Hawks in Flight

When I arrived at Muddy Creek Greenway the day I took the photo below hawks were flying low, and they were easily identifiable as hawks, as opposed to some kind of vulture or buzzard. I counted about 20.

Thirty minutes later more hawks had joined, and they were circling higher. I counted around 60.

When I saw them again after another thirty minutes, more hawks had arrived. I counted over 90, and took the picture.

A flock of birds circling like this is called a kettle. Perhaps something was brewing.

Hawks. Muddy Creek Greenway. 2/16/17.

3/02/17:  Last night as I stared at this photo before I posted it, the hawks began to sway/undulate/can't-find-just-the-right-word in their fixed positions. I thought, "Weird. Maybe the computer is still catching the picture."

I checked the photo a few minutes later and the hawks weren't moving anymore, until I stared at the photo in one spot with a certain focus - like trying to look beyond the birds. After a few seconds, the hawks appeared to be slightly moving again. I checked it about 7 more times over the course of an hour, and the optical illusion repeated.

I stared at the photo a couple times this morning, and the illusion didn't repeat which made me wonder if last night's occurrences had something to do with lighting or maybe that something is wrong with my eyes.

And now, this afternoon, the illusion has appeared again. Regardless of the cause, it's fun. :D

I wonder. Since the birds are at different heights and because of their quantity and positions in relation to each other, if the photo contains a 3-D effect, and if that is what causes the appearance of movement?