The Incredible Shrinking Birds

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Bergmann’s Rule

According to Science Daily, May 23, 2019, warm blooded animals – birds and mammals – are getting smaller. That seems bizarre until you put some pieces together.

Scientists have known about Bergmann’s Rule for many years. This “rule” is actually a general ecomorphological concept which says that as the environment gets colder, the body sizes of birds and mammals within a species or population gets larger. The reason for this is that as an animal gets larger, its volume increases faster than its surface area. That is, the inner bulk of the animal gets bigger faster than the outside skin. What this means is that the part of the animal that generates body heat, the inside, increases faster than the part that loses heat, the outside – the skin.

1″ on a side

Look at this cube, six sides, all one inch in length. Each side is thus 1 square inch and the total surface area 6 square inches (6 sides x 1 square inch). The volume is 1x1x1 or one cubic inch. Now, double the size of the side of the cube to 2 inches. Now we have 4 square inches per side and a total surface area of 4×6 or 24 square inches. The volume is 2x2x2 or 8 cubic inches. Notice that the surface area has gone up by a factor of 4 while the volume has gone up by a factor of 8.

2″ on a side


In summary, larger warm-blooded animals are more adapted to cold weather because their volume/body mass is larger and their surface area smaller than smaller animals. In other words, larger animals lose less heat than smaller animals and are thus better adapted to colder environments. Conversely, smaller animals of the same species are more adapted to warmer environments because they are able to lose excess heat with their larger surface to volume ratio.

Why is this significant? Well, the world is warming. Hot environments are getting hotter and cold environments less cold. The evolutionary pressure to adapt to the environment then means that animals will get smaller as the environment gets warmer. Even in 2011 we were aware of this phenomenon. See Live Science.

It’s not necessarily a negative that animals are getting smaller, but it is a definite sign that things are changing. What will happen when birds are smaller? Well, their diet will change – they won’t be able to capture and eat insects or seeds as large as they did before. Maybe their nests and eggs will be smaller. Perhaps they won’t be able to defend their territories as they did previously. On the other hand, they might be able to handle smaller food items, hide from predators, and use smaller nesting holes. We can only speculate.

The Fox Sparrow, found on the west coast of the U.S. from central California to northern Alaska, shows an increase in body size. There are other rules as well. Allen’s Rule says that appendages get shorter with colder temperatures, for the same reason as Bergmann’s Rule. Gloger’s Rule says that birds get darker in more humid environments. I’ll let you read Wikipedia to get the explanation for that and wonder how global warming will interact with that rule.

Fox Sparrow

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