Sex and the Birds: 8. Splendid Fairy-wren, monogamous and promiscuous

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The Splendid Fairy-wren (Malurus splendens) is a smallish, long-tailed and sexually dimorphic Australian species. The male in full breeding plumage is a brilliant iridescent blue with black colouration (above). The non-breeding male has a predominantly grey-brown plumage, looking like a female (below) but with a cobalt-blue tail, unlike the dark bluish-turquoise of the female’s.

This bird lives and breeds in small groups in the Australian outback. The breeding pair has a small band of helpers consisting of progeny from previous years. These helpers help defend the territory, feed the chicks and care for the fledglings.

The breeding pair remains together throughout the year. They are thus socially monogamous. However, they are also sexually promiscuous. Both the male and the female sneak out regularly to mate with birds in neighbouring groups. Using DNA technology, it has been found that up to three-quarters of chicks in a brood is fathered by male or males from outside groups (Rowley & Russell, 2007).

This is an excellent example of partnership without the constraint of sexual fidelity (Crump, 2009).

According to Crump (2009), most of the existing breeding territories are already taken up by breeding pairs. Thus new adult males are not able to set up their own territories. Adult female on the other hand are unable to find eligible bachelors with territories. So these adults remain within their family groups as helpers. Helpers also sneak out to mate with neighbouring males and females.

Should one of the breeding pair dies or leaves the group for one reason or another, replacement will mostly come from among the helpers. This leads to brother-sister, mother-son or father-daughter pairs and thus inbreeding. But then as most fertilisation comes from copulation outside the pair, inbreeding as a result of partnering with close relatives is kept to a minimum.

Credit: YC Wee (text), Eric Tan (images).

References:
1.
Crump, Marty, 2009. Sexy orchids make lousy lovers & other unusual relationships. University of Chicago Press, Chicago & London. 214pp.
2. Rowley, I. & E. Russell, 2007. Family Maulridae (Fairy-wrens). In: del Hoyo, J., A. Elliott & D. A. Christie (eds.). Handbook of the birds of the world. Vol. 12. Picathartes to Tits and Chickadees. Lynx Editions, Barcelona. Pp. 490-531.

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