had same issue once with a large tele [ 800 ] .the cause was moisture condensation between the front lens and a uv filter .removing filter did let the condensation out .[ cold damp day ]
had same issue once with a large tele [ 800 ] .the cause was moisture condensation between the front lens and a uv filter .removing filter did let the condensation out .[ cold damp day ]
Have you not heard of 'bird flu'? The fella just sneezed and the droplets were caught by the light
This theory is supported by the observation that the light area is not truly circular and you will note that it is trailing off towards the rear of the bird, obviously due to the wind direction at that time