Red Bird has a rather powerful ability, able to turn a monster of your choice into a random monster of the same cost. It can also turn into the same monster but it will be the default version of that monster. The effect will check the current cost of the monster and will end up causing the monster to stop existing (it doesn't get added into the dustpile nor does the Dust effect activate) if there is no monster with the same base Cost. (For example, using it on a monster at 15G with the help of cards like Inflation, Bratty or Echo Flower, as no monster has a default cost of 15G. The same applies to monsters at a cost of 19G or higher.)
On low cost monsters, this ability is a lot more random, but it can still help if you want to screw over the enemy by changing their Epics such as Flowey, Napstablook or Mettaton into something less powerful.
On high-cost monsters, you can use it on your own in order to heal them up, as most of them have around the same stats and a Magic, such as Asgore and Undyne. Without the aid of cost changing effects, these two are also the only ones that can turn into Frisk (33% chance), due to their 14G. This allows you to get a Frisk into your possesion for the rest of the game without having to go through the process of obtaining the card and lets you bypass the 1 DT limit in Ranked Mode.
You can also try to set high-cost monsters to 10G or 12G, so that you might have a chance to obtain Cool Papyrus (for 10G) or Casual Undyne (for 12G), as all of these have Haste or Charge. This allows them to attack again. However, trying to obtain a Cool Papyrus has a 75% chance to fail, as your 10G monster can also turn into Goner Kid, Clover, or Mettaton NEO.
- Red Bird was revealed on Twitter with 4G and was released in Beta 13.4 with a cost of 4G, but the Patch Notes of Beta 13.4 said its cost was 5G.
- It got nerfed to 5G in the next update, Beta 14.0.