Why are there few quail any more in the Northeast of the US?
Work in the early 1990s (https://pubs.er.usgs.gov/publication/5210546) (https://www.nwf.org/en/Magazines/National-Wildlife/1993/The-Case-of-the-Disappearing-Quail) suggests development (curtailing habitat), climate crisis (and associated water loss), and decline in insect populations (reducing food supply) as possible causal factors.

Insects are an important energy source for birds, generally. They help pollinate plants, which help feed birds, and they are also a food source, themselves. Their loss (https://en.m.wikipedia.org/wiki/Decline_in_insect_populations) particularly hits bird populations hard. Unless we change our agricultural practices, it is predicted that most insects may be extinct within a century. This would decimate many forms of life (ourselves, included with quail and other birds) that depend on them.