Wings. Birds are also the only animals with wings. Wings are modified forelimbs that allow birds to fly. Birds' wings are incredibly complex, and they have evolved to be lightweight and aerodynamic.
Beaks. Birds have beaks instead of teeth. Beaks are made of keratin, and they come in a variety of shapes and sizes. Beaks are used for a variety of purposes, including eating, preening, and defending themselves.
Hollow bones. Birds have hollow bones, which help them to be lightweight and fly. Hollow bones are also strong, and they help to protect birds' organs.
Air sacs. Birds have a unique respiratory system that includes air sacs. Air sacs help birds to breathe while they are flying. Air sacs also help birds to regulate their body temperature.
Crop. Birds have a crop, which is a sac-like organ that stores food. The crop helps birds to digest their food more easily.
Gizzard. Birds have a gizzard, which is a muscular stomach that helps them to grind down their food. The gizzard is lined with grit, which helps to break down food.
Cloaca. Birds have a cloaca, which is a single opening that serves as the outlet for the digestive, urinary, and reproductive systems. The cloaca also helps birds to excrete waste.
Eggs. Birds lay eggs. Eggs are a unique form of reproduction, and they are protected by a hard shell. Eggs contain everything that a developing bird needs to grow, including nutrients, water, and oxygen.
These are just a few of the things that make birds unique. Birds are fascinating and diverse creatures that have evolved to fill a variety of niches in the world.