Activate thermal visualization (not sure how it's called in anglo). Birds indicate thermals then.
Like Mathias said, and make sure it's set to "Natural"...then hunt for thermals.
Also, some 3rd party scenery designers add birds, as an earlier poster mentioned. Check out Bill Womack's Hawaii Dillingham X, where you can hear and see gulls flying about. In reality, gulls are quite rare in Hawaii...I think of them as terns, instead, but then I'm an obsessive birdwatcher.