Circular Polarizer

I am not really sure where to post this as I really need advise. Last week we went to the beach and I took a series of pictures of my boys feeding the seagulls. All the pictures turned out with a dark patch in the one top corner. Was this because of the circular polarizer that I had on? If so, what can I do in the future to avoid this happening? Any advise would be welcomed.
Thanks.

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That does help, thanks Andy. I had never had this problem before but I guess with the angle of the sun and I was taking the shots quickly to get the shot of the bird just before they caught the bread, so I didn't really catch it in the viewfinder. I had set the CP so that the picture would be lighter but I guess there were too many factors against me. I will just have to keep practicing with it.

 

Thanks Bob. When I bought the filter I did not think that there would be an art to using it. Yes the boys did have fun. But the seagulls were quite persistent and even after the had finished with the bread they kept trying to steal our chips - much to the amusement of the kids. And I too wish I was that age again.

 

Here are three seagulls desperately trying to steal some food from me. One of them eventually got up the courage to swoop down and grab a bite while I was taking the pictures.

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I think I did have the white balance set to sunny. Thanks Joshua. I will remember that in the future.