Little bird

Little birds, or small birds, are white birds with distinctive crests that are recurring ambient species in the Mario franchise ever since their debut in Super Mario 3D Land. Occasionally, they even serve a gameplay purpose. Their appearance seems to be based on, a family of tits, chickadees, and other with similar body shapes, with some species such as the crested tit sporting crests, though the colors of their feathers and beaks coincide more with those of the albino. 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.

Super Mario 3D Land
Little birds make their first appearance in Super Mario 3D Land, serving as ambient creatures and usually appearing in flocks. When Mario or Luigi approaches them, they fly away.

Super Mario 3D World / Super Mario 3D World + Bowser's Fury
In Super Mario 3D World and its campaign in Super Mario 3D World + Bowser's Fury, little birds can also be found in various levels. In this game, they can occasionally appear on the title screen, where they fly away when the main cast of characters run through it. In the Bowser's Fury campaign of Super Mario 3D World + Bowser's Fury,  can be seen throughout Lake Lapcat, and fly away if Mario or Bowser Jr. comes near, or if a Green Shell is kicked or thrown at them. All Cat little birds will also fly away immediately when the rain that signals Fury Bowser is about to rampage begins.

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. Other variants of little birds also appear in other kingdoms, such as a variant in the Wooded Kingdom. If Mario falls asleep in an area where little birds can be found, one may rest on his nose, though in areas such as the Metro Kingdom, many birds can be found near Mario when he is sleeping.

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 Mario's Figure Skating Spectacular, 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,, , , , , , Dragon Driftway, and Hyrule Circuit.

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.

Super Mario Party
In Super Mario Party, little birds fly across the screen during Social Climbers.

The Super Mario Bros. Movie
Little birds appear in The Super Mario Bros. Movie as wildlife in the Mushroom Kingdom.

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.