Environments on earth change all the time. Plants and animals with traits that help them survive are able to reproduce more successfully. A trait that helps an organism survive is called an adaptation. As environments change, plants and animals with the most helpful adaptations will survive. Only plants and animals that survive are able to pass their traits on to new generations.
An adaptation that is visible to the naked eye is called a macroscopic adaptation.
Example: Large beak size
An adaptation that isn't visible to the naked eye is called a microscopic adaptation.
Example: More effective digestive chemicals
In this image you can see different macroscopic bird adaptations that may be helpful in different environments.