Birds and Trees: An Inseparable Attraction

Let your mind wander to birds and you will invariably also think of trees, especially in eastern North America, where forests and forest-nesting birds dominate the landscape. The two kinds of organisms (birds and trees) are nearly inseparable. In ecology, symbiosis is the term used to describe an interaction between dissimilar organisms living together in a more or less intimate association. Ecologists have categorized several different types of symbioses depending on how the organisms interact and whether one or both of the organisms benefit from the association.

Commensalism is an association in which one organism benefits while the other is not affected. This is perhaps the normal relationship between birds (benefit) and trees (not affected). Trees benefit birds in numerous ways: they provide (1) ready perches for displaying, feeding, mating, preening, resting, and sleeping; (2) display posts from which to announce territories and advertise for mates; (3) branches and limbs for supporting nest structures; (4) micro-habitats—from bark to flowers to leaves—that harbor a wide variety of insects and other animals that the birds prey upon; (5) direct sources of food in the form of berries, buds, flowers, nectar, nuts, sap, or seeds; (6) crevices, nooks, and crannies in which to conceal and store food; (7) watching and listening posts from which to scan their surroundings for predators or prey; (8) shelter from wind, weather, and predators; and (9) cavities for nesting and roosting.

Mutualism is an association that is advantageous to both organisms. In the case of birds that feed on the berries, nuts, or seeds of trees, the relationship between birds and trees may be truly mutualistic. The birds benefit the trees by scattering the seeds of the trees far and wide. In many cases, tree seeds are more likely to germinate if they have first passed through the digestive tract of a bird. The importance of seed distribution by birds has been known for some time, and is currently the focus of a study on regeneration of tropical forests. Populations of insect-eating birds are known to increase in response to outbreaks of forest pests. Thus, in their role as insect predators, birds may benefit trees by helping to reduce the potentially devastating impacts of defoliating insects such as the spruce budworm (.pdf). The potential economic benefits of insectivorous birds was, in fact, one of the original arguments put forth for justifying passage of the Migratory Bird Treaty Act in 1918.