Climate change makes birds smaller

Even the wildest species of the Amazon rainforest are being impacted by human-caused climate change.

Hotter and drier weather conditions are reducing the body size but increasing the wingspan of rainforest birds, according to a study published in the journal Science Advances on November 12. These changes reflect nutritional and physiological challenges, especially during the dry season from June to November.

The Amazon species Platyrinchus coronatus is a "victim" of climate change. (Photo: Cameron Rutt)

This conclusion was drawn after biologist Vitek Jirinec and colleagues from the IERC ecological research center collected, tagged and measured more than 15,000 birds in the rainforest over the course of 40 years.

They found that virtually all birds have become smaller since the 1980s, losing an average of 2% of their body weight per decade. That means if a bird weighed 30 grams in the 1980s, it would weigh less than 27.6 grams today.

The team spent four decades collecting and measuring more than 15,000 birds. (Photo: Vitek Jirinec)

The data is not linked to a specific location but is collected from a large range of rainforests, meaning the phenomenon is ubiquitous. In total, Jirinec and his colleagues investigated 77 species whose habitats range from the lower forest layer (dark and cool) to the upper vegetation layer (sunshine and warmer).

Birds at the top fly more and are exposed to more heat, so there are the most pronounced changes in body weight and wing size. The team suggests that this is an adaptation to heat stress and energy stress caused by the depletion of food sources such as fruit and insects.

Changing weight and size is one way birds adapt to climate change. (Photo: Vitek Jirinec)

Longer wings and a reduced mass-to-wing ratio allow birds to create more efficient flights, just as an airplane with a thin fuselage and long wings can levitate with less energy. In contrast, a higher mass-to-wing ratio requires birds to flap their wings faster to stay aloft, using more energy and generating more metabolic heat.

How will birds in the Amazon cope with increasingly extreme climate conditions in the future? That is still an open question. The team added that what they observed in birds may also be true for other species around the world.