Ironically I removed my mention of the windscreen evidence. There are some citizen science surveys documenting the decline (e.g. a UK study found a 60% decline

Ironically I removed my mention of the windscreen evidence. There are some citizen science surveys documenting the decline (e.g. a UK study found a 60% decline over 20 years, or something like that). But it's a good example of a strong indicator but not the greatest quality evidence - geographically limited, concerns low flying population only, might be confounded by changes in car aerodynamics, verge / hedgerow maintenance etc.