Little bird

Little Birds are white birds with distinctive crests that are recurring ambient species in the Mario franchise ever since their debut appearance in Super Mario 3D Land. Occasionally, they even serve a gameplay purpose. Their appearance seems to be based off, a family of tits, chickadees, and other with similar body shapes, with some species such as the crested tit sporting crests. They are similar in behavior to other birds throughout the series, such as the Birds in Super Mario Sunshine, as well as the blue birds outside Peach's Castle in Super Mario 64.

Mario & Luigi: Bowser's Inside Story
A little bird named Birdley first appeared in Mario & Luigi: Bowser's Inside Story, serving as an ally and a guide to Mario and Luigi throughout the game. It is revealed later in the game that Birdley is the pet of Dr. Toadley and he can be acquired as a "special item".

Super Mario 3D Land
Little Birds make another appearance in Super Mario 3D Land, behaving as ambient creatures and usually appearing in flocks. When Mario or Luigi approaches them, they fly away.

Super Mario 3D World
In Super Mario 3D World, Little Birds can also be found in various levels. In this game, they can occasionally appear in the title screen, where they fly away when the main cast of characters run through it.

Super Mario Odyssey
In Super Mario Odyssey, the Little Birds can be found in the Mushroom Kingdom and in the Cap Kingdom, the latter wearing top hats. If Mario falls asleep in an area where Little Birds can be found, one may rest on his nose.

Mario & Sonic at the Sochi 2014 Olympic Winter Games
In Mario & Sonic at the Sochi 2014 Olympic Winter Games, Little Birds appear during the first portion of the Mario Figure Skating Spectacular event, which is based on Super Mario 3D Land. As always, they fly away when the skaters come near.

Mario Kart 8 / Mario Kart 8 Deluxe
A flock of Little Birds appear on the road in the following tracks in Mario Kart 8 and Mario Kart 8 Deluxe: Mario Circuit, Wii Moo Moo Meadows,  GBA Mario Circuit. SNES Donut Plains 3, N64 Royal Raceway,  3DS Piranha Plant Slide,  GCN Yoshi Circuit, Hyrule Circuit and Dragon Driftway.

When racers approach them, they fly away, though after a set period of time without any racers, they return to the track. These birds are removed in the split-screen modes of the game.

Captain Toad: Treasure Tracker
Players can interact with the Little Birds in Captain Toad: Treasure Tracker. Normally, they fly away when Captain Toad or Toadette approaches them, but if they stand by a Little Bird long enough, the Little Bird flies and lands on their heads, which is detailed in Episode 11 of the European The Cat Mario Show.

Mario Sports Superstars
 Little Birds are one of the objects encountered in the grass in the stable mode of Mario Sports Superstars'' sport, Horse Racing. When players pass into them, they fly away and reveal a ? crate, which reveals a Mario Token. When players obtain 20 Mario Tokens, they unlock the Mario Token Crown, a special horse accessory for the head.

Mario + Rabbids Kingdom Battle
In Mario + Rabbids Kingdom Battle, the Little Birds can be found in the Ancient Gardens. They behave like they do in the Super Mario series, in which they fly away when the party members are near them, only to return soon after the party members leave the area.

Trivia

 * Despite being endemic to Mario's world, Little Birds can also be found on the Hyrule Circuit in Mario Kart 8, which is set in the kingdom of Hyrule from The Legend of Zelda franchise.