Ilian, you seem to think I'm just "guessing" the rattlesnake story. I should dig up the source, but if my mind doesn't trick me (and I'm pretty sure it doesn't), it's established science and not just a ghost story fantasy (in contrast to some of the ideas raised here...). On the other hand, it's not always like that either - sidewinders can also steal the home of these rats after finishing off the original inhabitants.
Ilian Velikov wrote:is there such a thing as not hungry snake? As far as I know they are opportunistic and would eat suitable prey even if they've eaten already because they might not have another chance any time soon..
As far as I know, this is incorrect for most ectotherms. A (Belgian) berus eats less than 10 mice a year. Not exactly the menu of a voracious "never enough" animal. Of course, this will be different for racers which spend much more energy on movement, but in general, I think snakes may very often disregard potential prey items because of a full stomach. Digestion also takes quite a bit, and then there's of course still the fact that you cannot fit an infinite amount of prey items into the gut of any predator, so saturation has to set in always at a certain point.
Ilian Velikov wrote:If anything it might be the adder benefiting from the grass snakes's much better vision. I've noticed that whenever these two species bask together it's always the grass snake that first notices an approaching human/predator and flees alarming the adder that there's danger.
??? Vipers are not slower (or flee less quickly) because their eyes are worse. Their venom-based defence gives them less need for a speedy take-off. Also, I'd say their physiology (and maybe even anatomy?) is most likely just different, in relation to their sit-and-wait predation mode.