Flights of fancy: the evolution of plumage patterns in male and female birds

amb206 — Wed, 18 Dec 2013 07:39:21 +0000

Ducks, geese and swans are waterfowl, an order known to scientists as Anseriformes. Hens, pheasants, partridges and turkeys are game-birds (Galliformes). Both orders are famous not just for their flesh but also for their striking and elaborate plumages which are sought after as decorative flourishes. Some members of these orders show marked differences in appearance between the sexes: a phenomenon known as sexual dimorphism. Male and female mallards look so different that for many years they were thought to be different species. In other members of the same orders, there is little apparent difference in the plumage of males and females.

Research by Cambridge PhD candidate Thanh-Lan Gluckman, published today in the Biological Journal of the Linnean Society, looks afresh at similarities and differences in plumage in almost 300 members of the Anseriformes and Galiformes orders – and focuses on patterning between male and female birds rather than colour. She said: “ ̽��ֱ��colour of plumage has attracted much research interest, but the exquisite patterns of bird plumage, such as the spots of the guinea fowl and the barred patterns of ducks and turkeys, to just name a few, have received much less attention.”

Since the 1980s, differences in the appearances of male and female birds have been seen through a prism of genetic correlation. In other words, it was thought that female birds may have evolved similar patterning to males due to common genes but that female patterns would be subsequently lost as it is not beneficial.

“It was argued that male birds developed their spectacular colours and elaborate patterning as a result of their mating patterns – they used their plumage to compete for and attract females. On the other hand, female birds needed to blend into their surroundings in order to nest safely and protect their young – so they became drab and dull to protect themselves and their young from predators,” said Gluckman.

“My research looked at the plumage patterns of male and female birds on a separate and equal basis – and then went on to identify similarities and differences between them. By tracing the evolutionary pathways in the dimorphism of 288 species of waterfowl and gamebirds, I reconstructed the evolutionary history of plumage pattern sexual dimorphism, which allowed me to demonstrate that plumage patterns in females are not a result of genetic correlation. Essentially, what I found was that plumage patterning is remarkably labile – both male and female birds have the capacity to change between different types of patterns. What’s interesting is to consider what are the forces driving these changes in male and female plumage patterns – whether they have an environmental basis and/or whether they have a signalling function between birds of different sexes or within the same sex.”

As early as 1780, the Philosophical Transactions of the Royal Society of London published a paper by John Hunter proposing that plumage differences between the sexes were driven by sexual selection. Ever since, the prevailing view of sexual dimorphism has been one of showy males strutting their stuff to win over demure females. ̽��ֱ��predominant explanation put forward to explain how differences in dimorphism evolved hinges on mating habits; males of polygamous species (those with more than one mate) had developed spectacular plumage in order to attract a maximum number of females, while monogamous species (those with one mate) retained similar plumage.

Gluckman said: “Previous research has shown that the traditional argument that differences in plumage between the sexes stem from differences in breeding systems doesn’t always hold up. In many putatively monogamous species, the plumage of the males is significantly different to that of females and, likewise, males and females in many polygamous species have the same type of plumage. This suggests that plumage is not exclusively an outcome of breeding habits – but is a matter of function in a highly complex way.”

In her study of patterning, Gluckman looked at the variations between the sexes of the same species and across species in order to build a picture of the pathways to similarity and differences between male and female bird plumage patterns. She used a classification of four broad types of patterning: mottled, scaled, barred and spotted. Birds exhibit a fabulous number of variations and combinations of these visual patterns in females as well as males.

“By emphasising similarities as well as differences in plumage patterns between male and female birds, rather than whether one sex is the same as the other, I found that sexual dimorphism in the plumage pattern of birds is highly nuanced and that there can be multiple types of sexual dimorphism. In expanding the definition of sexual dimorphism, and reconstructing evolutionary history, I found that changes in sexual dimorphism could be due to changes in males and/or females. In addition, the plumage patterns of birds seem to transition easily between different types of dimorphism, which is congruent with adaptation to fluctuating social and environmental conditions,” said Gluckman.

Thanh-Lan Gluckman is a PhD candidate in the Evolutionary genetics group at the Department of Zoology at the ̽��ֱ�� of Cambridge. She carried out this research during her MPhil in the Department of Zoology, ̽��ֱ�� of Melbourne, Australia.

