Hi Nancy,
There are only 3 things you can adjust, an in Av mode, you're already adjusting one of them (Aperture; usually wide-open to get the shutterspeed as high as possible). After that, the camera will always give you the highest shutterspeed possible for the ISO you currently have selected (the same ISO you'd have to select with manual metering mode anyway), so going to manual metering really doesn't gain you anything; at best it'll give you the same exposure that Av would have given you (which you can bias up or down using Exposure Compensation), but at worst, it's likely to give you significant exposure variations as you track the bird across the sky (ie all risk, no reward)
It's really easy to get shutterspeeds as high as you like shooting manual -- the only problem is they're likely to just be black frames unless you balance the exposure by other means ... and when light is changing rapidly (as when you're tracking across the sky), the camera can do that a lot faster than we can.