The biological taxonomy system can be a confusing and sometimes imperfect method for categorizing life, but 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, and five kingdoms represent all of the complex organisms that fit within Domain Eukarya. These kingdoms constitute the majority of multicellular organisms on the planet and can include everything from oak trees to apes to the common flu virus. Here are the five kingdoms:
- 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 will 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 Protist: 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 most cases, they may require a serious scientific microscope to even observe. Members include sea lettuce, kelp, and varying amoeba species.
- Kingdom Monera: The Monera Kingdom is uniquely primitive in that its organisms are monocellular. They fall under two classifications – eubacteria and archaebacteria. But not all bacteria fall under this kingdom. Thanks to their ability to photosynthesize, some types of blue algae belong under the classification of Kingdom Plantae.
- Kingdom Plantae: Kingdom Plantae used to cast a wide net, but it continues to be one of the most diverse and expansive kingdoms even now that Fungi and Protista are now categorized under their own families. The key distinguishing factor between plants and members of the other kingdoms is their ability to photosynthesize. This is thanks to the presence of chlorophyll in these organisms. This process allows them to absorb all of the nutrients they need through the absorption of sunlight and carbon dioxide.
- Kingdom Animalia: Kingdom Animalia is a categorization that encompasses all animals. These are some of the most sophisticated organisms on the planet, and they distinguish themselves from the other kingdoms in a number of ways: their advanced mobility, their habits for absorbing and processing nutrients, and their methods of reproduction being the most common characteristics. Many of the rules that define the kingdom are broken by more than one species, and that can make this kingdom both incredibly diverse and sometimes challenging to properly categorize organisms within the kingdom.
Defining the Features 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 in nature to see how they stand up to analysis.
- 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 understandably incapable of photosynthesis.
- 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. Despite their awkward appearances, penguins are adept swimmers that can spend long periods of their life underwater. The equally goofy-looking ostrich is capable of reaching a top speed of 43 miles per hour, and their six-inch long talons are capable of gutting a living creature. That’s not counting the vast numbers of birds who are capable of flight. Not all animals are capable of self-sustained movement – with sponges being notably immobile – but birds meet the criteria of self-movement with flying colors.
- With only a few exceptions, sexual reproduction is the norm for both animals and plants. And some of the species most prized by birdwatchers have developed their unique plumage thanks to sexual selection. From the peacock to the various birds of paradise to the Mandarin duck, there’s a distinct pattern of males developing colorful and flamboyant coats while females maintain more muted colors. This too is a result of sexual selection, as it makes mothers less noticeable to predators.
- Animals are all multi-cellular organisms, and that allows them to have incredibly complex physiologies. This is especially true of birds, who actually pack more cells into their skulls than mammals despite their brains being significantly smaller. Cellularly, birds are most closely related to reptiles thanks to the fact that they share dinosaurs as a close ancestor.
- Aerobic respiration is present in all animals and 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 is a navigation method that requires a whole lot of energy to sustain.
Birds: Animals Or Not?
Ultimately, it’s important to keep in mind that birds don’t belong to Kingdom Animalia simply because they share all the characteristics of an animal. They share all the characteristics of an animal because they share a common ancestor with all other animals on the planet. Yes, birds are considered animals. They share that distinction with organisms as wide-ranging as the salmon, komodo dragon, gorilla, and mouse.
Fortunately, taxonomy allows us to narrow things down 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 are 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
The total of discovered bird species is roughly 10,000, but there are some characteristics that are common regardless of what species you’re talking about. Creatures resembling modern birds first appeared 60 million years ago, but they’ve taken many evolutionary turns since then. In most cases, these features remain because they’ve proven to be beneficial across a variety of landscapes and for birds with otherwise unique physiologies.
- Birds developed feathers for the same reason that mammals developed fur: to better regulate their temperature to match external conditions. But feathers also help make flight easier and can even develop as a part of sexual selection. Many birds are missing feathers in notable places, but you won’t find a living bird that doesn’t at least have some feathers. But vultures, turkeys, and kiwis are all notable for their sparse or unusual feather patterns.
- Rheas, cassowaries, and emus are just some birds that can’t fly – but that doesn’t mean they don’t have wings. Wings are a characteristic shared by all birds, and many have become altered 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. While some mammals like the flying squirrel are capable of gliding, birds are the only animal capable of true flight.
- All the bones in a bird’s body aren’t hollow, but the primary ones 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. Their lungs can expand into their hollow bones when they inhale deeply.
- 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 developed alongside the growth of brains in birds. The beak developed as a means of protecting this growing gray matter, but today’s birds use their beaks for everything from foraging to self-defense to mating.