From the Super Mario Wiki
A Nitpicker is a flying enemy that first made its debut in Donkey Kong Jr. Small Nitpickers appear in Level 2, flying at different levels and dropping eggs on Donkey Kong Jr. Large Nitpickers appear in Level 4, but do not drop eggs on Donkey Kong Jr.
The two forms of Nitpickers make a reappearance in Donkey Kong for the Game Boy. They first appear in stage 2-9 in the Forest. They fly out of their nest and cross the screen horizontally. When they fly over Mario, they stop and try to drop an egg on him. Touching the eggs or the Nitpicker itself kills Mario. The second variety of Nitpickers only appear in stage 9-4 in the Tower. They have a different appearance, but the same patterns as the first variety.
The Mario vs. Donkey Kong series features Nitpickers in a more modern appearance, with yellow feathers and an orange beak.
They appear again in Mario vs. Donkey Kong 2: March of the Minis where they behave in the same way as in the last game, however this time around they appear to be more mechanical than in previous appearances.
Nitpickers reappear again in Mario vs. Donkey Kong: Minis March Again! but only in two boss battles.
Nitpickers made an appearance in the Captain N: The Game Master episode "The Lost City of Kongoland", where they're depicted very differently.