You already have several answers, but I want to add a different one. A bird that was very common when I was a boy is dying now. Why doesn't it just move a few

You already have several answers, but I want to add a different one. A bird that was very common when I was a boy is dying now. Why doesn't it just move a few km northwards, to where the temperature is right for it? It doesn't because it needs craggy islands in a shallow sea and the right temperature range. A lot of species can move, but that doesn't mean that there are many suitable new locations. What we have now has been optimised for the current geography and climate over a long, long time. Shifting all the locations 100km isn't likely to produce as many geography/climate matches.