Question

WHY IS it harder for smaller birds to maintain their body temperature (link to SA and volume). I would think that the larger you are the more heat you lose no?

Answer 1

or what do they have to do differently as compared to larger birds

Answer 2

I don't know what your text materials have to say on this topic, but you might find some information among these articles:

https://www.google.com/search?q=WHY+IS+it+harder+for+smaller+birds+to+maintain+their+body+temperature&ie=&oe=

Answer 3

Loss of body heat is a function of surface area per unit volume.

The higher an organism's volume, the higher its metabolic activity and internal heat production.
The higher an organism's surface area, the quicker it loses heat to the atmosphere (humans do this by sweating - evaporative cooling, plants do this through transpiration)

Smaller birds, and smaller mammals, have a very high surface area to volume ratio.
This means that they lose heat extremely quickly, and cannot keep heating their bodies efficiently enough to make up for that most heat, due to a lower volume compared to their surface area.