Ducks

Ducks and Mallard Ducks are critters. As all other critters, they have 5 health, no defense, and are often killed instantly by other enemies or the player. They can usually only spawn in the middle third of the above-ground world, or in areas near an NPC house if water is present. They only appear in pools of water, regardless of their size or if they are naturally generated. They quite frequently make a "quack" sound when nearby.

Ducks will passively wander the surface until they fall a certain distance or are submerged, at which point they will switch to their alternate flying form, where they will fly in a given direction for a certain amount of time before landing and switching back to their passive form. Despite their spawn conditions, they do not prioritize water whenever landing.

Ducks can be caught using a Bug Net. They can be carried around in the inventory until placed (released), deleted, or crafted into a Duck/Mallard Duck Cage. Caged Ducks can be used as furniture and cannot be released once crafted.

The Duck Cage occupies a 6×3 space. The Duck inside may follow the following patterns randomly:
 * Run from one side to the other of the cage
 * Briefly clean itself with its beak while at the right side
 * Sit down and stay in place

Trivia

 * When Ducks quack, there is a chance the sound will be a very obvious human voice saying the word . It was pulled from a royalty-free site because the Re-Logic team found it amusing. The sound was included purposefully as an "Easter egg".
 * Ducks, Mallard Ducks, and Bunnies are the only critters that only spawn in the middle-third of the above-ground world.
 * Ducks are one of six critters that do not have a gold variant.
 * Due to a potential oversight, whenever a Duck "transforms" into its flying variant, it is considered a new entity and will no longer be considered a critter, causing it to occupy spawn slots as normal, thus preventing hostiles from spawning. Furthermore, due to their flying variant being considered a different entity altogether, statues ignore their existence when calculating the limit of concurrent entities existing in the world at once. This can be exploited by placing statues submerged, or using a water pump to cause Ducks to instantly switch to their flying form, enabling endless spawning and potentially hitting the physical entity cap.
 * This bug is already fixed in the 1.4.0.1 update.