Black Background on Images - Examples

I've seen my D300 do this a couple of times, with different lenses now. I'm thinking the sensor is overexposed as I pan towards the cloudy sky and turning the background black.

Here is a sequence of shots during the day, no flash. When the bird got a little lower with a non-cloudy background, the photo was corrected.

Any ideas?

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Matt Shetzer
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D300, 17-55mm, 12-24mm, 60mm macro, 200-400mm, 500mm