There is no filter that will do what you want. The problem is dynamic range. The sky is very bright and the underside of most birds is very dark. Asking your camera to be able to expose both at the same time is like trying to hear the clock in your Rolls Royce while Motorhead are playing in the back seat.
You have to decide which you want focused correctly - sky or bird. Most people will pick 'bird' as the answer, so we'll go for that.
You could try using spot metering. That will work well if your bird is 18% grey and you can keep the spot centred on the bird. Unfortunately most birds are darker or lighter, and they tend not to fly in nice straight lines.
If the light is steady I'd really recommend shooting in manual mode. Point the camera at something like tarmac, grass, gravel - something close to 18% grey, or a bit darker. With the aperture set to around f8 or f11 (for good depth of field and optimum lend sharpness) check out the shutter speed to get the meter about a stop to the right of centre. If that shutter speed is fast enough (preferably around 1/1000s) that's great. If it's not then adjust ISO until it is fast enough. If you start going into stupidly high ISOs then you'll need to open the aperture - or call it a day and come back when there's more light.
Now put the camera into manual, change the aperture, SS and ISO to the values you've already determined and take a test shot of any bird in the sky. Chimp and check it's exposed properly. Adjust settings as required.