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?

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

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=

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.

The difficulty for smaller birds to maintain their body temperature compared to larger birds is primarily due to their higher surface area-to-volume ratio. This concept relates to the fact that, as a 3-dimensional object grows larger, its volume increases more rapidly than its surface area.

In the case of birds, the surface area is where heat exchange occurs, while the volume represents the internal heat generation and storage. Smaller birds have a relatively larger surface area in proportion to their volume, leading to higher heat loss compared to larger birds with lesser surface area in proportion to their volume.

To understand this concept, let's take a closer look at the surface area and volume ratios in small and large birds:

Surface area: Birds lose heat through their bodies' surfaces. Smaller birds have a relatively larger surface area compared to their volume due to their compact size. This larger surface area means that more body heat can be lost to the environment.

Volume: The volume of a bird represents the internal heat generation and retention. Larger birds have a comparatively larger volume due to their overall size, which allows them to generate and store more heat internally.

When considering heat loss and heat generation, smaller birds have a greater challenge in maintaining their body temperature due to their higher surface area-to-volume ratio. They experience more heat loss through their relatively larger surface area, which can be detrimental in colder environments.

It is important to note that larger birds can also lose heat, but their larger volume enables them to generate and retain more heat internally, helping them maintain a suitable body temperature more effectively.

In summary, smaller birds struggle to maintain body temperature because their relatively larger surface area-to-volume ratio causes more heat loss compared to larger birds with a smaller surface area-to-volume ratio.