There is a considerable amount of sexual dimorphism in great-tailed grackles (Quiscalus mexicanus), a type of blackbird. Sexual dimorphism is the difference in form between individuals of different sex in the same species.
Male great-tailed grackles can grow up to about 43 cm, including a tail that is almost as long as the body, and are jet-black in colour with a violet-blue iridescent sheen to the feathers. Females, on the other hand, are significantly smaller at about 33 cm, and are mainly brownish-black, with a pale brown throat and belly.
Sexual dimorphism is mainly attributed to sexual selection. Longer tail length has always been associated with territory acquisition and social mate attraction, in combination with size.
There can also be another explanation to sexual selection. Larger males have the advantage in acquiring territories, and females preferred to settle on the territories of larger males, probably because larger males were able to acquire trees with the most desirable nest sites.
Recently, it came into question whether animals also use glossiness, where their hair or feathers reflect light like a mirror, to signal to the opposite sex.
In a recent study conducted by PhD researcher Mr Matthew Toomey and colleagues from Arizona State University, Tempe, US, birds were captured, with photographs taken of them and their tails measured before being released. The glossiness of the bird’s feathers in each photograph was calculated using a reflectance spectrophotometer and computer software.
According to Mr Toomey, it was found that male great-tailed grackles were significantly glossier than females. Another discovery was that males with the glossiest feathers also had the longest tails. One possibility arising from this study is that glossiness may play a role in visual signaling and glossier males may be more attractive and better competitors for mates.
Glossiness can be a potential step towards the evolution of iridescence where selection for glossiness can result in a refinement of the microstructure of a feather.
Long tails are no longer enough to attract mates? Further research will be required to prove this theory.
“Female birds find males with glossy feathers more sexy,” by Jody Bourton. BBC, 23 March 2010. URL: http://news.bbc.co.uk/earth/hi/earth_news/newsid_8577000/8577316.stm (accessed on 3 April 2010).
Kristine Johnson, Emily DuVal, Megan Kielt & Colin Hughes, 2000. Male mating strategies and the mating system of great-tailed grackles. Behavioural Ecology, 11(2): 132-141
William A. Searcy and Ken Yasukawa, 1981. Sexual Size Dimorphism and Survival of Male and Female Blackbirds (Icteridae). The Auk, 98(3): 457-465
William A. Searcy, 1979. Sexual Selection and Body Size in Male Red-Winged Blackbirds. Evolution, 33(2): 649-661
“2 of 2 Great-tailed Grackle (Quiscalus mexicanus), Male,” by Michael “Mike” L. Baird, mike at mikebaird.com. URL: http://www.flickr.com/photos/mikebaird/4495734086/in/photostream/ (accessed on 5 April 2010)
“Great-tailed Grackle (Female) (Quiscalus mexicanus),” by Mike Baird, BairdPhotos.com. URL: http://www.flickr.com/photos/mikebaird/339193969/ (accessed on 5 April 2010)
“Quiscalus mexicanus,” by Pablo Lèautaud. URL: http://www.flickr.com/photos/pleautaud/2662384817/ (accessed on 5 April 2010)
“Quiscalus mexicanus,” by Pablo Lèautaud. URL: http://www.flickr.com/photos/pleautaud/3217071624/ (accessed on 5 April 2010)