Related anecdote: I moved to a small town in the West Midlands in the UK from Barcelona last year. One of the things I first noticed was an absence of nature sounds (but you can always hear a motorway, no matter how far you walk :( ). This is compared to living in downtown Barcelona in El Born. I finally figured it out after going back for a visit to Barcelona that there are just substantially fewer birds here, or at least in the area I live. I wonder if that's because there is less food for them?