The dazzling array of colours that birds display – some that humans can’t even see – are more than awe-inspiring eye candy; their evolution over time is a burgeoning topic in avian biology.
The functions of feather colour include thermoregulation, climatic adaptation and a fine balance between sexual signalling and camouflage – but there is still much to learn.
Rainbow lorikeets and their kin have particularly stunning hues of vivid oranges, yellows, reds, greens and blues, but how do they manage to stay hidden from predatory lizards and hawks?
That’s a conundrum pursued by Jon Merwin and colleagues from the American Museum of Natural History, who identified a suite of evolutionary processes acting on feather colour in Australasian lorikeets as reported in the journal BMC Evolutionary Biology.
In gaudy peacocks and dazzling birds of paradise, bright colours and extreme feather ornamentation come about by sexual selection; while the males display, dull coloured and cryptic females can hide in the undergrowth and raise the young, safe from predators.
But in the lorikeets of Australasia, sexes are monomorphic, and both sexes equally colourful. Moreover, the pattern is broadly the same, with various riotous colour patches on front and face, and the wings and backs green.
Could mosaic evolution, where different traits evolve independently of one another, be at play here?
The brush-tongued parrots, known as the Loris or Lorikeets (Loriini) are a tribe with more than 100 species, offering researchers an opportunity to conduct a detailed comparative analysis of the drivers of plumage colour evolution.
Merwin and senior author Brian Smith took into account that, like many birds, lorikeets have the ability to see in ultraviolet light. Accordingly, the pair collected colour data from museum specimens using visible light and UV light photos of 98 museum specimens from across the lorikeet family.
“The range of colours exhibited by lorikeets adds up to a third of the colours birds can theoretically observe,” Merwin says. “We were able to capture variation in this study that isn’t even visible to the human eye.”
The colour patches on the lorikeets ranged from vibrant ultraviolet blue to deep crimson red and black, varying in size and placement across taxa, and yet were easily defined and compared across species.
Various models used in phylogenetic analysis applied to the patches allowed for detailed comparison of the various traits across evolutionary timescales.
Smith and Merwin discovered that the patches on the wings and back remained largely the same, or evolutionarily conserved across time. These patches are mostly green, a perfect colour for camouflage in the lorikeets’ leafy forest habitat.
Meanwhile, areas of plumage on the birds’ face, head and front varied wildly in colour across phylogenetic timescales. These patches are used in communication and species recognition, and are observed as a burst of rainbow colour across the Lorikeet clade.
The researchers conclude that at a phylogenetic scale, lorikeet plumage has evolved in correlated regions, with subsets of traits subject to different evolutionary pressures, a pattern suggesting that mosaic evolution is at work.
Merwin says the study also demonstrates the usefulness of a good museum collection. “The idea that you can take colour data from museum specimens, infer patterns, and gain a larger understanding of how these birds evolved is really amazing.”