BRD is what the biology arm of USGS was called 15 years ago. It's now the ecosystems mission area, which includes things like the bird banding lab that manages all of the waterfowl banding permits and data across the country, the national wildlife health center that does a bunch of work on CWD and avian flu, ungulate migration stuff, piles of salmon and other fish work, the fish and wildlife cooperative research units, etc.
State agencies might be able to absorb some of the work, but what about, for example, trying to map and understand migrations that span multiple states (or even countries), how a wildlife disease spreads across the continent, or how to manage waterfowl harvest as birds migrate through multiple states and countries?
Some legislation, such as the American Conservation Enhancement Act (from the last Trump administration and reauthorized last year), actually identify USGS as being responsible for coordinating science and research across state and federal agencies.
For you bean counters, check out USGS's green book from last year:
https://www.doi.gov/sites/doi.gov/files/fy2024-usgs-greenbook.pdf-508.pdf. Whopping $400m to ecosystems. That's like $2 per taxpayer and not even a rounding error compared to $6-7t total fed spending.