Story related - this will unlock early in the game when you find 25 "shiny things", as requested by the crow. Some are obvious, some are more well hidden, none are difficult. A good way to get them is to jump into bins / trash cans, you'll usually get 2-3 shiny things when you exit.
When you have 25, head back to the crow and he'll exchange them for a fish - which gives you the ability to scale smaller walls using the plants.
WARNING This one seems to be glitched. If you get a fish from somewhere else before getting it from the crow, the achievement may not pop.