Are Birds Animals?

The biological taxonomy system can be a confusing and sometimes imperfect method for categorizing life. However, it can also be an effective way for us to better understand the flow of evolution throughout history. It’s commonly recognized that birds rank among the closest direct descendants of dinosaurs, but many people wonder if they can be classified as animals.

We’ll work through the defining characteristics of Kingdom Animalia so you can get a sense of whether or not birds are animals, and then we’ll work our way down to discussing the unique characteristics that distinguish a bird from the rest of the biological world.

Defining the Animal Kingdom

A Kingdom represents the second-highest division in biological taxonomy. Five kingdoms represent all of the complex organisms that fit within Domain Eukarya, which include organisms with cells that have a membrane-bound nucleus. These five kingdoms constitute the majority of multicellular organisms on the planet and include everything from oak trees to apes.

Kingdom Fungi

Members of the Fungi kingdom have no direct means of locomotion, and they typically absorb the nutrients they need from dead matter in their environment. All mushrooms fall into the fungi kingdom, as do molds and yeasts. Fungi typically reproduce by releasing their reproductive materials in highly resilient spores. It’s an approach that allows many fungi to survive even in inhospitable conditions.

Kingdom Protista

Protista distinguishes itself from the other kingdoms by primarily consisting of single-celled organisms. Most of these organisms don’t have cell walls and can absorb nutrients into their body by either consuming matter or through photosynthesis. In many cases, they require a scientific microscope to observe. Members include various amoeba species, while organisms like sea lettuce and kelp are now classified in different groups according to modern taxonomy.

Kingdom Plantae

Kingdom Plantae continues to be one of the most diverse and expansive kingdoms, even though fungi and protista are now categorized into their own families. The key distinguishing factor between plants and members of the other kingdoms is their ability to photosynthesize. This is due to the presence of chlorophyll in these organisms. The process allows them to synthesize their food by using sunlight to turn carbon dioxide and water into sugars and oxygen.

Kingdom Animalia

Kingdom Animalia encompasses all animals. It is estimated that there are around 9 million animal species, but the exact number is unknown. These are some of the most sophisticated multicellular organisms on the planet. Organisms in the kingdom Animalia are characterized by their advanced mobility, ability to breathe oxygen, processes for consuming organic material, and methods of reproduction. Unlike plants and fungi, these organisms lack cell walls. This kingdom includes both vertebrates and invertebrates.

Defining Characteristics of Animals

The most accurate way to figure out if birds are animals is to evaluate the different characteristics of an animal. Let’s compare them to birds to see how they stand up to analysis.

Nutrition

Animals primarily rely on heterotrophic nutrition. Unlike plants or fungi, animals need to ingest other living things to meet their nutritional needs. This is also the case for every bird on the planet. Whether we’re discussing a vulture that feasts on festering roadkill, a chicken pecking at seeds in the yard, or a hummingbird making a dinner of nectar, every bird needs to eat to survive. They’re also incapable of photosynthesis.

Navigation

Most animals are capable of self-propelled navigation. This can take the form of swimming, flying, or walking, and this navigational diversity is reflected across the full spectrum of bird species. Penguins are adept swimmers that can spend long periods of their lives underwater. The ostrich cannot fly, but it can run up to 43 miles per hour. Of course, the vast majority of birds are capable of flight.

Reproduction

Animals typically reproduce sexually. They produce haploid gametes, or sperm and eggs, which combine to create a diploid zygote during the fertilization process. Diploid means that the cells contain two sets of chromosomes. The zygote goes through a series of distinct developmental stages, resulting in an embryo. Birds reproduce via internal fertilization, where sperm is transferred to the female’s cloaca and the eggs are fertilized inside the body before being laid.

Cells

Animals are all multicellular organisms, which allows them to have incredibly complex physiologies. This is especially true of birds, which pack more cells into their skulls than mammals, despite their brains being significantly smaller. On a cellular level, birds are most closely related to reptiles, due to the fact that they share dinosaur ancestors.

Respiration

Aerobic respiration is present in all animals and is the key to properly absorbing nutrients from food. The oxygen that animals inhale breaks down sugar into energy, which can then be used by the body. Birds in particular tend to demonstrate especially efficient levels of aerobic respiration. That’s a necessity born from evolution, as flying requires a great deal of energy to sustain.

Are Birds Animals or Not?

Yes, birds do belong to the kingdom Animalia, as they share all the characteristics of an animal. However, this is because they share a common ancestor with all other animals on the planet. They share that distinction with organisms as wide-ranging as salmon, komodo dragons, gorillas, mice, and sea sponges.

Taxonomy allows us to narrow things down even more by moving further down the evolutionary chain. Like most of the more advanced species in the animal kingdom, birds belong to Phylum Chordata, which includes animals that have a vertebra or develop the evolutionary predecessor to a backbone at some point in their development process.

The Unique Characteristics of Birds

There are over 11,000 bird species with common characteristics. Creatures resembling modern birds first appeared around 66 million years ago, but they’ve taken many evolutionary turns since then. In most cases, these features remain because they’ve proven beneficial across bird species with otherwise unique physiologies.

Feathers

Birds developed feathers for the same reason that mammals developed fur: to better regulate their temperatures to match external conditions. Yet feathers also help make flight easier and can even develop unique coloration as a part of sexual selection. Birds such as vultures may have sparse feathers, but every living bird has at least some feathers.

Wings

Rheas, cassowaries, and emus may not be able to fly, but that doesn’t mean they don’t have wings. Wings are a characteristic shared by all birds, but some have been adapted to better suit life on land or in the water. The wings of the emu help it keep its balance while running, and penguins have developed appendages that more closely resemble flippers than wings. Except for bats and insects, birds are the only other animals capable of true flight.

Hollow Bones

Not all of the bones in a bird’s body are hollow, but the primary bones are. This allows their bodies to be lightweight enough to support flight, but many of these bones are reinforced inside to make them far less brittle. These hollow bones have also evolved to accommodate the enormous respiratory needs of birds. Air sacs, connected to the lungs, expand into the hollow bones to help with gas exchange.

Beaks

One thing birds and turtles all have in common is the presence of a beak without teeth. This beak developed millions of years ago as dinosaurs transitioned into birds. It’s believed that the beak not only developed as an adaptation for birds’ specialized diets, but also to achieve faster embryo growth and shorter incubation times.