The results of a study on birds in the Amazon rainforest has shown that birds have gotten physically smaller over the last four decades, and scientists believe that a warming planet may be the reason.

This study is an example of climate change human actions globally affecting a fundamental thing such as the size and shape of these birds in the middle of the Amazon, the symbol of terrestrial biodiversity, ecologist Vitek Jirinec told NBC News . Jirinec was a lead author on the study, which was published in the journal Science Advances on Friday.

The paper analyzed measurements and weigh-ins of 77 different bird species since the early 1980s. Nearly all of the bird species have gotten lighter as time goes on. On average, most species became about 2% lighter every decade, which is significant for such small creatures.