Feathers: Birds have feathers, which are unique to them. Feathers are complex structures made of keratin, a protein also found in hair and nails. Feathers provide insulation, waterproofing, coloration, and lift for flight.
Beaks: Birds have beaks instead of teeth. Beaks are made of keratin and vary in shape and size depending on the bird's diet and lifestyle. Beaks are used for grasping, tearing, crushing, and probing for food, as well as for preening and nest-building.
Endothermy (Warm-bloodedness): Birds are endothermic, meaning they can generate their own body heat to maintain a constant internal body temperature, regardless of the external environment. This allows birds to be active in a wide range of habitats and climates.
High Metabolic Rate: Birds have a high metabolic rate, which means they need to consume a lot of energy to power their flight and maintain their body temperature. This is why birds often have a rapid heart rate and respiration, and may spend a significant portion of their time foraging for food.
Flight: Most birds are capable of flight, with the exception of some flightless species like penguins and ostriches. Birds have wings that are modified forelimbs, and they use their powerful flight muscles to generate lift and propulsion.
Laying Eggs: Birds reproduce by laying hard-shelled eggs, which are incubated by one or both parents until they hatch. Bird eggs contain all the nutrients and resources needed for the embryo to develop and hatch.
Care of Young (Parental Investment): Birds exhibit varying degrees of parental care after their young hatch. Some birds, like altricial species, require extensive care and feeding from their parents, while others, like precocial species, are more self-sufficient and can find food and move around soon after hatching.
Complex Social Behavior: Many bird species are social and form flocks, colonies, or other social groups. Birds communicate through vocalizations, visual displays, and body language. Social behavior can be important for finding food, defending territories, and finding mates.
Migration: Many bird species undertake long-distance migrations between breeding and non-breeding grounds. These migrations are often triggered by changes in day length, temperature, and food availability.
These are some of the basic characteristics that define Class Aves and distinguish birds from other vertebrate groups.