This post is one of several on birds at the Arboretum. We’re highlighting birds in our landscape as part of the 2018 “Year of the Bird”, celebrating the 100th Anniversary of the Migratory Bird Treaty Act.Although the showy spring flowers are long gone, brightly colored fruits, and the colorful birds that consume them, have taken their place on the grounds. Crimson cherries, purple plums, red serviceberries, and blueberries are just a few of the summer fruits that attract and contrast with the red of northern cardinals and American robins, orange Baltimore orioles, purple grackles, and yellow goldfinches. These colorful floral and faunal displays are actually intertwined, connected in ways surpassing their shared vibrancy.
For example, many bird species actually depend on the chemicals in colorful fruits and berries to enhance their flashy plumage, while many plants depend on birds to spread their colorfully packaged seeds. A bird’s feather color is primarily derived from two types of chemicals, melanins and carotenoids. All birds synthesize melanins themselves, resulting in feathered hues of blacks, browns, grays, and purples. Since melanin also stiffens feathers (giving them greater durability) these colors are ubiquitous on the wingtips and flight feathers of many species, from tiny warblers to eagles. Humans produce melanin as well, evident in the diversity of our skin and hair colors.
Carotenoids, on the other hand, can only be derived from plants or the organisms that eat them and are responsible for shades of red, orange, and yellow. For example, male northern cardinals (Cardinalis cardinalis) metabolize the carotenoids in fruits and berries and turn them into the brilliant shades characteristic of the species. Those that eat the most carotenoid rich foods develop the brightest red plumage, which helps signal their fitness levels to potential mates as well as to rival males. As a result, northern cardinals, and many other colorful bird species, strongly prefer carotenoid rich fruits and berries over other available foods and in return spread the undigested seeds of these plant species far and wide.
It is important to note, though, that some colors defy easy explanation. Blue, for example, long puzzled ornithologists since the color is not linked to any internal or externally synthesized chemical. That is because the blue color of a feather is result of refraction and reflection by a unique structuring of keratin, the proteins of which feathers are composed. In other words, the brilliant “blues” of bluejays, eastern bluebirds, and tree swallows are owed to a trick of the light derived from the physical properties of their feathers.
Although these causes of feather color may seem straightforward enough, nothing is simple, especially in this increasingly interconnected world. For example, research on two invasive plants, Lonicera tatarica (Amur honeysuckle) and Lonicerca marrowii (Marrow’s honeysuckle), show that some native birds who eat their berries develop atypical plumage. For example, when native cedar waxwings eat the red fruits of L. marrowii during feather development, they form an orange tail band (instead of brilliant gold) that may complicate their future success at attracting mates. Similarly, research suggests that male northern cardinals that eat the berries from L. tatarica develop bright red plumage regardless of their actual physical fitness. This may make it difficult for females to determine which potential mate is actually a healthy mate and which is a less fit, but very shiny red, imposter.
Fortunately, neither species appear threatened by these unintended morphological changes; the affected birds are in the minority and the species do quite well in urban areas. However, they do provide a illustrative example of the direct connection between fruit and plumage color and between birds and plants in general. On another level, the connection between berries and feathers illustrates how the decisions we make regarding plants in our urban and suburban landscapes can influence local ecosystems, sometimes in ways we don’t readily observe or understand.
If you are now wondering how to choose plants that are beneficial to birds, consider buying native plants, which native birds have evolved with, whenever possible and creating year round habitat in your yard by planting a diversity of trees, shrubs, and herbaceous plants. If you are wondering how you can learn more about birds, please check our events calendar for upcoming classes and bird walks. And if nothing else, come to the Arboretum this summer to enjoy the plants in the landscape and the colorful fruits, and birds, that adorn them.