Anywhere a bird goes is its home range; within that home range may be a defended area which is the territory. A territory serves to spread out competing individuals into their own spaces and thus reduce competition. Robert Ardrey in the Territorial Imperative, argues that territoriality is innate in all animals, including humans, and entitles his first chapter “Of Men and Mockingbirds”, appropriate since the Northern Mockingbird is known for its especially territorial behavior. The defended space could be the foraging area, a wide area around the nest, the nest itself, a place for a male or males to display, or a roosting area. Territoriality among most species occurs during the breeding season but some birds hold winter territories to protect their food supply. Food is often a major factor in determining whether or not territories will be held, how long they will be held, and their size. Food supply is variable because of its distribution, density, predictability, renewal rate, the diet of the birds and the population size of the users, so territorial defense could change as well.

            I have a long time colleague who became a well published and respected botanist. One day he and some students took 500 containers of flowering plants by truck to Yosemite National Park. They placed the plants on the ground in a predetermined grid. Within five minutes, Anna’s Hummingbirds had established territories around several of the flowers. Apparently nectar was scarce, so the hummingbirds were searching for sustenance and did not set up a defended area until they determined it was worthwhile. In a study of Calliope and Rufous Hummingbirds in Nevada, researchers found that Rufous Hummingbirds fed mainly on one species of flower and were highly territorial, defending their food source vigorously. The smaller Calliope Hummingbirds, faster fliers, did not hold territories. Instead, they simply raided the territories of the Rufous Hummingbirds which fed on flowers at heights eight in. (20 cm) or higher above the ground. The Calliope hummers would zoom in below the Rufous Hummingbird feeding zones to feed on lower flowers. The smaller size and higher metabolic rate of the Calliopes made territoriality too expensive for them so they robbed the territories of the Rufous Hummingbirds.

            There are several species of flycatchers in western North America in the genus Empidonax. – the Dusky, Gray, Willow, Alder, Pacific-Slope, Hammond’s, and a few more; they are notoriously difficult to tell apart as they are morphologically and behaviorally similar. The expectation is that they would strongly compete in areas where their ranges overlap, but they do not. There could be several reasons like an abundance of insects or predation pressure, but studies indicate that the flycatchers probably maintain both inter-and intraspecific territories during the breeding season to reduce competition.

            Territories are generally measured as covering a two-dimensional area, rather than as a three-dimensional volume although they are 3-D in reality. So let us expand this notion of space by examining foraging styles to give us insight as to how avian communities work.