Fairy-wrens are a family of small, active, insectivorous passerine birds endemic to Australia and New Guinea. They are known for their beautiful plumage and complex social behaviors. One of the most intriguing aspects of fairy-wren biology is the “secret note” sung by males of some species to advertise their territory. In this article, we will explore the mysteries of the fairy-wren’s secret song and what it reveals about these charming little birds.
What are fairy-wrens?
Fairy-wrens belong to the family Maluridae and consist of around 15 species in the genera Malurus and Clytomyias. They are found in a variety of habitats across Australia, from grasslands to forests to mangroves. Fairy-wrens are small, ranging from 9-15 cm in body length, with rounded wings and long, pointed tails. Their most distinctive feature is their colorful plumage, with breeding males donning bright blue, purple, red or black feathers depending on the species. Breeding females and non-breeding males have more subdued brown, grey and white plumage. Fairy-wrens are social, living in groups with a dominant breeding pair and several helpers that assist with raising young. They feed on insects and other small invertebrates.
Blue fairy-wren
The blue fairy-wren (Malurus cyaneus) is perhaps the most common and recognizable fairy-wren species. Native to southeastern Australia, males are known for their striking bright blue plumage set off by a black throat and lores. Females have grey-brown upperparts and paler underparts. This species prefers open scrubby habitat near water. Blue fairy-wrens live in social groups with a dominant breeding pair and up to 5 helpers, often offspring from previous years. Helpers assist the breeding pair by collecting food and participating in predator mobbing.
Superb fairy-wren
Another charismatic species is the superb fairy-wren (Malurus cyaneus). Found across southern Australia and parts of Tasmania, males are deep blue above with a black mask and throat, and violet-blue below. Females are grey-brown with pale blue tails. This species favors wetter habitats like reedbeds and marshes. Superb fairy-wrens have an elaborate courtship ritual in which males pluck flower petals which they then display to females from sheltered perches. Superb fairy-wrens have a polygynandrous mating system where males and females may have multiple mates in a season.
The secret song of the fairy-wrens
In addition to their normal vocalizations and calls, male fairy-wrens of several species sing a special soft, high-pitched song that remains mostly inaudible to humans. This “secret song” is sung from within the nest or territorially from secluded perches. It was first discovered in superb fairy-wrens in 1969 and has since been documented in variegated fairy-wrens, red-winged fairy-wrens, splendid fairy-wrens, and blue fairy wrens.
Acoustic properties
The secret song of fairy-wrens is acoustically very different from typical bird vocalizations. It has an extremely high frequency between 4-9 kHz, near the upper limit of human hearing which peters out at around 20 kHz. For comparison, most songbirds sing between 2-4 kHz. It also has an exceptionally low amplitude of around 16-36 dB at a distance of 10 cm. This soft volume combined with the high frequency makes the secret song very difficult for humans to detect and appears to function to avoid eavesdropping. The song has a highly stereotyped syntax and ranges from a few seconds to over a minute in length.
When do they sing it?
Male fairy-wrens sing their secret song year-round but peak during the breeding season. It is primarily sung within the nest, often while incubating eggs. Males will sit in the domed grass nest and sing softly into the interior. The enclosed space amplifies and concentrates the song. Males also sometimes sing softly from hidden perches within their territory, choosing concealed locations like dense foliage or cavities. In contrast, the main advertisement song of fairy-wrens is belted out loudly and conspicuously.
Who is the audience?
Due to its stealthy nature, the secret song appears to be targeted at a nearby, intended audience rather than broadcast widely. Studies have shown the main listeners are the male’s mate and close social group. Females will often quietly approach singing males. When another male encroaches on his territory, a male will increase his secret song tempo andamplitude. Eavesdropping predators and parasites seem unable to detect the song, potentially preserving the territorial message.
Why sing so secretly?
Why do fairy-wrens sing this covert song? Scientists have proposed several hypotheses:
Teirrory defense
By using a private channel, males can signal their presence and ownership of a territory without advertising to undesirable eavesdroppers like predators and brood parasites. This allows them to discreetly but securely broadcast tenancy. Consistent with this theory, males increase singing when an intruder appears.
Mate guarding
The hidden song also functions in close-range mate guarding. Males sing frequently to their mate while she is fertile, apparently to advertise that he is vigilantly guarding her. The female often quietly approaches in response. Lower amplitude may allow the pair to communicate without distracting predators.
Bonding
Singing inside the nest may help males bond with their mate and form pair associations. The acoustic properties of the nest interior allows the male to deliver an intense signal to the female at very close range. This may reinforce pair bonds between breeding pairs who share in nest construction and reproduction. Females have been observed making soft vocalizations in duet with mates.
Fertility signals
Soft songs may allow males to provide honest signals of their condition and fertility to mates at extremely close range. High performance songs may indicate the health and fitness of the male. Similarly, females may be able to subtly signal fertility to males through their movements and response calling.
How do they sing so softly?
Researchers have investigated the morphological adaptations that allow fairy-wrens to produce such low amplitude vocalizations:
- Smaller syrinx: The vocal organ in birds, fairy-wrens have smaller syrinxes compared to relatives.
- Weak muscles: The syrinx muscles that control song lack strong striations for power.
- Soft beak movements: Bills are opened only microscopically during soft song.
- Resonance: Nest architecture enhances resonance and discretely amplifies output.
These adaptations allow fairy-wrens to strategically dampen their vocal output to avoid unwanted listeners while discreetly communicating with intended receivers.
Significance and conclusions
The secret song of the fairy-wrens provides intriguing insights into their complex social world and communication strategies. By conveying signals in private channels, they are able to discretely deliver territorial proclamations, court mates, bond pairs, and potentially assess physical condition. Their tactical use of near-ultrasonic frequencies and extreme amplitude reduction allows neighbours and mates to be signalled without alerting unwanted eavesdroppers. This system reveals the sophisticated social strategies of even small, common backyard birds. Biologists continue striving to unlock the mysteries of the fairy-wrens’ secret songs and what they can tell us about animal behaviour and adaptation.