The Amazon rainforest, is as undisturbed a place as most people can imagine, but even there, the effects of a changing climate are playing out. Now, research suggests that many of the region's most sensitive bird species are starting to evolve in response to warming.
Birds are often considered sentinel（哨兵）species-meaning that they indicate the overall health of an ecosystem-so scientists are particularly interested in how they're responding to climate change. In general, the news has not been good. For instance, a 2019 report by the National Audubon Society found that more than two-thirds of North America's bird species will be in danger of extinction by 2100 if warming trends continue on their current course.
For the new study, researchers collected the biggest dataset so far on the Amazon's resident birds, representing 77 non-migratory species and lasting the 40 years from 1979 to 2019. During the study period, the average temperature in the region rose, while the amount of rainfall declined, making for a hotter, dryer climate overall. According to the report on November 12 in the journal Science Advances, 36 species have lost substantial weight, as much as 2 percent of their body weight per decade since 1980. Meanwhile, all the species showed some decrease in average body mass, while a third grew longer wings.
Because of the study's long time series and large sample sizes, the authors were able to show the morphological（形态学的）effects of climate change on resident birds. However, the researchers themselves are unsure and wonder what advantage the wing length changes give the birds, but suppose smaller birds may have an easier time keeping cool. In general, smaller animals have a larger rate of surface area to body size, so they dissipate more heat faster than a bigger animal. Less available food, such as fruit or insects, in dryer weather might lead to smaller body size.