Birds-of-paradise are a family of small to medium-sized forest birds found in the New Guinea region. These birds are among the most beautiful creatures on Earth. Male birds-of-paradise are notorious for their extravagant feather ornaments, complex calls, and shape-shifting dance moves — all evolved to win a mate. According to a new study, preferences of female birds-of-paradise drive the evolution of physical and behavioral trait combinations that may also be tied to where the male does his courting — on the ground or up in the trees.
The lesser superb bird-of-paradise (Lophorina minor). Image credit: William Matthew Hart.
“Female birds-of-paradise evaluate not only how attractive the male is, but also how well he sings and dances,” said study lead author Dr. Russell Ligon, a researcher at the Cornell Lab of Ornithology.
“Their preferences for certain combinations of traits result in what the researchers call a ‘courtship phenotype’ — bundled traits determined by both genetics and environment.”
Dr. Ligon and colleagues examined 961 video clips and 176 audio clips in the Cornell Lab’s Macaulay Library archive as well as 393 museum specimens from the American Museum of Natural History.
They found that certain behaviors and traits are correlated:
(i) as the number of colors on a male increase so do the number of different sounds he makes;
(ii) most elaborate dancers also have a large repertoire of sounds;
(iii) males that display in a group — called a lek — have more colors to stand out better visually amid the competition.
Signal diversity indices vary widely across birds-of-paradise: to facilitate interpretation of variation in signal diversity across the family (Paradisaeidae), Ligon et al generated scaled diversity scores for each signal type (color, behavior, sound) to create (left) a composite metric of overall signal diversity (heat-mapped to the phylogeny) and (right) stacked bar plots illustrating variation in the relative diversity for each signal type for each species. Image credit: Ligon et al, doi: 10.1371/journal.pbio.2006962.
Because females judge male quality based on a combination of characteristics, the study suggests that males may be able to evolve new features while still maintaining their overall attractiveness to females – there’s room to ‘experiment’ in this unique ecological niche where there are few predators to quash exuberant courtship displays.
The researchers found that where a bird-of-paradise puts on his courtship display also makes a difference.
“Species that display on the ground have more dance moves than those displaying in the treetops or the forest understory,” said study co-author Dr. Edwin Scholes, leader of the Cornell Lab’s Bird-of-Paradise Project.
“On the dark forest floor, males may need to up their game to get female attention.”
“Above the canopy, where there is less interference from trees and shrubs, we found that males sang more complex notes, where they are more likely to be heard,” he said.
“But their dances were less elaborate — perhaps a nod to the risks of cutting footloose on a wobbly branch.”
The study was published in the journal PLoS Biology.
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R.A. Ligon et al. 2018. Evolution of correlated complexity in the radically different courtship signals of birds-of-paradise. PLoS Biol 16 (11): e2006962; doi: 10.1371/journal.pbio.2006962
