Bird banding programs have tagged and recorded millions of individual birds, resulting in important discoveries about bird populations and migration. The US Geological Survey continues this work which contributes to our understanding of avian longevity, survival rates, populations, and migration.
An enduring concept resulting from banding programs is the migration flyway, common north-south flight corridors used for seasonal migration. Terrific online visualizations like Audubon Migration Explorer and Movebank show how birds traverse the migration routes connecting North and South America, Africa and Europe/Asia, and Australia and Asia, among others.
The images below show four flyways outlined in Migration of Birds, published in 1950 by the Fish and Wildlife Service (US Department of the Interior). The banding program provided the data needed to develop this concept of migration pathways utilized by numerous bird species.
There is better understanding of the global flyways, but the basic concept is the same: birds use common north-south pathways that traverse the globe as they migrate between breeding and wintering grounds.
World's flyway map, Pacific Birds Habitat Join Ventures with information sources from International Wader Study Group/Pacific Birds Habitat Join Ventures, Copyrighted, All Rights Reserved - Used by Permission, https://www.fws.gov/media/worlds-flyway-map
This global interconnectedness means that local or small habitat changes can have a big impact on birds, especially if those areas are within a flyway. A study of wetlands and migratory waterbirds showed that changes in a single but important wetland could impact a species' entire biogeographical population. Changes to bird populations would also have economic impacts - other researchers found that ceasing wetland protections in the US would have negative economic impacts due to decreased wildlife viewing ($489 million) and hunting ($32 million).