Biology & Ecology

What are feathers made of?

Feathers set birds apart from all other animals. They are made of keratin, which is the same material that your toenails, a horse's hoofs and an antelope's horns are made of. Some birds may have up to 25,000 feathers, comprising up to 20% of its body-weight, and two or three times heavier than its skeleton.

They do not grow randomly all over a bird's body, but sprout in specific areas known as feather tracts.  Each tract generates feathers of particular shapes and sizes for specific functions. Different feathers have specific functions, including assisting flight, insulation, camouflage and attracting a mate, but they are not specialized in flightless birds such as emus or penguins, on which the feathers are the same shape all over the body. There are three basic types of feathers: contour feathers, down feathers, and filoplumes.

Colours

Since they first appeared on the earliest birds, feathers have developed a wide variety of colours. The combination of different coloured feathers forms different patterns, and on some birds these patterns form a camouflage which helps to make the bird blend into its surroundings, and this helps to makes it less obvious to predators. Tawny Frogmouths, for example, look so much like a broken-off branch that they are often completely overlooked by people walking right underneath them; and the markings on a Spotted Nightjar look so much like the leaf litter that it roosts in that they are almost impossible to find. It is no mistake that young birds, less experienced in evading predators, are generally duller than adults, and birds which incubate the eggs or brood the chicks (often the female) are usually dull and plain, so they do not draw attention to the nest. In some species, the colours of feathers or the patterns they form are used by males to attract a mate, often performing a display which serves to accentuate the effect of the feathers, some of which may be highly ornamental.

As feathers become worn, they become less efficient in assisting in flight or whatever other function they serve, and they need to be replaced. The process of replacing worn feathers is known as moulting, and occurs about once a year. Feathers are moulted gradually over a few weeks. When moulting, some birds find it difficult to fly. The feathers of moulting penguins do not offer sufficient insulation against the cold of the ocean depths, and penguins may sit on a beach for a week or two while their new feathers develop. Replacement feathers sprout from small pores in the skin (which are known as follicles) where the old feathers grew.

Types of Feathers

Contour feathers

The contour feather is the most familiar type of feather. These are the feathers which cover the body of the bird. They consist of a central shaft, known as the 'rachis', and a stiff vane projecting from either side. These vanes are made up of a series of 'barbs', which to the naked eye look like a series of hairs or stiff fibres, but when viewed more closely each barb has many smaller barbs, or 'barbules', branching off it. Each barbule has a tiny hook (barbicel) at its end which is used to attach itself to other barbicels. It is this ability to hook the barbs together which makes each vane a smooth surface. Birds are often seen preening their feathers by running individual feathers through the beak. This refastens any barbules which may have come undone by zipping them up, just like a zipper.

Contour feathers have evolved into all sorts of shapes and sizes. The flight feathers of the wings (remiges) are modified contour feathers which are uneven, with the vane on the leading side narrower than the vane on the trailing side, which, due to its aerodynamic qualities, make the bird capable of flight. The tail feathers (rectrices) are also modified contour feathers. On some species these have evolved into all sorts of varied shapes and sizes. For example, the rectrices of the tiny Southern Emu-wren are so elongated that they are more than twice as long as the rest of the bird, and the rectrices of the Superb Lyrebird have become modified into fantastic shapes which resemble a lyre, and these are shimmered and waved during spectacular mating displays. At the other end of the body, the crest on the head of the Sulphur-crested Cockatoo is also formed by modified contour feathers.

Down feathers

Down feathers have the same basic stricture of contour feathers, except they lack barbules, so that the barbs do not interlock, and do not form flat vanes. The air spaces between the barbs of down feathers trap tiny pockets of air, and are used to insulate the bird. Chicks are often covered only in down feathers before their contour feathers develop.

Filoplumes

Filoplumes are simple, hairlike feathers which grow around the base of contour feathers, and comprise a thin rachis topped with a few barbs at the distal end. They are shorter than the contour feathers, and their function is not fully understood, but may be related to maintaining the contour feathers in the correct position.

