Bird Diversity assesses the number of species and their relative abundance. Birds play important roles in ecosystems, respond quickly to environmental disturbance, and generally have lots of data available compared to other taxonomic groups. Thus, birds are useful indicators for assessing ecological conditions and overall ecosystem health. This indicator assessed bird diversity using Simpson’s Diversity Index. Generally, a higher diversity of birds is an indicator of a healthy ecosystem.
How is it scored?
Christmas bird survey areas that at least partially occurred within a watershed were combined to calculate Simpson’s Diversity Index for that watershed. Simpson’s Index is calculated as 
This value was multiplied by 100 to calculate a watershed’s score. Watershed scores were weighted by their area to calculate the overall score for southeast Michigan.
Data source: Audubon Christmas Bird Count for 2020
(https://www.audubon.org/conservation/science/christmas-bird-count).