In 1958, there was a classic study of an avian community done by Robert MacArthur of warblers in a white spruce forest of the coast of Maine. He discovered that warblers of different species used different areas of a pine tree to feed in. One original paper can be seen here.
The perennial question is: do birds prefer different foods which are found in different places or do they prefer to feed in different places (to avoid competition) and thus eat different foods?
Probably a combination of the two.
Birds segregate themselves by habitat and food, but only so much segregation is possible and thus the number of species is limited. This is exemplified by island biogeography (MacArthur and Wilson, 1967). Studies of bird communities on islands show two main trends: the size of an island and the number of species found on it are positively correlated and the further the island is from a large land mass the smaller the number of species that occur there. The reason is that the number of species on islands is a balance between the number of new immigrant species arriving and the rate at which other species die out or leave. There is an alternative explanation; the number of species on an island is due to the richness of the habitat which is related to the plant species on the island. Isolated patches of habitat, such as woods, ponds, or mountain tops can be considered as ecological islands.
There is considerable evidence to indicate that the vegetation structure of the habitat determines the bird diversity of that habitat.
Martin Cody did a detailed analysis of grassland birds
which shows how the difference in vegetation structure can change the bird community. Each short grass area of the world contains 3 types of foraging birds: probers, launchers, and foliage gleaners. In very short grass, birds can probe the ground for insects (meadowlarks, starlings) or launch into the air for insects (horned larks). They divide the habitat horizontally.When the grass is slightly taller and denser, both probing and launching become more difficult, but we begin to add foliage gleaners, so we have three types here. When the grass becomes tall, probing and launching become nearly impossible, so we are left with only foliage gleaners, who divide the resource vertically as well as horizontally. Cody found this system in both North America and Chile – with totally different species of birds.
The species of vegetation doesn’t matter as much as the structure of the vegetation. That is, a many-layered forest of a few species of vegetation will provide habitat for more bird species than a more simply structured forest with many more species of plants.
What is diversity? A combination of numbers and equitability. The simplest measure of diversity is species richness, but this doesn’t tell the whole story. In other words, we can count the number of species and get an indication of species diversity, but it’s the distribution of the entire population among the species that is a better measure. There is a way to calculate species diversity from population data, with diversity figures generally running from zero (no birds) to about four – such as you would find in an equatorial rain forest.
More structurally varied habitats have the greatest species diversity. Species diversity also increases with size of the habitat. For a tenfold increase in area, bird species numbers increase by 1.6x to 2.5x.
What is important about being able to measure species diversity? It’s a way of quantifying as well as qualifying baseline data of a particular habitat and it works for all species of organisms, structures, and habitats. A change in the data over time tells you that something is happening in the habitat. It also gives you a way to predict what you will or should find in a habitat. It provides a way to identify “indicator” species which may signal environmental degredation. Like canaries in a mine, birds can be valuable indicators of what is happening to the environment.