Assuming the rogue is using a missile to shoot the robin I'd still tell the rogue's player to roll to hit, if only for the potential of an amusing miss and the much more likely loss of one unit of ammunition.Let's go go a massive extreme. You are the GM. You have 5 PCs in your game, all 20th level. They are walking through the forest. You narrate that they see a standard in-no-way-extraordinary robin off in the distance (maybe you want to drop a hint it's a peaceful and non dangerous area, but the reason is unimportant). The rogue decides for no apparent reason to attack the robin and kill it.
If you actually have a combat in this situation, with initiative, stealth checks, attack rolls, etc it would be a pointless combat.