All species of birds have backbones because birds are vertebrates. The animal kingdom can be organized by grouping classes of organisms together based on key physical characteristics. Whether or not an organism has a backbone or spinal column is one means to classify animals.
Vertebrates are animals that have a skeletal system that includes a backbone. All vertebrates share a similar body plan that consists of the spinal cord with a central nervous system at the top, a digestive tract below and an anus at the end. Birds, mammals, reptiles, amphibians and fish are all vertebrates.
Invertebrates, on the other hand, are animals that do not possess a backbone and have varying body structures. Some classes of invertebrates are jellyfish and sponges, arthropods, insects and worms.