Birds perform an essential service to plants by carrying seeds away from the parent plant to other locations. Seed dispersal over a wide area is vital, because seedlings that germinate below their parent are usually doomed as a result of competition with each other and the parent for sufficient light and water.
Unlike rodents, such as squirrels and mice, which destroy seeds by chewing them with sharp teeth, birds swallow plant seeds intact. Seed germination is improved by the scarification (scratching of the seed coat) that takes place as the seed passes through the gizzard before being deposited in nitrogenous fertilizer, far from parent and sibling plants.
Because birds are important to plants, the plants have developed fruits that are attractive and conspicuous to birds. For example, the fruits of bird-distributed plants typically have single, hard seed that are no more than three-fifths of an inch in diameter, the largest size that a seed-eating bird can swallow. Most bird-distributing fruits are bright red, a color that is attractive to birds. In contrast, orange, yellow, and green fruits generally signal unripe fruits with immature seeds. Some plants that rely on certain birds for seed dispersal appear to disregard the red color rule by having fruits that are blue, black, or white. Virginia creeper, poison ivy, and wild grapes all depend upon birds to distribute their blue or white fruits. In these plants, enzymes prematurely break down the green chlorophyll in the leaves, which allows underlying yellow, red, and orange to show through.
The fruit of over 70 percent of bird-distributed plants ripen in the fall, which is just in time for migration. In New England, most shrub and trees ripen in August and September, coinciding with the migration of the thrush and cedar waxwing. The same plants will have ripe fruits a month later in the Carolinas, providing migrants with continuous food on their southbound flight.
Trees and shrubs that produce fruits with high fat content are attractive to birds at this time. These fruits have twice the energy value/unit weight as carbohydrates, and they help the birds build up essential deposits of fats, which permit them to stay air born during the long flights.
The message for the bird gardener is to plant a variety of trees, shrubs, and flowers that will benefit birds throughout the four seasons.