Yes, Grouse (partridge) in Maine are still plentiful and this year was a banner year for most areas. While the hunting is still good there has been a decline in numbers in the state that correlate in large part with a change in forestry practices in Northern Maine and an increase in sub-divisions in Southern Maine. That changed about 30 years ago. No, you can not tell the sex of a Grouse by looking at it although some will claim you can by the length and band on the tail. Color variation is just that, a variation.
Yes, foxes and coyotes can catch Grouse and do. I find piles of feathers fairly frequently in areas with high concentrations of birds. Although, I would say nest predation is a bigger concern.
I hunted the mountains of East TN hard from 2006-2009 with two well-experienced dogs (GSP and English Setter) and had minimal success. While the cover around here might be the best it's been in years it's still severally lacking what is needed to produce sustainable numbers for hunting. Then again I am spoiled growing up in and getting to hunt in Northern Maine every year.
I am no biologist but from my experience, these are reasons why Grouse numbers decline.
1. Change in forestry practices
2. Urban sprawl/loss of habit
3. Domestic/feral Cats
4. Poor rearing conditions in the spring (cyclical)
While there is a lot of speculation that turkeys affect grouse numbers I am not sure you would get a state biologist to admit it with all the money turkey hunting can generate.