system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do birds especially smaller ones like robins survive?

I have been watching my bird feeder today and couldn't stop thinking. This little bird flies from tree to tree to the feeder to get 1-2 sunflower seeds then flutters away to the far end of the yard. How many calories does a robin burn flying say 100 meters, and how many calories do they get from a single sunflower seed? 

Bigger birds that glide most of the way makes sense, but smaller birds that have to flap their wings all the time I just can not make sense.
gpt
Birds are incredibly efficient fliers, so calories matter more for keeping warm. They puff the feathers on their bodies trapping air and using their skin to warm it up. They also have lower body temperature than mammals do so less calories are needed to warm them up. They also, if flocking birds, huddle together for warmth, bask in the sun and find little holes to live in during the winter. 

Birds like robins actually fly South so they end up in places that they can tolerate.