Bird Surface-to-Volume Ratios

The activity around bird feeders is very high during the winter. The birds are more challenged to keep their energy levels and internal temperatures high enough to survive. We are also challenged and are refilling the feeders more often. Why are those little birds so busy? Today, I offer you a simple science lesson to explain why they eat you out of house and home when it gets cold.

The smallest birds are the most active feeders. Little ones such as Fox sparrow and Chickadee are non-stop feeders. The activity level is less for birds with larger volume such as this Turkey. The big ones are more casual about their eating habits. It is an inverse relationship.

The behavior is related to the ratio of the surface area of the bird to the volume of the bird. The bigger birds have more of their mass and volume guarded internally from the frigid temperatures near their skin. They are able to maintain their internal body temperature more easily. They retain their heat better from the metabolism of the food they eat. Since heat transfers less quickly from their body volume to the surroundings, they can be more leisurely about eating. The tiny birds have very little internal volume. They lose heat quickly as it transfers through to the outside. They must eat more often to maintain their internal temperature.

Here is an illustration of the effect of surface area and volume. It uses cubes 1 meter on a side. It works for cubes of any size. All that matters is the ratio in the end. You can try it yourself with sugar cubes or toy blocks. Let’s assume each cube face is a square 1 meter by 1 meter. There are six exposed squares on each cube. There is a total of 6 square meters for all the faces. Each small cube volume is one cubic meter. The ratio of surface to volume is 6:1.

The middle set has a lot of faces that aren’t exposed. Notice how the ratio of surface area to volume has decreased by half to 3:1. The third set on the right has a ratio of even less at 2:1. Scale this up to 10x10x10. You have a surface area of 600 and a volume of 1000 for a ratio of 0.6:1. So, the benefit to the larger animals is the reduced surface area exposed compared to their core volume where their heat energy is localized. They lose less heat proportionately from their core.


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