Splendid fairy wrens (Malurus splendens) can be found in south-west, central and inland eastern Australia. Males – such as this one at Lake Cargelligo, New South Wales – are easily distinguished from females by their distinctive colourings: mostly blue, with a black band across their back and head. This blue turns iridescent when the male is looking for a mate. The intensity of this iridescence is dependent on a good diet and serves as an indicator of health and fitness. Females do not require this iridescence in their own plumage and so are much less brightly coloured, displaying fawn-coloured bodies and blue tails.

Though rosellas are brightly coloured, they are surprisingly difficult to spot in forests. Crimson rosellas (Platycercus elegans) produce interesting examples of hybridisation which have piqued the interest of researchers at Deakin University. The project is investigating the occurrences of hybridisation in some geographic locations and the reasons behind the wide spectrum of colour variations that exist (from pale yellow through to the deep crimson for which the birds are famous). Previously it has been observed that yellow and crimson rosellas will produce orange offspring. Orange offspring are most commonly found in South Australia, where the crimson distribution from eastern Australia meets the yellow distribution from South Australia.

The macaws of South and Central America – such as this red-and-green macaw (Ara chloropterus) – are among the brightest and most colourful birds in the world. The plumage of this species is mostly scarlet red, with blue feathers around the tail and flight feathers on the wings. They also have green bands on the upper wings and yellow-red colouration underneath them. Though these colours seem incredibly bright, they blend in surprisingly well with the leaves, flowers, fruits and shadows of the rainforest.

Lesser flamingos (Phoenicopterus minor) – pictured here at Lake Nakuru in Kenya – can be found in East Africa and parts of South Africa, Madagascar, and India, where foods that give their plumage brilliant pink hues are plentiful. Flamingos feed on algae and small shrimps which contain carotenoid pigments. The flamingo can metabolise these pigments in such a way that it gives bright colour to their feathers. These pigments are not permanent, however, and the colour fades over time if the birds don’t eat the right nutrients. Because of this, plumage colour can indicate the health of the bird.

Rather than using pigments, peacocks (Pavo cristatus) use structural elements of their feathers to reflect light in different ways. The distinctive tail feathers are made up of tiny melanin cylinders that create structures which vary in complexity. The structures shift the light in different ways, depending on how they are latticed together, giving variations in colours and patterns that we can see with the naked eye. The feathers appear to shimmer when they move because of the changing angle. Research has shown that brighter colours are consistent with healthy immune systems, therefore these colour displays play an integral part in helping the females choose a suitable mate. Similar structural colours are also found in some beetles and butterflies.

Male regent bowerbirds (Sericulus chrysocephalus) are glossy black with bright orange-yellow feathers on their heads shoulders and wings, while females are light brown with mottled markings and grey bills. Males take four years to adopt their bright adult plumage and look like females until then. During courtship, males fan out their tails and spread their wings to reveal the bright colour underneath. Flapping wings, while also chattering and wheezing, can enhance the effect.

Female zebra finches (Aeniopygia guttata) are attracted to males with the brightest beaks because they are the healthiest birds. Males of this central Australian species that have the brightest red and orange beaks get the highest levels of carotenoid nutrients from the foods they eat. Carotenoids are pigments found in plants and seeds that help to boost the immune systems of these birds. Birds with dull beaks are less likely to have access to the best food resources and are more likely to be looked over by females.

There are two main ways birds create colour in their feathers. The first is to use pigments, and the second is to use the colours created when light refracts from the physical structure of the feather. Sometimes – such as in these macaw feathers with yellow pigment and blue iridescence – they employ both methods at once.

Gouldian finches (Erythrura gouldiae) have become popular with bird enthusiasts worldwide for their bright and varied plumage. Found in very few numbers in the north of Western Australia and in parts of the Northern Territory, this bird has one of three different head colours: black, red or gold – gold being the rarest variation. They have purple chests, and the colour of their backs can be yellow, green or blue. Females are typically duller than males, but the differences are slight. Juveniles are predominantly olive green. Their bills vary in colour during the breeding season, changing from white with red or yellow to pearl (males) and dark grey (females). These colour variations are largely genetic, similar to eye- and hair-colour in humans.

White-winged fairy wrens (Malurus leucopterus) in mainland Australia are mostly a blend of dull brown and cream or off-white, with a blue-grey tail. That is until the breeding season, whereupon the plumage of the dominant male of the social group transforms into a gleaming blue, with white and dark blue-grey patches, and a blue-black tail. Other males are brown and when the dominant male dies, another male in the group becomes blue. Juveniles appear similar to adult females, however immature males have the black bill of their adult counterparts.

Rainbow bee-eaters (Merops ornatus) can be found during the summer in forested areas in most of southern Australia and Tasmania. They migrate north during the winter into northern Australia, New Guinea, and some of the southern islands of Indonesia. Males and females are similar colours although females are slightly duller in appearance.

Eclectus parrots (Eclectus roratus), from the rainforests of north Queensland, are unusual among parrots for having such marked sexual dimorphism. Males are bright green with red under their wings. The top half of the beak is a brilliant orange-yellow, while the lower half is black. Females on the other hand are bright red and blue with black beaks. Work from the Australian National University has shown that the differences are at least partly due to the fact that the sexes spend time in different habitats – males are often in the forest canopy where they need to be well camouflaged from eagles, while females spend more time in or around their nest holes, which are lower down in the trees. The brilliant yellow of the male’s beak is an indicator of how healthy an animal is. Bright beaks are attractive to females.

The incredible plumage of the peacock is an example of sexual selection in evolution, where traits that the opposite sex finds attractive are amplified. This can occur even when the traits are disadvantageous in other ways – such as the peacock’s tail, which can make the animals more obvious to predators.

Male satin bowerbirds (Ptilonorhynchus violaceus) have both blue iridescent plumage and patches of UV colour on their tails which are invisible to us, but attractive to female bowerbirds. During courtship displays, males of this eastern Australian species will attempt to position themselves at the best angle for the female to view their plumage to full effect.

The yellow beak of male mallard ducks (Anas platyrhynchos) indicate to potential mates the duck’s immune status, and the quality of his sperm. Males’ beaks range in colour from dull green to bright yellow – males with brighter yellow beaks have sperm with more effective antibacterial properties, which increases reproductive success and the likelihood of having healthy offspring. Mallards are common in temperate and subtropical regions of the Northern Hemisphere, and have been introduced to Australia and New Zealand.

Males and females of the blue tit – found in Europe and North Africa – were thought to bear the same colouration, but recent studies have shown that this only how they appear to the human eye. Most birds have a larger variety of cells for detecting colours in their eyes than humans, which means they can clearly see light in the ultraviolet (UV) range that is invisible to us. We now know that many birds also have feathers that reflect UV light. Male blue tits (Parus caeruleus) have crests and patches of plumage that strongly reflect in the UV range. The brightness of the UV colour depends on testosterone levels. Older and more dominant birds give off brighter colours, and are more attractive to females.

The azure kingfisher (Alcedo azurea), an Australian rainforest dweller, dazzles in iridescent purple and orange.