http://jeb.biologists.org/content/178/1/39.full.pdf rates their muscles at 100-120 W/kg, and ~25% of body weight as flight muscles.

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.