Yet a Ruby-Throated Hummingbird can fly 450 miles (20 hours) nonstop over water when migrating, which I've always found amazing.
So, 3.5 grams of hummingbird would be ~0.1 Watt, or, for a 20 hour trip, 2Wh of capacity. On the other hand, a kg of hummingbirds could give you 600 Wh, or 2-4 times what LiOn gives you.
I would go for hornets or other nasty insects, though. That gives less hassle with the law and animal rights activists.