Dawn in the Amazon. Amongst the mist drifting over the treetops, the forest’s myriad inhabitants are stirring. Howler monkeys greet the sun with their deep throated calls, joined by the songs of millions of birds, their feathers flashing in the fresh light. It is a scene that has remained unchanged for millions of years1 – until now.
Slowly but surely, the birds of the Amazon are shrinking.
This bizarre discovery was first highlighted in 2021, when scientists released the results of a remarkable 40-year study tracking 77 bird species deep into the undisturbed parts of the rainforest. They had found that the birds were slowly getting lighter, their bodies shrinking, while their wings got longer. Why might this be? The scientists came to an extraordinary conclusion – Amazonian birds are shrinking not due to habitat loss, hunting or human disturbance, but due to climate change.
But how could climate change cause birds’ bodies to shrink? To find out, we need to go back to 19th century Germany.
In 1847, German biologist Carl Bergmann noticed that, within animal families or even within species, those living in hotter places tended to be smaller.
One example of this can be seen in the bottlenose dolphin – dolphins living in warm seas off the coast of Florida are smaller than those living in the North Sea.2 It is not clear why this pattern (sometimes called Bergmann’s rule) exists, but many biologists think it is to do with temperature regulation. As animals get smaller, all their cells become closer to the body’s skin. This makes it easier for heat to diffuse out, preventing the animal from overheating. Conversely, being larger means most cells are further away from the skin and therefore heat cannot diffuse so easily, keeping large animals warm in cold climates.
Might this mean climate change will cause animals to get smaller?
In the deep past, animals have indeed shrunk as the climate warmed. 55 million years ago, there was a sudden temperature increase called the Palaeocene-Eocene thermal maximum. Crocodiles and palm trees encircled the Arctic Ocean, and the sweltering continents were filled with dwarf horses the size of dogs3. But that warming happened over a much slower timescale than today’s human-induced climate change, so it was unclear whether any species would have had the time to evolve smaller bodies.
To investigate, a group of scientists in Brazil took measurements from 77 rainforest bird species over a 40-year period. They chose undisturbed Amazonian rainforest because these areas are (relatively) free from human influence and so reflect a natural system. Most Amazonian birds are also non-migratory and therefore do not expose themselves to lots of different temperatures, all of which might impact their body size. The researchers also checked if temperatures had actually increased due to climate change over the study period – which it had! With some parts of the year being 1.65 degrees warmer compared to the 1980’s.
Sadly, some of the forests they worked in 40 years ago have now been felled, but despite this the scientists built an impressive database of measurements from over 11,000 individual birds.
The researchers’ findings were pretty clear. Most species had experienced significant declines in body mass, by up to 1.8% per decade. However, strangely those same species had also evolved significantly longer wings!
But what was particularly interesting was that the changes in size and shape weren’t random!
The changes were especially pronounced after hotter-than-average dry seasons, when food and water become scarcer. You see, the Amazon has two seasons – one hot and dry, the other warm and wet – and it’s after those intense dry spells that the birds decreased in size most dramatically.
Shrinking body mass is one thing. But why are their wings growing?
The answer to this requires some physics! In these hotter-than-average dry seasons, resources are scarce, and any energy birds can save makes them more likely to survive. Longer wings provide more lift at slower speeds, so the bird does not have to waste energy flapping quickly to get around. This, coupled with lower body mass further potential lift, reducing the energy needed to get airborne!
So, let’s have a quick recap!
Researchers have found that within a human lifetime, birds in the Amazon rainforest are changing shape! Their bodies are getting smaller, and their wings are getting longer. And, although this could simply be an adaptation to improve temperature regulation, as Carl Bergmann described, there is also reason to believe these changes are due to a climate-induced decrease in resources such as food. Which is a poignant reminder of how powerful climate impacts can be, with the potential to starve animals living in Earth’s richest ecosystem!
How can we find another way?
It is bizarre and terrible to think that human emissions are causing rainforest birds, some of whom may have never seen a human, to change shape. Here in the UK, we are not easily placed to directly help the Amazon in its plight. But there are individual actions we can do to help.
One of the easiest things to do is eat less beef. Beef is the most carbon-intensive meat to produce, with each beefburger emitting 6 kilograms of CO2, the equivalent of burning 6 milk-bottles’ worth of crude oil!5 But demand for cheap beef is also the primary driver of deforestation in the Amazon: fast food chains such as McDonald’s have been implicated in importing beef from deforested land4. Therefore, if you do choose to eat beef, try to buy British and locally sourced.
Another crucial action is to listen to the Indigenous people who call the Amazon home. They have lived in the rainforest for thousands of years and have the best understanding of what is happening there.6
Lastly, as individuals, we can help by discussing these issues, raising awareness for the plight of our rainforests and simply remaining interested! The Amazon is the most biodiverse place on Earth, and a powerful climate regulator – what happens there impacts us all.
Article written by Ollie Ray
Primary Paper:
Vitek Jirinec et al., 2025, Morphological consequences of climate change for resident birds in intact Amazonian rainforest. Sci. Adv. 7, 2025. https://www.science.org/doi/full/10.1126/sciadv.abk1743
Other references:
- Greenpeace staff (2020) Biodiversity and the Amazon rainforest, Greenpeace [website]: https://www.greenpeace.org/usa/biodiversity-and-the-amazon-rainforest/
- Favilla A.B., et al. (2024) Examining Bergmann’s rule in a cosmopolitan marine mammal, the bottlenose dolphin (Tursiops spp.), Marine Mammal Science, Vol 40, Issue 4: https://onlinelibrary.wiley.com/doi/10.1111/mms.13130
- Gingerich P.D. (2006) Environment and evolution through the Paleocene-Eocene thermal maximum, Palaeontology series, Vol 21, Issue 5, May 2006, https://www.cell.com/trends/ecology-evolution/abstract/S0169-5347(06)00093-0?large_figure=true
Saget, S. et al. (2021) ‘Comparative life cycle assessment of plant and beef-based patties, including Carbon Opportunity Costs’, Sustainable Production and Consumption, 28, pp. 936–952. doi:10.1016/j.spc.2021.07.017 Available at: https://www.sciencedirect.com/science/article/pii/S2352550921002165?ref=pdf_download&fr=RR-2&rr=8b7c433ad8a463b4
- Leading Burger Supplier sourced from Amazon Farmer using deforested land (2019) The Guardian. Available at: https://www.theguardian.com/environment/2019/sep/17/leading-burger-supplier-sourced-from-amazon-farmer-guilty-of-deforestation
- Wasley, A., Heal, A. and Campos, A. (2019) ‘Leading burger supplier sourced from Amazon farmer using deforested land’, The Guardian [website]: https://www.wwf.org.uk/updates/ways-help-amazon-rainforest
