why does it look like a bird is flying in the very far background?

It's a small group of dead pixels on the left side NavCam camera (L-NavCam)

We only usually see those when that section of the upper left tile of a 4 tile NavCam captures the sky

Here are a pair of raw upper left NavCam tiles from Sols 1833 and 1835 that I have annotated, note the dead pixels in the same location in the sky.

If you go back through the image server you'll find the same dead pixels on L-NavCam images, I've include an example from December 13, 2023 (Sol 1000)