Relationship Between Plants and Birds
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.
Attractive Fruits
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.
Fruit For Migration
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. |
