Wednesday, December 26, 2018

Berries, Red; Birds, Red!

A robin’s red breast is an “honest signal” of mate attractiveness. In males, an indication of good health. In females, that she will product healthy eggs. While their black and brown feathers are colored by melanin, the same pigment that colors our skin and hair, the red is recycled from red berries. Late December is a great time to woods-walk in search of berries, obvious against winter’s white and dark tints. The species we see locally are winterberry, Japanese barberry, multiflora rose, burning bush and Oriental bittersweet.

Winterberry berries stay until robins or cedar waxwings
show up. Click on photos to enlarge.
Winterberry is a native, deciduous (leaf-shedding) relative of holly. Berries are round, clustering at branch tips. Cultivars are available from nurseries. Like holly, there are male and female plants, so at least one male is needed in a planting if the female plants are to have berries. Cardinals and other non-migrating birds don’t eat winterberries. In past times, when robins migrated south, these shrubs would retain the berries throughout the winter – hence the name. Now that flocks of robins stay the winter the berries rarely make it through January.

Japanese barberry is an invasive, naturalized from ornamental plantings. Sales now banned in Massachusetts. In wooded areas, barberry is a low, thorny shrub festooned with oval, red berries. It displaces native shrubs. Deer avoid eating it because of the thorns. Burning bush, also an escaped ornamental, is also banned in MA. It forms dense thickets three to nine feet tall, with small, rust-hued berries. Multiflora rose has modest white flowers in spring, branch-ending clusters of small, dark-red berries in fall. Together, these three can dominate a forest understory.

Winterberries covered with ice
Last/worst is Oriental bittersweet, a woody-stemmed, vining invasive that can blanket treetops with a haze of red/orange berries. It is a tree-killer. If there is any on your property, cut the stems as close to the ground as possible. While many bird species will consume multiflora rose and burning bush berries, winterberry and bittersweet are left for the robins (and the occasional flock of cedar waxwings). Examples of all five can be seen along the Assabet River Walk trail, which has signed entrances at Colbert Avenue and Concord Street. As a bonus, there is a decorated spruce tree 1/3 mile in from the Colbert Avenue end.

Back to birds. The addition of these invasive plants provides enough forage that some robins choose to not migrate south. Males especially, will remain here, to be closer to territory they intend to claim in the spring. These are not the plump and placid birds of summer. Rather, they move about in jittery flocks of 15 to 30 birds descending on winterberry like raucous starlings. Cardinals do not migrate and do not winter-flock, but they do eat other types of red berries and incorporate the color compounds into feathers (males more than females).

Robins do not always migrate, instead staying north as long as
there is food. This photo taken in 2011, winterberry bush.
From the plants’ perspective, birds eating berries is a means to making more plants. This explains why fruits and berries undergo veraison (turning color) when they do. Once the seeds are ready, the pulp becomes sweeter and the skin colorful, as a ‘please eat me’ signal. It is the reds, yellows and oranges of these skins – courtesy of compounds called carotenoids – that robins, cardinals, orioles, basically any bird of reddish feather, incorporate into feather color. Goldfinches get their yellow the same way. Insectivores such as red-headed woodpeckers get their share from plant compounds those insects ate.

Color matters only if an animal (or its predator) can see color. Insects, fish, amphibians, reptiles and birds see color, perhaps better than we do, including ultraviolet hues we cannot see at all. Birds are caught in a dilemma – be a brightly colored male to secure a mate, but then be at more risk of being seen by a predating bird or snake. Females, meanwhile, evolved more toward camouflage colorations that allow them to stay on the nest without being detected. Some species, such as mallard ducks, molt twice a year, the males brightening up for mating season, then going drab for the rest of the year.

Male Mallard duck in courting/mating colors.  
Most mammals have poor color vision or none at all (marine mammals), a theory being that during millions of years of early evolution mammals were mostly nocturnal, eyesight giving up color-detecting cone cells in favor of dim-light functioning rod cells. The old-world primates from whom hominids descended in effect re-invented color vision about 35 million years ago, the better to see ripened fruits and berries. Color blindness is far more common in men than women. One theory was that in the hunter-gatherer era, men needed to see movement while women were looking for ripe fruit, but the reality is that the genes coding for color vision are on the X chromosome, so that women who inherit one mutated X chromosome and one normal will not be afflicted, whereas men, who have only one X chromosome, are.  

Oh, and yes, a “dishonest signal” is lying, to either dissuade predators or fool potential mates. The tasty butterfly that has evolved a wing pattern and coloration similar to a different species that is distasteful is an example of Batesian mimicry. People who color their grey hair are just hoping for the best.

The genesis of this column was a 2009 observation that robins were no longer migrating south for the winter, to then become harbingers of spring

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