Features of the Class Aves
They are endothermic vertebrates with internal fertilization. They have a single excretory and reproductive orifice called a cloaca. The body is covered in feathers
Morphology of Birds
There are 4 main types of feathers each with their own specific function.
Flight Feathers are structurally strong with a strong shaft (quill) and a stiff vane (see diagram).
Contour Feathers have a thinner shaft and designed to overlap giving the bird an aerodynamic form and to insulate the body to prevent to the loss of body heat.
Soft down feathers are fluffy feathers that lie under the contour feathers and are specifically to trap body heat.
Filoplumes are hair-like feathers and the function is not known but some theorize that they act as small sensory organs.
The feather is a long shaft: the thick end is embedded in the flesh of the bird whilst the remainder of the shaft has barbs (branch-like rods) that come off at regular intervals. Each barb has a network of barbules that interlock with those of the adjoining barbs. The interlocking gives the feather strength and a solid foundation for flight.
Birds are constantly preening using a ‘preen gland’ which yields an oil. The bill picks up the oil and spreads it over the feathers making the feathers water resistant, reducing fungal growth and maintaining flexibility. As the bill runs along the shaft of each feather it brings the loose barbules back together. Note that, amongst others, darters and cormorants lack the preen gland; they are constantly diving for their prey and oil would increase their buoyancy.
Dust bathing has been adopted by many bird species; inter alia doves, sparrows, starlings and many ground species, to dislodge various parasites and other unwanted materials from the feathers.
Moulting is the removal of old or damaged feathers. Moulting varies depending on species however most birds moult only once a year. Birds that come into breeding plumage will moult twice a year. Flight feathers are normally only moulted once a year. Some species, such as the female grey hornbill, moults all her feathers at once whilst incubating. Most birds moult incrementally where they lose 1 of the 10 primaries, 1 of the secondaries, etc at a time as to not affect flight performance. Other species such as ducks undergo a full moult of all flight feathers and for this period they are flightless.
Bathing has many forms depending on the species. Some birds rely on rain collected in the leaves of trees (bulbuls, white-eyes and sunbirds) shaking their bodies amongst the leaves allowing the water to ’spray’ onto their feathers whilst others indulge in a full bath in shallow pools (babblers, robins, starlings and weavers).
Birds are frequently seen crouching over an area infested with ants with their breast feathers puffed out allowing the ants to crawl all over them. In some instances ants are picked up in the bill and dragged over the body and feathers. This ‘anting’ is thought to have a twofold purpose: it rids the bird of parasites and clears the feathers of unwanted sticky substances.
Birds produce sound from an organ called the syrinx which is similar to the vocal chords of humans. The air inhaled goes down the wind pipe through the syrinx where it travels through bronchial tubes into each of the two lungs. The air divides still further into the air sacs. As the bird pushes air from the air sacs and the lungs it passes over vibrating membranes in the syrinx. Variation in the song is achieved through variations in the openings in the syrinx.
Birds do not have any external pinna to assist with sound collection. They simply have an opening. Although they can hear sound at the same distance as humans, their ability to interpret sound is much more advanced. That which sounds like a single note to humans may sound like a melody of many notes to a bird.
It is often the fine detail in the quality of song that we as humans are unable to distinguish that determines which male will be chosen by a female of that species.
Owls have excellent hearing and often rely entirely on their hearing and not their sight to capture prey. The ears are placed asymmetrically on the skull and this allows for better triangulation of sound to determine the exact location of the prey. In experiments where some owls had their ears covered and others had the eyes covered, the owls that were using their hearing had much better hunting success!
The facial discs of many owls such acts as a parabolic reflector where the entire face becomes like one large external ear. Barn owls have a moveable flap of skin at the ear-hole that can move and assist in determining the direction of a sound.
Shapes of Birds
Birds come in all shapes and sizes. The shape and size is determined by the birds niche and each bird is specifically designed for a specific feeding type. While all species are different there are broad categories. These include:
Waders have relatively long legs and necks with long probing bills with small unspecialized 3 toed feet.
Raptors have short necks, powerful legs that are equipped with very powerful feet and claws known as talons. The bill is hooked to aid with ripping off flesh.
Seed-eating passerines are medium to small birds and have short conical bills that have good crushing abilities. They have the typical passerine foot structure with 3 toes forward and 1 back. The foot is specifically adapted for perching.
Non-seed eating passerines are usually medium to small birds with longer bills that are adapted for catching arthropods. The foot structure is the same as the seed-eating passerines.
Near-passerines are usually medium size birds similar in structure to passerines but with a different foot structure. Many are have a zygodactyl foot structure where the 3rd toe is reversible and it can have 3 forward facing and 1 back or 2 forward and 2 back facing toes.
Territorial behaviour is seen in many bird species. The more pronounced territorial behaviour can be seen in birds of prey. The area defended by a breeding pair will depend mainly on food availability. The more food that is available in an area the smaller the territory that needs to be defended. Birds of prey use a combination of aerial displays and calling to display ownership of a territory. Bateleur and Fish Eagle are particularly vocal in advertising territory. Smaller passerines and non-passerines declare their areas with their calls as they patrol their territorial borders such as the black-collared barbet. The mere advertising of a territory is enough in most birds to prevent the need for fighting. However, in some species such as the babblers, there are frequent loud abusive exchanges on territorial boundaries that often end in a fight between dominant birds from each flock. Other species practice distinctive displays such as flight (red-crested korhaan). Birds, as a rule, do not use scent marking for territory (unlike mammals).