Research published today looks at the evolutionary pathways to differences in bird plumage patterns between males and females – and concludes that birds are able to adapt their appearance with remarkable ease.

In expanding the definition of sexual dimorphism, and reconstructing evolutionary history, I found that changes in sexual dimorphism could be due to changes in males and/or females.

Thanh-Lan Gluckman

Images via Wikimedia

Waterfowl and gamebirds

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Cuckoos impersonate hawks by matching their 'outfits'

fpjl2 — Thu, 17 Oct 2013 09:11:54 +0000

New research shows that cuckoos have striped or “barred” feathers that resemble local birds of prey, such as sparrowhawks, that may be used to frighten birds into briefly fleeing their nest in order to lay their parasitic eggs.

By using the latest digital image analysis techniques, and accounting for “bird vision” - by converting images to the spectral sensitivity of birds - researchers have been able to show for the first time that the barred patterns on a cuckoo’s breast may allow it to impersonate dangerous birds of prey. This might enable cuckoos to frighten other avian hosts into leaving their nests exposed.

̽��ֱ��latest findings, published today in the journal Animal Behaviour, expand the cuckoo’s arsenal of evolutionary deceptions, which include egg mimicry and chick mimicry that allow it to trick other birds into incubating its eggs.

Importantly, the study shows that a wide variety of cuckoos have adapted different plumage patterns depending on the area they inhabit so that they match a local bird of prey species.

While scientists have previously looked at links in plumage patterns between the common cuckoo and Eurasian sparrowhawk, the new research shows that this type of impersonation of a more dangerous animal – called ‘Batesian mimicry’ – may be far more widespread in cuckoos. In addition, the dangerous bird of prey that cuckoos resemble goes beyond sparrowhawks to include such raptors as bazas and harrier-hawks - depending on the species prevalent in the cuckoo’s neighbourhood.

“There is no benefit in looking like a dangerous species your target is not familiar with,” said lead researcher Thanh-Lan Gluckman from Cambridge’s Department of Zoology.

“We first established similarity in plumage pattern attributes between cuckoos and raptor species, and then showed that cuckoos look nothing like species from a different geographical area.”

̽��ֱ��cuckoos also use their crafty ‘hawk impression’ to allow them to fly ‘under the radar’, undetected as they scope out potential nests in which to deposit their parasitic eggs.

“ ̽��ֱ��barring on their plumage helps cuckoos conceal themselves while searching for potential nests, then when they approach, the host of the nest may mistake a cuckoo for a raptor coming to get them – giving them unfettered access to lay eggs,” Gluckman said.

While previous studies have focused on Batesian mimicry in the common cuckoo and Eurasian sparrowhawk, this is the first time that the plumage patterns of cuckoos have been analysed using digital image analysis techniques. ̽��ֱ��study suggests that this form of mimicry may be widespread among many cuckoo species, and that they may be mimicking a variety of different types of birds of prey.

̽��ֱ��researchers were surprised to find no pattern matching between cuckoos and raptors that live in different geographical areas, showing that the visual similarity is highly localised to species in the immediate vicinity.

“These findings underscore the importance of using digital image analysis to objectively quantify plumage patterning in mimicry – it is important not to make assumptions about even simple patterns such as these,” added Gluckman.

“We hope this encourages other researchers to examine the function of barred plumage in parasitic cuckoos and raptors the world over.”

Another interesting finding is that of the African cuckoo-hawk, a raptor so named because of its visual resemblance to cuckoos. This study objectively shows that the naming was an apt one, given that a local cuckoo matched the African cuckoo-hawk in all of the pattern attributes measured.

One of the earliest observers of the cuckoos’ invasive guile was Ancient Greek philosopher Aristotle, who noted some 2,300 years ago that it “lays its eggs in the nest of smaller birds”.

Evolutionary trick allows cuckoos to mimic the plumage of birds of prey, and may be used to scare mothers from their nests so that cuckoos can lay their eggs. Mimicry in cuckoos may be more much more widespread than previously thought.

There is no benefit in looking like a dangerous species your target is not familiar with

Thanh-Lan Gluckman

Thanh-Lan Gluckman/Gabriel A. Jamie

Left: Cuckoo compared to cuckoo-hawk plumage. Right: Cuckoo

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̽��ֱ�� of Cambridge - Thanh-Lan Gluckman

Flights of fancy: the evolution of plumage patterns in male and female birds

Cuckoos impersonate hawks by matching their 'outfits'