Things with squirrels are a bit more complex. While the two species do fight over the same resource, the main reason for the decline of red squirrels is disease.
The grey squirrels carry squirrel-pox, which can be fatal to the reds. That's always the explanation I've heard cited for the decline in red squirrels in the UK at least:
I wonder if the grey squirrels adapted this behavior because of previous encounters with red squirrels, displacing them in the process, or if it's just "bad luck" for the red squirrels to run into grey squirrels who just happen to perfectly counter their survival tactics?
In more remote places and in the mountains where the winters are harsher you still see the small red native squirrels though.
Someone's related blog: https://www.miss604.com/2015/03/grey-squirrels-of-stanley-pa...
Quote: So how did this squirrel get here just over 100 years ago? Some reports say that the squirrels were a gift from the Mayor of New York City in 1909 but Jolene from Stanley Park History found that in 1909 Vancouver Park Boar Chair, Charles Tisdall, wrote to various American cities in search of purchasing “grey squirrels” for Stanley Park.
And here I was thinking milder winters would benefit the red squirrels because they would have a better chance getting through those without food supplies/fat reserves. Guess it's actually more of an even playground in terms of food supplies, where things like the grey's parapoxvirus tip the balance in favor of more greys surviving.
Are harsher winter conditions harder on the grey squirrels? Maybe their fat-reserves give them a mobility disadvantage that becomes crippling in harsher winter conditions? Or maybe those remote places are so remote that grey squirrels merely haven't made it there, yet.
Afaik most of these are usually invasives species, displacing local populations, which is happening in the weirdest ways and places.
In Germany, our local ladybug species are on the verge of going extinct because an imported Asian ladybug species emits some kind of spores killing them, and the Asian ones generally being sturdier.
Eastern grey squirrels, brought to the European mainland from the UK, are also increasingly replacing local red squirrels. Red squirrels get through winter by hiding food supplies and staying lean and mean, while grey squirrels build up fat reserves.
What ends up happening is that the grey squirrels sniff out the food supplies of the red ones and clear them out to fill up their own fat reserves, that leaves the red squirrels with no food supplies to get through the winter and so they starve.
But I agree with your general sentiment, I remember nature being quite a bit "buzzier" when I was a kid, I always thought I felt that way because I might have paid more attention to things like insects as a kid, but I guess my feeling wasn't all that off.