To be honest - as it was rather angry I didn't really get much time to notice its eyes at the time of taking the photograph - but now that you mention it - think you are probably right - it does have what could be described as " puppy eyes "
Until you mentioned chickens - I'd always assumed the reason why I see a lot of snakes was because - I live in a fairly remote mountainous area with rocky stony ground - which here is thought to be their natural habitat - however although I don't keep chickens myself or have a major rodent problem - some of my neighbors - who although some considerable distance away from me - do keep chickens and other wildlife - but saying that I don't just get snakes visiting my garden but every other kind of reptile too.
You can actually tell a snake is either a constrictor or a viper by looking at its head. Vipers have a distinct and angular shape of the head while constrictors have a rounded shape. The eyes are also a good indicator. Vipers have slits while constrictors have "puppy eyes".
That could also be a reason. Maybe the reptiles just wander off into your yard. I have no idea how you can live with that. I'd probably have a mini heart attack every time I rake the leaves or something.