I would wait until the full cluster is ripe. You will notice that they will get plumper and even more purple/black as they get a little older. The birds should leave enough for you, even if they gorge themselves. But, watch your bushes daily as ripeness can happen in a moment. I make elderberry jelly and syrup every year. It's delicious.

Wait until the clusters are fully ripe if you can. Unripe berries are toxic and should NEVER be eaten, raw or cooked. Some clusters may ripen unevenly. If the birds are beating you to the berries, you'll have to pick the clusters half-and-half, and pick off only the fully-ripe berries for use, discarding the green or partially-ripe ones.

Elderberries freeze very well, so you can gradually build up enough supplies by freezing the ripe berries until you have enough for your needs. I just put mine in a plastic bag or container after picking them off the stems (painstakingly!). No need to thaw them before cooking them.