Migration in birds refers to the annual movement of birds along a predictable path. The birds undergo this journey to maximize feeding and breeding opportunities.
Our migratory birds in southern African belong to 3 groups.
Altitudinal migrants- These are resident in southern Africa but move higher into the mountains in summer and back to the lower mountains in winter.
Intra-African Migrants- These birds migrate to tropical Africa for our winter and visit southern Africa for our summer. They can be breeding or non-breeding migrants. These include our migratory kingfishers and bee eaters (excluding the European bee eaters) and many more.
Palaearctic Migrants- These come from the area referred to as the western Palaearctic and includes Europe and much of Asia. These birds are long distance migrants. They tend to have longer wings in relation to their bodies to aid with flying such huge distances. These include black kite, Steppe buzzard, barn swallows and many others.
Migration routes are predictable and birds follow routes that have fewer long distance water crossings because there is no thermal updraft. They also include important stop-over points where the birds can feed and replenish some of their diminishing reserves.
How do birds find their way?
It has been proposed that migratory birds have an element called magnetite in the brain which aids as a compass, giving the bird an indication of north. This has yet to be conclusively proven.
Birds use the sun and the stars to navigate the same way we do. The use of land-marks is important on learned migration.
Prominent species you need to Identify
Protective and Alert Behavior: Hissing is used in aggression. Age Estimation: Juvenile colouration lasts 1 year. Sexual Dimorphism: Male with a dark head and white ear patch. Female ear coverts are not white. Diet: Bees eggs, larvae and wax and hawks other insects such as flying termites in flight Honeyguides are the only vertebrate in Africa that can digest beeswax. They do so through as symbiotic relationship with bacteria in the digestive system. Habitat: Riparian woodland, broad-leafed woodland, savannah and others. Noct/Diurnal: Diurnal. Gegariousness: Solitary except when breeding. Migratory Status: Resident
Protective and Alert Behavior: None. Age Estimation: None. Sexual Dimorphism: Male is red on the head, nape and malar stripe. Female has a dark speckled forehead and a brown throat, malar stripe and cheek patch. Diet: Forages mainly on the ground for termites and ants. Uses its sticky tongue to capture the insects. Habitat: Noct/Diurnal: Diurnal. Gegariousness: Solitary or in pairs. Migratory Status: Resident
Protective and Alert Behavior: Has a Krrrrrr call given when a predator is in the area. Will fly over and circle the area where the predator is. Age Estimation: Juvenile’s have smaller bills. Sexual Dimorphism: Male has a heavier bill than the female. Diet: Invertebrates and small vertebrates including snakes, rodents and small birds. Also feeds on seeds and fruit. Habitat: Dry open savannah and woodland. Noct/Diurnal: Diurnal Gegariousness: In pairs or small family groups. Migratory Status: Resident
Protective and Alert Behavior: None. Age Estimation: Juvenile’s have smaller bills. Sexual Dimorphism: Male has a slightly longer bill with a black patch at the base of the lower mandible Diet: Mostly invertebrates. Also takes some fruit. Habitat: Wooded savannah and broad-leafed woodland. Noct/Diurnal: Diurnal Gegariousness: Pairs or small family groups. Migratory Status: Resident
Protective and Alert Behavior: None. Age Estimation: Eye turns from dark to yellow at 2 years and at this age the first red appears in the throat and facial skin. The red is only complete at between 4-6 years. Sexual Dimorphism: Male has larger bill with a more pronounced casque. Female has a violet patch in the bare red throat skin. Rarely males may also have a small blue throat spot. Diet: Invertebrates and vertebrates up to the size of young scrub hares and mongooses. Group members will attack larger prey in a gang. Habitat: Prefers open grassland, savannah and open woodland. Noct/Diurnal: Diurnal with rare feeding on moonlit nights. Gegariousness: Dominant male and female with helpers and young. Usually 5-6 birds in the group. Migratory Status: Resident
Protective and Alert Behavior: A loud “churr” is used as an alarm. Some sources state that the foul smell of the hole-nest acts as a defense against predation. Age Estimation: None. Sexual Dimorphism: Female is slightly duller than the male. Diet: Small invertebrates caught while probing. Habitat: Woodland. Noct/Diurnal:Diurnal. Gegariousness: Found solitary or in pairs. Migratory Status: Resident
Protective and Alert Behavior: None Age Estimation: None. Sexual Dimorphism: Tails streamers slightly longer in males. Diet: Invertebrates and small vertebrates. Habitat: Savannah and dry woodland favored. Noct/Diurnal: Diurnal Gegariousness: Solitary or in pairs. Migratory Status: Resident
Protective and Alert Behavior: None. Age Estimation: None. Sexual Dimorphism: Male has 2 bands across the chest. The upper one is broad and almost broken in the middle. The lower one is thin and consistent width. Female has a single broad incomplete band across the chest. Diet: Mostly small fish but also includes some invertebrates including insects and crustaceans. Habitat: Open, slow flowing or still water. Noct/Diurnal: Diurnal. Gegariousness: In pairs or closely knit family parties. Migratory Status: Resident