Translucent UFO discovered on picture

On January 28, 2015, a man was in a national wildlife refuge in Titusville (in the State of Florida) to take photos of birds. Later the same day, when reviewing pictures, he discovered a translucent object.

Witness statement:

I am submitting the attached photos because I unknowingly captured an odd looking object in the sky when photographing a bird. I was taking pictures at the Merritt Island National Wildlife Refuge outside Titusville, FL with two friends. Ordinarily I wouldn't take a picture of a bird this far away but my friend loves kingfishers and he said he can never seem to get close to one, so we better get one while we can.

None of us saw anything at the time. When I was reviewing my photos later that day, I saw this odd artifact in two photos taken a second apart. The camera was set on slow continuous shoot. It does not appear on the first two images in this series. It is not on later images, since we got in the car and changed locations. These were taken with a Canon DSLR D70 camera with a 100-400mm zoom lens with a UV filter. I think the camera was pointed due south. I have uploaded the four original images.