Scientific research has been corrupted but we can still make simple observations. London pigeon eating people’s left over  processed food full of seed oils, versus country pigeon eating  its natural diet.
https://m.primal.net/HiYl.jpg
https://m.primal.net/HiYk.jpg 

 I see a similar phenomenon with crows. In Vancouver they are quite small and scrawny. But in rural Alberta they are big as chickens!