According to new studies, many birds in the Amazon rainforest have become smaller as temperatures have increased. The difference hasn't been obvious, but it has been significant enough that some scientists have suggested it's a universal response to climate change.
But new research finds that the body size reductions aren't happening across the board with some large - brained birds having much less significant changes.
For the study, researchers studied some data on about 70, 000 birds that had died when they crashed into buildings in Chicago from 1978 to 2016. They added data on brain volume and lifespan（寿命） for 49 of the 52 species of migratory birds（候鸟） in the original study.
They found that birds with very large brains had reductions in overall body size that were about one-third of the reductions noted in birds with smaller brains. They thought that in birds, the species with big brains are the ones that build tools, manage to survive in tough environments, live longer, invest more time and energy into raising babies, and end up surviving better in the wild.
Researchers aren't certain exactly how warmer temperatures might lead to decreasing body size in birds, but they are considering two possible explanations, which could even be happening at the same time. First, natural selection might be favoring birds that can dissipate heat better. This is because smaller birds have higher ratios（比例） of surface area to volume, so being small can help birds stay cool. Second, warmer summers might have less food available for birds at the time when they are feeding their babies. In that case, birds might be getting smaller because of decreased food over the years.
The findings don't suggest that climate change is having zero impact on bigger-brained birds, but researchers believe these findings can info rm us of climate change and help set conservation priorities.
