red kite

I find metering of birds against sky difficult as, unless you are lucky (or using spot metering and skilled at keeping the bird framed 'just so') the camera ends up metering for the sky and the bird comes out badly underexposed.

By far the easiest and most reliable technique I have found is to simply put the camera in manual mode, point it at some bit of foliage roughly the same colour/shade as I expect the bird to be, thump the green button to set exposure then merrily snap away at the flying birdies without having to worry about exposure or whether they are against a blue sky, or white cloud or brick building etc. as background.