Is Wildlife Shrinking?

Believe it or not, the largest cause of bird mortality in the US is collision with buildings. Glass covered structures that reflect sunlight are a confusing obstacal for migrating birds who interpret the glass sides of tall building as open air space. In Chicago, the Field Museaum collects the victims of collisions for measurement and add them to the ornithology collection. These make up about 20% of the collection by now, over 100,000 killed in four decades. A curious data point emerged from this collection of 35,000 dead birds: they have been shrinking—and rising global temperatures are likely to blame, according to a 2019 analysis of measurements. And it is not just birds that appear to be getting smaller. A growing body of research says that many wild creatures are shrinking--from cold blooded amphibians to warm blooded mammals.

An ecological principle known as Bergmann's rule may be at work producing this phenomenon. It states as originally formulated that within a genus populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. This rule can be seen in penguin populations around the globe. [see chart] Temperate species tend to be smaller in size, while those inhabiting Antarctica are the largest. This rule has also been found to operate over geologic time. Temporary, reversible dwarfing of mammals has been noted during two relatively brief upward excursions in temperature during the Paleogene( (66-23 million years ago)

Measurements made in Chicago show the length of birds' lower leg bones (an indicator of body size) shortened by an average of 2.4% and their wings lengthened by 1.3%.  Scientists say that just as in the pass we can expect evolutionary changes to improve adaptation to a climate warmer by 1.5℃  Other studies on birds, deer, rodents, insects, and fish show similar patterns.  As with any rule in biology, its is a generalization of the prevalent facts, and their are examples of variations and contradictions.  The point being that natural systems are really complicated. Even if theory and lab research suggests that animals consistently shrink under warming, the exact result of climate change is not hard and fast in Nature, the greatest laboratory. Rather than study shrinking dead birds in a museum, perhaps it would make more sense to save energy, reduce warming, and conserve bird lives simply by turning off those lights.