The unmatched promiscuity of Superb Fairy-wrens is assisted by the color signals displayed by brightly colored males and the songs of less colorful females.
By Rex Graham
The species is Australia’s black, white and blue dabs of iridescence in the bush.
Fairy-wrens, like other birds, use “extra-pair” sexual liaisons as an important part of their reproductive strategy. Female fairy-wrens sing to stake territorial claims, and scientists are decoding how some of those musical claims are stronger than others.
Superb Fairy-wren signals
On Australia’s Kangaroo Island, where severe 2007 fires scorched large areas of brush, territorial fairy-wren females undoubtedly feel pressure from claim-jumpers. The birds don’t migrate. With less breeding habitat to go around on the island, females sing heartedly to signal fitness. Heard at a distance, the song itself can minimize close-range encounters with unwanted strangers. The females may also use song to woo a mate, advertise their reproductive status or even function as a temporary romantic “come hither” invitation to a neighborhood male.
On the island, the largest females signal their territorial claims with songs that are twice as long as those of smaller females. “Female song rate and complexity predicts reproductive success,” Austrian and Italian researchers said in 2016 in the journal Frontiers in Ecology and Evolution.
Hidden meaning in ‘chit-and-chet’ songs
The study was unique because most other studies of “sexually dichromic” species in which males are more colorful than females focus on the males and ignore the females.
The promiscuity of Superb Fairy-wren females is unique among all songbirds, or members of the Passerine order. Female fairy-wrens pair with a dominant male, but allow less dominant male “helpers” to participate in the feeding and care of her offspring. However, the paternity of fairy-wren offspring is not what scientists expected: 76% of a female’s chicks are sired by males outside the family group.
Most promiscuous songbird
“This is the highest known frequency of extra-pair paternity of any bird,” a team of Australian and British scientists reported in 1997 in the journal Molecular Ecology. Since that landmark study, many others using genetic-fingerprint analyses of chicks and eggs have measured various rates of “extra-pair paternity” in virtually all songbirds.
Promiscuity may arise as a way to mate with males in better health than their own mates. Females may also use extra-pair paternity to add genetic diversity to their broods, or use it as a form of “reproductive insurance” in case her mate is infertile.
Of course, males stray, too. Some may wait until their mates are no longer fertile. A female’s long hours at the nest incubating eggs may also provide an opportunity to father more chicks. It’s possible that the increasing fragmentation of forests, grasslands and other habitats could reduce the rate of extra-pair matings because birds of many species are spread thinner over greater distances.
Strategic neighborhood sex
Males may seek hook-ups outside the matrimonial unit as a way to have more offspring. On the contrary, a female may copulate with an older male neighbor as a way to improve the odds that her young will also live longer. The priority of each male and female in an extra-pair mating seem to vary dynamically and independently.
“Studies about how changes in the social context shape their decision-making can provide us with valuable information for a better understanding of such phenomena at both the individual and population level,” a team of researchers from Switzerland and Spain said in 2015 in the journal Animal Behavior.
‘Extra-pair’ double duplicity
Indeed, since the 1990s, more studies have focused on why “socially monogamous” female songbirds mate with so many males, and vice versa. The entire field of extra-pair mating has exploded with new theories and ideas. Some researchers also argue that the appearance (to humans) of drab females may miss the real reasons they may be attractive to the opposite sex.
Researchers were surprised that the songs of female Superb Fairy-wrens, not their feathers, were linked to vigorous, more-attractive-to-males body condition.
“This is one of the first studies investigating multiple signals in a female songbird, suggesting that plumage features and song performance might underlie different selection processes,” said the authors of the study in Frontiers in Ecology and Evolution. “Our study revealed that song length is related to a trait reflecting quality and supports the idea of song as a sexually selected trait in female passerines.”
— Rex Graham (@TopBirdingTours) September 9, 2016