(Note all achievements unlock at the end of the game only not during the actual gameplay)
During the 1st walking section (and possibly the second as well) try and picture the pink bird whilst its in flight with the camera.
Unlike the other bird achievement the camera will NOT change colour to indicate a good shot. Sometimes the game may play an audio cue when you meet an achievement condition but its not very consistent.
Providing you snapped the pink bird in flight the achievement should unlock at the end of the game.