very smooth looking ojbects and resembled a manta ray or a butterfly. The way they flew was very controlled and not just with the wind, they would bank and change altitude, dip and bank again in large loops uncoordinated with each other but in the same general direction.

I ran back inside to grab my camera but had to swap lenses so it took me an extra second to get out to the roof (via the window) I was able to snap 2 photos before they flew out of sight to the East just North of my house.

The building that blocked my view is only 5 floors and I was on the 3rd floor room so they had to be very low. I went outside and around the block to the north but I could not see them.