I think Sentret has replaced Pidgey as a biome marker.

I live on a residential loop of houses and apartments. It is a Pidgey rich, Weedle light biome. A typical poketracker radius can have 15-20 Pidgeys at a time. I noticed today there are a comparable number of Sentret and only 2-3 Pidgey.

I speculate that other gen 2 pokemon will become the new biome markers and we will see far fewer Pidgey, Weedle, Spearrow, etc.