escape responses are reduced on remote islands, because predators are scarce or absent there, and natural selection under reduced predation favors prey that do not waste time and energy developing and performing needless escape.

There may be a learned behavior involved here, rather than a genetic or instinctive behavior.

Prey in groups can learn from their peers' mistakes, "he didn't run fast enough," for example.

There is a conceivable test for this, but it can't be done in the wild for conservation reasons.

You'd need to expose island prey to mainland predators, and mainland prey to island predators, in groups large enough for the "learn from others' mistakes or personal mistakes" effect to be possible.

If the island prey change their behavior, then it is a learned response, and they are delaying flight on the island simply because they've learned when to run and when not to run vs their natural predator.

So this is an additional alternative hypothesis that needs testing