Difference between an insect a bird and a mammal?

Insects are invertebrates with exoskeletons, three main body segments, and typically six legs, while birds are warm-blooded vertebrates with feathers, beaks, and the ability to lay eggs. Mammals, another group of vertebrates, are characterized by having fur or hair and mammary glands that produce milk for their young. Additionally, insects undergo various life stages through metamorphosis, birds have a unique respiratory system and hollow bones for flight, and mammals possess a more complex brain and higher cognitive functions.