The ability to fly is a major feature of most species of birds, though some birds such as emus and penguins are incapable of flight. Although some other animals, such as bats, are able to fly, birds have many highly evolved characteristics which allow them to fly more efficiently. The first of these is that birds are generally light-weight, and birds have many features of their skeletons which minimize their weight. Birds' bones are usually not solid, but are hollow or contain air sacs, which allow air to circulate to make the bird lighter. These bones are supported by internal struts which brace the bone to give it extra strength. Other weight-reducing features of a bird's skeleton are that it has few joints within its skeleton; they lack teeth; and feathers are also light weight. Birds also have exceptionally well-developed breast muscles which allow them to flap their wings.

How do birds fly?
 

It seems obvious, but birds fly by flapping their wings. It takes lots of strength to flap a pair of wings, so birds have developed large muscles for the task, and the size and shape of the sternum (breastbone) of a bird is enlarged to attach the muscles to the skeleton. It is, however, the shape of their wings which provides the lift birds need to become airborne. The upper surface of each wing is convex in shape and slightly longer than the underwing, which is concave. As the bird flaps its wings, air is forced both over and beneath each wing, and the difference in air pressure caused by the shape of each surface forces the bird upwards. All birds that can fly have convex and concave surfaces on their wings, but the overall shape of the wings varies greatly, and determines the type of flight that the bird does. Birds with long, pointed wings, such as Peregrine Falcons or Fork-tailed Swifts, are capable of high-speed flight; those with shorter, more rounded wings, such as Superb Fairy-wrens or Brown Goshawks, generally do not fly very quickly but are able to manoeuvre deftly between obstacles such as trees or shrubs in the forest; and birds with long, narrow wings, such as Shy Albatrosses or Australasian Gannets, can glide for long periods without flapping their wings at all.

Whether it's a flower, a berry, a seed, some nectar, a fish, a worm, a crab, a mouse or even another bird, there is a bird that eats it. There are few forms of life that birds do not eat.    The best way of finding out what a bird eats is to look at the shape of its beak. With so many different types of food, many birds have beaks that are adapted to specific types of food, or specific methods of foraging. By having differently shaped bills and feeding on different food sources, birds are able to exist in the same habitats and not compete with one another for food.

Nectar

Different species use different methods to obtain nectar from flowers. The long, slender, slightly curved bill of the Eastern Spinebill is used to probe deep into the base of long, tubular flowers to get to the sweet, energy-rich nectar of plants such as correas. Some other species have become adapted to feeding on nectar of different plants, but have a different strategy. The Swift Parrot, for example, often forages on nectar in open flowers, especially those of flowering eucalypts. Its bill is shaped differently to those of honeyeaters: it has a typically parrot-shaped beak, but instead its tongue is shaped like a brush. The Parrot wipes its tongue around the eucalypt flower to gather nectar and pollen. Because its bill is the wrong shape it cannot feed in long tubular flowers like those used by Spinebills, and has become specialised in feeding in eucalypts.

Seeds

To feed on seeds birds require a beak that is capable of exerting sufficient pressure to break open the hard seed husk to expose the nutritious endosperm within. Finches, such as the Zebra Finch, forage on seeds, usually of various types of grasses, and have bills which are stout and conical, perfect for the task. For seeds that are contained within tougher seed-pods, such as those of banksias, a tougher bill is required. Though cockatoos and other parrots often feed on grass seeds, they are also capable of extracting seeds from hard, woody seed-pods. It is the shape of their bills which allows them to do it. The stout, curved bill of the Yellow-tailed Black-Cockatoo, for example, allows the bird to exert enormous pressure on seed-pods to split them open or tear them apart to expose the seeds.   The strong curved bill also allows Cockatoos to tear branches open to expose insect larvae that may be tunnelling into the wood.

Meat

The hooked bills of hawks, eagles and owls are perfectly adapted for tearing flesh from prey they have caught. The sharp point of the bill allows the bird to pierce the flesh, and its curved profile allows the bird to exert great pressure to tear it away from the prey item. The size of the bill gives an indication of the types of prey that each species takes. For example, the Nankeen Kestrel and Southern Boobook both have small hooked bills which only allow them to feed on small items, such as grasshoppers, moths and mice. Larger species, such as the Powerful Owl, have much larger beaks which enable then to peck at and tear the flesh from larger animals, such as possums and gliders, and the Wedge-tailed Eagle has a powerfully hooked beak which it uses to tear the flesh from kangaroos after first piercing the tough skin.

