Where I grew up there was one of the biggest murder of crows in Europe (I can't remember what the term was) and there is an estuary nearby.
In the morning hundreds of crows would leave the area to fly over the estuary and I guess go to the richer farmland, they would fly back around dusk, retrospectively it was crazy to see, but even then it was appreciated.
Fast forward roughly 20 years, and that area has been built on so much you might be lucky to see a murder of 5 crows. There has been so much building, even across the estuary that now in the area the most noticed birds are seagulls sitting on rooftops and raiding bins. Magpies are flourishing too.
Staying there during the summer means being woken to magpies attacking cats or other domestics animals and their horrible cakakakakakaa at 4am through to 6am.. I can't help but think that the crows used to keep them at bay due to their massive numbers.