Has come out too dark. Why?

Not sure if this is in the correct section to start this. But oh well.

Wildlife/bird photograph isn't my area but I decided to give it ago today.

Can anyone help explain to me as to why it's come out so dark? It was in a forest but on the edge of the car park so the trees weren't as dense. It was taken at 1/320 sec, f/5.6, ISO1600 with a sigma 70-300mm lens at 300mm.

Any help will be appreciated to give me a idea what to do next time to improve.

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It's dark because it is underexposed, 1/320th @ f/5.6 / 1600 ISO can be a bit optimistic in forest settings as the light can appear better than it actually is. You were in shutter priority, so leaving the camera to set the aperture. If your lens is wide open at f/5.6 then the camera opened up as much as it could but couldn't open up enough to get a correct exposure.

I would expect that to be the situation (next time, check if the aperture value is blinking, that means that it can't be opened enough for correct exposure, then slow your shutter speed until it stops blinking).

Also, check that you haven't got any exposure compensation dialled in, as that could be the culprit.

In future, you can check exposure as soon as you take the shot, by checking the histogram, in the shot above it would be bunched to the left, with a big gap to the right hand side. That indicates underexposure and you need to give it some more.