Fruit Colors and Fruit Flagging

Since birds have good color vision but little sense of smell, you would guess that fruits that are designed to attract birds as a means of dispersal of seeds should be brightly colored with no odor. So how well does nature confirm to this prediction? Think about the wild fruits you are familiar with and how they might be dispersed.

The waxwing shown eating a red serviceberry fruit is more specialized for fruit eating than most birds. But a wide variety of birds eat fruit when other foods such as insects are in short supply. The serviceberry or shadbush is an example of an early fruit that is carbohydrate rich and must be eaten fairly soon or it decomposes. Blackberries that start green, then turn red and then black as they mature are an extreme example of the “sugar fruit” that must be eaten when ripe. Why do you think they change color- is it simply to indicate to birds when the fruit is ready to eat? Green fruits are often poisonous and/or unpalatable to discourage their consumption before the seeds are ripe. Other fruits such as those from the holly will last most of the winter and are rich in lipids.

The brilliant red colors of many fruits such as the northern spicebush and the southern stopper illustrate the value of advertising to attract bird consumers. Yet some fruits eaten by birds actually turn from red when unripe to a dark color when ripe. Look at the fruits of the black cherry and the firebush. Perhaps the method of presentation helps to attract birds to these fruits. The black cherry offers its fruits at the end of twigs and the firebush presents sprays of ripe fruits among its brilliant red-orange flowers.

Yet some fruits eaten by birds can be unusual colors. Fruits of the amazing beautyberry are a startling light purple color and are quite unlike any other fruits in the southern coastal plain. A bush covered with these fruits can be strongly defended by some territorial birds such as mockingbirds.

Some of the most remarkable strategies used by plants to attract birds to their fruits involve advertisement by bright colors, not of the fruit itself but adjacent leaves or stems. Such fruit “flagging” (see studies by Dr. Stiles at Rutgers) is not that uncommon if you look at the possible purpose of red leaves and stems in those plants that have fruits that are not easily seen. For example consider the red fall leaf colors of Virginia creeper that provide a blaze of color while the fruits are a bluish black color. The sassafras has a dark fruit but the stem is red. Similarly the fruit of pokeweed which is avidly consumed by birds is dark, but the stems are bright red. A similar scenario applies to the dark fruit of the black gum tree which are advertised by the bright red colors of its fall leaves.

So as birds fly south and insects hunker down for the cold weather, enjoy the amazing variety of fall fruits and consider how their distinctive differences and similarities in color may be designs to attract birds to eat the fruits and disperse the seeds.

Bill Dunson, Englewood, FL & Galax, VA
wdunson@comcast.net