Fish

The Darter specializes in eating fish. It captures them by swimming through the water, and when a fish is detected, the Darter coils its long neck like a spring, then darts forward with its long, sharp beak to spear the fish. Another fish-eating bird is the Australasian Gannet, which catches fish in the sea by diving spectacularly from a great height, head-first into a shoal of fish which are caught in its sharp bill.

Insects in bark

The beak of the Varied Sitella looks like a pair of tweezers, and the bird uses its bill exactly like tweezers when it probes into cracks in the bark of tree-trunks or branches to winkle out small invertebrates hiding there. Small flocks of these birds can be seen spiralling head-first down the trunks of trees, conscientiously investigating every fissure, crack and knothole. Striated Thornbills and Silvereyes also have tweezer-like bills which they use to pick tiny insects from the surface of the foliage or bark of trees. Robins, such as the Hooded Robin, pounce onto insect prey among the leaf-litter on the ground, and grab it in their bill before returning to a perch to eat it.

Insects in the Air

Some birds catch insects in the air. Swallows, swifts, frogmouths and nightjars all have rather wide bills (and mouths) which allow then to catch insects while flying. Species such as the Welcome Swallow and the White-throated Needletail, catch insects during the day, and their quick, acrobatic flight allows them to actively chase insects through the air, but nocturnal species, such as Tawny Frogmouths and White-throated Nightjars, are much less manoeuvrable when hunting flying insects in the dark.

Invertebrates in the Mud

Birds which forage for invertebrates in the mud have the most varied bill morphology of all. Because mud may support a wide range of potential invertebrate prey, varying from crabs to worms and tiny gastropods, different types of birds may all forage together on mudflats without competing with one another. Eastern Curlews have long, downcurved bills which allow them to probe deep into the mud to extract crabs or worms from their long burrows; Royal Spoonbills have remarkable spoon-shaped bills which they use to extract invertebrates by sifting through liquified mud; stilts have long slender bills which they use to peck tiny invertebrates from the surface of the water; oystercatchers use their strong bills to lever open the shells of bivalves and hammer limpets from rocks; plovers and stints have shorter, finer beaks which they use to peck at tiny gastropods and other invertebrates from the surface of moist mud. By foraging using different methods, many species are able to be seen foraging together on mudflats without competing directly with one another.

Generalist Diet

The Pied Currawong is omnivorous, and plucks berries from shrubs, snatches insects from the leaves of plants, probes into soil for insects and their larvae, pulls bark from branches to expose invertebrates from underneath, sometimes snatches eggs or nestlings from nests, and even eats carrion (the meat of dead animals). To cater for this, its bill is more generalized than many other species. It is robust, strong and pointed, with a small hook at the end. These attributes allow the species to forage on a wide variety of food items, but not specialize in any particular type of food.

When breeding, birds have many different interactions with the animals and plants around them, as well as with other aspects of the environment. To breed successfully, birds require a place to build a nest and materials to build it with, they need enough food for their young, and they need to protect their young from potential predators or other sources of danger.

Breeding season

Birds usually breed at the time when there is optimum chance for their chicks to survive. This usually coincides with the period when food for the young birds is most abundant. Clearly this varies from region to region, and from species to species, and is inextricably linked both to the feeding ecology of the bird, and the prevailing climatic conditions. Some species undertake extensive movements between regions (and some even move between continents) to experience suitable breeding conditions, while other species remain in the same area and wait for favourable conditions.

Some shorebirds (also known as waders) only breed when there is an abundant supply of insects for their young to eat. To do this they undertake regular migration between Australia and Siberia. After spending the Australian summer feeding on the coastal mudflats, they form great flocks and fly through eastern Asia to breeding grounds in the Siberian tundra, only stopping off at a few key sites to 'refuel' on invertebrates along the way. They time these movements so that they arrive just as the frozen wastes are thawing out, and there are clouds of insects swarming everywhere.  They lay their eggs among the stunted vegetation, and the chicks that hatch are able to gorge on the insects.

The breeding season of some species of waterbirds, such as the Blue-billed Duck, is determined by the season, and they breed each spring, regardless of the conditions.  During droughts, however, the size of their broods is usually much smaller than during wetter years, and the levels of survival are low.  Some other species, such as the Freckled Duck and the Black-tailed Native-hen, are far more discerning (or more opportunistic), and usually breed after the wetlands have been flooded. This is a response to an increase in abundance of food, such as aquatic vegetation, invertebrates and fish, which is directly triggered by the inundation, and it ensures that there is plenty of food for the ducklings to eat.

