Small Amazonians

Birds are sensitive indicators of environmental change. A recent study of understory birds of the Amazon rainforest over a timespan of 40 years and 77 species shows again that most birds are adapting to the current drastic environmental changes by becoming smaller. By zooming in on non-migratory birds living in intact rainforests with limited local human presence any observed changes in morphology can be considered the result of global trends in environmental change. The decrease in body size seems to be the result of Bergmann’s rule, which states that that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. Smaller animals have a higher surface area to volume ratio than larger animals, so they radiate more body heat per unit of mass, and therefore stay cooler in hot climates. The researchers were surprised to find that while their average weight dropped by 3 to 4%, wingspan actually increased by 2 to 3%. Although the mechanism behind this increase is poorly understood speculations are that larger wingspan may allow for more economic flight. It also increases surface area to volume ratio.

The morphological change in the Amazonians corresponds with a 40-year trend of increasingly hot and dry conditions during the rainforest’s drier season. As it turns out the research also showed that it is not so much the higher local temperatures that directly allow predictions of abundance of a species, but precipitation. More unpredictable rain patterns have led to more unpredictable availability of food and drinking water. Smaller body size, as well as greater wingspan seems to be a way to deal with unpredictability. If a species has the ability to shapeshift it has a greater chance of survival.