What do you imagine when you hear the word “feather”?
It’s likely that you imagine a tail or wing feather. But did you know there are 6 major types of feathers on a bird’s body? Each of the major types has specific form and function.
Let’s start by defining two important terms in regards to feather structure. A feather is either pennaceous or plumulaceous. Pennaceous feathers are what most people imagine to be a “typical” feather. Pennaceous feathers are flexible and consist of the following parts:
- Vane- the flat surface of a pennaceous feather
- Barbs- hundreds of stiff filaments that attach to a rachis
- Rachis- a relatively solid structure that extends down the middle of the feather
- Ramis- a central shaft which has slender branches on either side
- Barbules- the slender branches on either side of the ramis.
- Barbicels- tiny hooklets that attach the barbules together and create the flatness of the vane
Plumulaceous feathers have barbs that are loose and fluffy. Their barbs have rami that are less stiff and the barbules are usually either reduced or thinner. As a result, plumulaceous feathers cannot hold anything except a delicate, rounded form. When you see a chick with down, you are seeing an example of plumulaceous feathers.
Feathers grow out of and remain attached to a feather follicle in the epidermis. If you’ve ever seen the bumps on a plucked chicken, then you have seen feather follicles.
Now that we’ve discussed the basics of feather structure, let’s examine the 6 major feather types.
1. Down Feathers
These feathers are entirely plumulaceous, making them soft and fluffy. Down feathers act as insulators that allow birds to manage their internal body temperatures by thermoregulation. There are 3 types of down feathers.
- Natal Down temporarily covers the entirety of a hatchling’s body. Birds that are precocial (able to feed/tend to itself immediately after hatching, therefore not relying on a parent) tend to have more natal down since they have to maintain their temperatures on their own. Altricial birds (who rely on a brooding parent) have sparse natal down since they receive heat transferred by a parent.
- Body Down lies under the contour feathers of many adult birds. These are more common in waterbirds such as penguins, loons, and ducks. Ever see a duck or goose line its nest with feathers? These are body down from their breast.
- Powder Down are unique because they grown continuously and disintegrate at the tips to produce a keratinous “powder”. They help make feathers waterproof, and are only found in certain kinds of birds, such as pigeons and herons.
2. Contour Feathers
These feathers give a bird their characteristic shape and make up its exterior. The top section of these feathers are pennaceous, while the bottom section is plumulaceous. Many birds can use oil from the uropygial gland at the base of the tail to help their contour feathers repel water. Contour feathers have a variety of functions including aiding in thermoregulation, streamlining the body during flight, and social displays.
3. Flight Feathers
Flight feathers make up the majority of the tail and wing feathers. They are almost completely pennaceous. The anterior and posterior edges of flight feathers are asymmetrical. The leading edge (a.k.a anterior edge) is typically thinner than the trailing edge (a.k.a posterior edge). This feature allows flight feathers to stabilize under the pressure of air currents during flight. These feathers are usually the stiffest and largest feathers on a bird. There are 2 types of flight feathers:
- Remiges originate from the wings and attach to the bone. The feathers of the outer wing are called primaries and the feathers of the inner wing are called secondaries. The number of primary and secondary feathers vary by bird, but typically a bird can have between 9-12 primaries and 8-32 secondaries.
- Rectrices form the tail surfaces/airfoil of a bird. The central pair of rectrices attach directly to the tailbone. Like remiges, number of retrcies vary by bird size, but is typically between 6-32.
Coverts are smaller contour feathers that overlap the wing and tail feather and create the streamlined shape that is important to the aerodynamics of flight.
Semiplumes are the intermediate form between the contour and down feathers. Their barbs usually lack hooks, so their vanes are not pennaceous. They occur at the edges of contour feather tracts and complement the insulation of down feathers.
Bristles are highly specialized and lack barbs along most of its length. Their rachis are very stiff and they are almost exclusively found on a bird’s head. The most common type are rictal bristles, which commonly project at the beak’s base. Many birds that are insectivores, like flycatchers, have rictal bristles.
Filoplumes are hair-like feathers and are the smallest of all the feather types. They have a rachis but few or no barbs. They’re usually hidden underneath the contour feathers. Instead of feather muscles, they have sensory receptors in their skin near the follicles. Filoplumes aid birds in detecting changes in feather position caused by body movement or wind.
Feathers are an amazing adaptation that is unique to birds. Knowing the types of feathers helps us appreciate how complex and special birds are.