Honeyeaters usually breed in spring, which coincides with the flowering of many native plants.  Honeyeater chicks, however, do not eat nectar, so why do they hatch when the plants are flowering?  The answer is twofold.  As the weather warms up in spring, many insect larvae hatch and turn into flies, moths, wasps and the like, and it is these insects that are fed to the honeyeater chicks.  Insects are often attracted to the flowers to feed, and many of them depend on taking nectar or pollen to maintain their population. Honeyeaters (and other insectivorous species) are able to catch unsuspecting insects attracted to the flowers as they feed. Another advantage to this timing is that nectar provides high levels of energy for the adult honeyeaters, and this allows them to chase flying insects extremely energetically, and increases their chances of a successful pursuit.

Nests

There are almost as many types of nests as there are types of birds. Some species barely build a nest at all. The nests of some resident shorebirds, such as the Pied Oystercatcher, simply consist of eggs laid in a shallow scrape in the sand, though some have a shell or two, or a strand of seaweed as decoration. Most parrots, cockatoos, owls and many ducks nest at the bottom of tree hollows, where the eggs are laid among a few pieces of rotten wood.  Some other species of birds, such as the Australian Hobby, refuse to build a nest at all, and use old nests built by crows or other raptors.  Most birds, however, build a nest to lay their eggs in. 

Many birds, from gulls to eagles to honeyeaters, use a variation on a simple cup-shaped nest, made from intertwined sticks, grass or bark.  A few species have taken these designs a little further.  Most species of robins build a neat woven nest of grass, but tastefully decorate the exterior with moss or lichen. The Northern Fantail builds a simple cup-shaped nest, but with an amazingly long tail hanging beneath it. The Zebra Finch has added a roof to its cup-shaped nest to turn it into a sphere of grass with a roof to protect the eggs and young. The Spotted Pardalote also builds a spherical nest, and places it at the end of a long tunnel which is excavated into a cliff or bank of soil. The Yellow-rumped Thornbill builds a spherical nest, but with a fake nest on top to fool any passing cuckoos that may want to lay their eggs. The Yellow-bellied Sunbird builds an oval-shaped nest which hangs by a long cord made from bark, grass and fibres, all intertwined together, and there is often also a similar tail which hangs below the nest. The Mistletoebird also builds a nest which is made from spider webs and plant down, and looks like a small bag or purse hanging from a branch.

Probably the most unusual nests belong to the megapodes, such as the Orange-footed Scrubfowl.  After building a nest which consists of a huge mound of soil and organic material such as leaf litter scraped from the floor of the forest, they lay their eggs in the huge pile of debris. As the organic material rots down, like a compost heap, it heats up, and it is this process which incubates the eggs.  The adult birds usually stay close by so that they can scrape some material away if the nest becomes too hot, or scrape extra onto it if it cools down, and in this way the temperature of the eggs is regulated and maintained.

How Birds Protect Eggs

All species of birds lay eggs. Because each egg contains the embryo of a developing chick it is full of protein, and eggs are highly sought after by all sorts of predators which find them a nutritious food-source.

Most eggs that are laid in open cup-shaped nests have shells that are patterned with spots, blotches or streaks which act as camouflage to protect them from potential predators. Some are so well disguised that even when you are looking for them they are almost impossible to find.

The eggs of birds which nest in hollows or tunnels, or in nests with a roof, are well hidden from view, so they have no need for camouflage, and are often white. Other birds hide their nests by building them in dense shrubs where they are difficult to notice, or in prickly bushes which make it difficult for predators to move through the spines to get to the eggs, and other species build their nests in the outermost foliage of trees where most predators cannot reach. 

Another way that birds protect their eggs or chicks from predation is by their behaviour.  Some species, such as the Australian Magpie and the Masked Lapwing, aggressively swoop at intruders near the nest, which is a very effective deterrent. Other species, such as the Spotted Nightjar, are so well camouflaged that when an incubating bird is approached it remains on its nest, keeping its eggs hidden, and most predators will walk past, unaware that the nest or bird was there.

Some species nest in large colonies, where the presence of many birds means that there is an increased likelihood that any predator will be detected quickly. An offshoot of this is that some species of birds nest cooperatively, with a number of birds looking after a single nest.