How much does an average bird weigh?

Well, the smallest bird is the bee hummingbird at 1.6 g, the largest is the ostrich at 157 kg. (or 157,000 g). So a simple average would be 78,500.8 g (78.5008 kg).
For a more accurate average, you would need to round up every single bird on the planet, weigh them all, total that weight, and divide by the number of birds. Get started now, it could take a few weeks.