Until that time, do still practice on them higher up - with a 600mm lens you should get something, and the practice will get you ready for the good days. In particular:

• Tracking
• Best times in any circle to take the shot (sun, and facing)
• Behaviours to watch for - patrolling or soaring on thermals, or breaking into straight flight, so you know where they’ll be
• Address dark bird on bright sky issues (post-processing RAWs luminance curve, or exposure compensation) - you’ll notice my sky is a blown out white - it wasn’t on the day :)
• Camera settings that work for you

And practice panning with pigeons, crows or anything else in view