Scientists from the University of Arizona offer an explanation of why birds change their plumage color over the course of evolution. The reason, they believe, is the evolution of colored pigments in bird feathers throughout North America. The work was published in Nature Communications.
There are two theories. First, the mechanisms that allow organisms to adapt well to their current environment and the mechanisms that allow them to change adaptations are different – the latter is suppressed as organisms become better and better suited to their current conditions. Therefore, adaptations are only activated when the environment changes.
Second, the mechanisms that force organisms to fit into the current environment themselves change during evolution.
It is difficult to distinguish between these possibilities because in evolutionary biology we necessarily study the processes that took place in the past, the events that we missed. Instead, we conclude that we have overlooked the comparison of species that exist today. While this approach can tell how well current organisms fit into their current environment, it cannot tell how they got here.
Alex. Badyaev, professor of ecology and evolutionary biology at the University of Arizona
Badyaev and his team tried to observe the adaptation to new conditions in action, paying particular attention to the mechanisms involved. To do this, they chose a domestic finch. This bird has spread throughout most of North America over the past century and now occupies the largest ecological range of any existing bird species.
These birds color themselves by eating and integrating pigmented molecules called carotenoids into their feathers.
Carotenoids are large molecules, they fill the feathers of birds, as a result, their appearance and structure changes. In feathers where structural integrity is important, such as temperature-regulating downs or flight feathers, mechanisms developed that inhibit feather growth by incorporating carotenoids. For this reason, flight or down feathers are almost never colorful in any species of bird. At the opposite end of the spectrum, ornamental feathers benefit from being colorful and developing mechanisms that alter their structure to allow for greater carotenoid incorporation and improve their appearance.
The sources of carotenoid pigments in domestic finches vary. In local desert populations, finches get their pigments from pollen and cactus fruits, while in urban populations they get them from newly introduced plant species and bird feeders. In northern populations, they contain pigments from grass seeds, buds, and berries.