The Unique Characteristics of Birds
What makes a bird...a bird?
We have all seen birds...they are everywhere, in our backyards, gardens, and hedgerows. You can even find birds in cities. They bring life and color to our world, as well as beautiful songs and sounds! But what, by the technical definition, is a bird? Biologists like to classify organisms into groups, and if you enjoy birding, you probably like to classify too (I know I do).
This lens seeks to answer the question "what is a bird?" by outlining the physical characteristics that are unique to birds and the Class Aves. Together, these characteristics identify birds as a monophyletic group. You may be surprised to learn that the two features most commonly associated with birds, flight and feathers, are not entirely unique to birds! Bats are mammals that can fly and some dinosaurs had feathers. :)
Hint: there may be a quiz at the end!
(Photo: the beautiful mug of a King Vulture, courtesy Flickr Creative Commons)
Clickable table of contents for this page:
Do you enjoy watching birds?
Bird Topography - from Wikimedia
All living birds have feathers, and no other animal alive today has feathers. However, feathers are not completely unique to birds. Did you know that some dinosaurs had feathers? However, since there are no living feathered dinosaurs, we can say that feathers are a unique characteristic among birds today.
Feathers can be found in just about every color imaginable! All of a bird's feathers collectively are called its plumage. Feathers are not only used for flight, they actually have many purposes, including: attracting mates, territorial dominance, regulation of body temperature, camouflage, and flight. There are also many types of feathers which serve different functions. The four main types of feathers are semiplumes, filoplumes, bristles, and powder.
Neat Bird Facts:
An American Robin can have up to 3,000 individual feathers.
A Bald Eagle has 7,000 feathers.
The number of feathers a bird species has varies depending on bird size!
Toothless in modern birds
The jaws of a modern bird are covered with a horny sheath --- which means birds have what's known as a horny beak. Many early birds had teeth, but no modern birds do. Why? Birds need to be as light as possible in order to fly effectively and teeth would add weight to a bird's skull. Also, a horny beak is much lighter than bone.
Many bird species have a bill that is specialized, which enables a particular bird to eat certain foods. Raptors have a hooked bill for ripping and tearing meat, seed eaters have a heavy bill for crushing seeds, and hummingbirds have a long narrow beak for sucking nectar from flowers. These are only three beak adaptations, there are countless others!
Roseate Spoonbill and its amazing bill shape (Image: wikimedia commons)
Bird Bill Shapes
My favorite bird field guide
Commonly called the WISHBONE
We've probably all played the game at one point in our lives --- two people grab hold of either tail of a wishbone (usually from a turkey) and give a tug. The person who comes up with the largest half gets to make a wish! But what really is a wishbone and do other animals have them? A "wishbone" or furcula is actually two fused clavicles. Other groups of animals have clavicles, but not furculas. We call our clavicles collarbones. No other living animals aside from birds have a furcula, but some dinosaurs did.
Why do birds have wishbones? Like many features of the bird's anatomy, the furcula is an adaptation for flight. When a bird flies, the furcula acts as a little spring to help power the flight strokes and is also believed to help birds breathe during flight. Because it is a characteristic unique to birds, all birds have a wishbone --- even birds that do not fly.
Image: Flickr Creative Commons via Dwayne Madden
There are around 10,000 different bird species on earth
The fastest bird is the Peregrine Falcon, which has a stoop speed of up to 270 mph!
The Spine-Tailed Swift is the fastest horizontal flier, reaching speeds of up to 105 mph.
Fastest bird (and animal) on the planet
Commonly called Hollow Bones
Birds that can fly have some pneumatic or hollow bones. Some of the pneumatic bones in flighted birds include: the pelvic girdle, some ribs, the humerus, and the femur. Hollow bones do not contain marrow --- the bones have air-filled canals and are strengthened by criss-crossed struts. Some flightless birds, including penguins and ostriches, have only solid bones. Some dinosaurs also had pneumatic bones.
In order to fly birds need to have a lightweight skeleton. A large number of heavy, marrow-filled bones would prevent birds from flying, which is why hollow bones are such a useful adaptation. Another flight adaptation is the fusion and shortening of certain bones in the bird's skeleton, which also reduces weight. We see this in bats as well.
Great Horned Owl in flight (notice the gopher in its right-hand talons)
Image: Flickr Creative Commons via Anita Ritenour
The Hallux - A special toe
Another characteristic unique to birds is the hallux.
What on earth is a hallux? It is just a special name for a bird's big toe! Most birds have a backward facing first digit (the hallux) and three forward facing digits. In most birds, the hallux can be used to grasp perches and other things. Not every bird has a useful hallux though, birds that only fly occasionally (like chickens) have a hallux that is farther up on the leg and never touches the ground --- otherwise this special big toe would slow these birds down when they run. Other birds, like woodpeckers, have two toes that face forward and two that face backward, this enables them to cling to vertical surfaces.
The claws on a bird's feet are actually specialized scales. Claws grow constantly and are kept short from daily use. Raptor claws are usually called talons.
The Ostrich is the largest bird in the world at 200-350 lbs
Largest bird egg ever is from the extinct Elephant Bird @ 2.6 gallons or 220 chicken eggs
The smallest bird in the world is the Bee Hummingbird at 0.056 oz.
The Vervain Hummingbird lays the smallest egg = the size of a pea
Most birds have nine
Birds do not have a diaphragm like mammals. Instead, birds have air sacs! Air is moved in and out of a bird's respiratory system through pressure changes in the air sacs. The air sacs act as bellows, when they expand air rushes in! A bird's air sacs even extend into some of its bones --- the humerus, femur, vertebrae, and skull. Air sacs also help to keep birds cool by expelling heat, this is useful because birds do not sweat. Again, this characteristic is unique to birds.
Thanks to air sacs, a bird's respiratory system is more efficient than the mammalian respiratory system, meaning birds can transfer more oxygen with each breath. Birds do have lungs, which are also much more efficient than a mammal's lungs, but a bird's lungs are different in many ways.
Few birds can rival vultures when it comes to soaring. This is a Turkey Vulture seen drying its dew-dampened wings in the morning sunlight. These birds are capable of soaring for hours on thermals (rising columns of warm air).
Image: Flickr Creative Commons via Anita Ritenour
Other Bird Characteristics - While these traits may distinguish birds from many other animals, most of them are not unique.
1. Fusion and reduction of bones
2. Flight (Most. Flightless birds are called Ratites.)
3. Songs and/or calls
5. Small size
6. Bipedalism (digitigrade foot posture)
7. Forelimbs modified for flight
8. Endotherms (commonly known as "warm-blooded" meaning birds maintain their body temperature through metabolic heat)
9. Single aortic arch which bends right (this is UNIQUE)
Did you learn something from my lens? - American Robin
Anything you didn't already know about birds on this page?
This page was awarded with a Purple Star for quality content on 7 July 2011. Thank you to my readers, nominator, and Robin!!! (:
So I can visit some of your lenses! :D