Populations of interacting species are called communities. Birds that do the same kinds of things – fill similar niches – comprise a guild (leaf-gleaners guild, fish-eater guild, bark-prober guild, etc.).
In their habitats, birds deal with three major factors: competitors, vegetation structure, and trophic levels. Vegetation structure is the best predictor of avian diversity. Also see the Biogeography Page.
Birds of same species or sometimes of different species will compete to some extent with each other. Since no two species can occupy the same niche, the numbers of similar species in a habitat are limited. Birds, and other organisms, have evolved ways to lessen competition in various ways:
1. Geographic replacement/separation. Have non-overlapping and sometimes complimentary distributions. E.g. Spotted Owl/Barred Owl; Eastern and Western Meadowlarks, Scrub Jay and Blue Jay, Yellow-billed and Black-billed Magpies, Grackle and Brewer’s Blackbird. May have different ranges, as above, or may have different altitudes- Scrub and Stellar’s Jay, House and Purple Finch.
2. Between-habitat segregation. Found in different portions of the same geographic area. Chestnut Backed and Mountain Chickadees, ducks and geese.
3. Within-habitat segregation. The same area in the same habitat are used differently. Yellow-headed and Red-winged Blackbirds; Whimbrels and Sanderlings; Kingfishers and swallows.
Lack (1971) studied the winter feeding sites of three species of tits in winter. Different species of tits spent differing amounts of time in different aspects of the habitat. MacArthur (1972) studied four species of ant-thrushes in South America and discovered that they used different levels within the vegetation for feeding. So, a major reason birds segregate themselves within a habitat is because of competitors – probably competitors for food.
Birds can also segregate themselves temporally. Since the breeding season is the most potentially stressful time of the year, let’s look at breeding times.
Month Bushtit nests (Northern California) Titmice Nests
Feb 103 0
March 495 73
April 200 195
May 61 62
June 0 21
And something similar is true for species all over the world. The interesting thing is that the breeding season of the entire avifauna of a geographic area is about 1.35x the average season of the component species. In other words, add up all the breeding lengths of all species – some nest over a period of 6 weeks and some nest over a period of maybe 3 months. Take the average of these and multiply that average by 1.35 and you’ll come up with the total time over which all the birds in that area breed. For example:
In Lapland, the average breeding length for all the species is .96 months. It takes 1.32 months (40 days) for all species to breed. 1.32/.96= 1.37.
West Java 9.89 months total season and the average nesting season per species is 7.54 months. 9.89/7.54=1.31.
Costa Rica 6.62/4.53=1.46
So there is a shift in the nesting season of about 30% the world over.
Birds are also different sizes and come in large, medium, and small.
Ash-throated Flycatcher 166 mm Ringed Kingfisher 381mm
Black Phoebe 147 mm Green and Rufous Kingfisher 212mm
Willow Flycatcher 121 mm Pygmy Kingfisher 132m
Schoener looked at hundreds of species of insectivorous birds in different areas of the world. Lumped all into temperate or tropical groups. He found that there are more species and a larger range of bill sizes in the tropics because there are more arthropods in the tropical forest and larger ones as well than in the temperate zones. Why do larger sizes allow for different food choices? Because larger birds can handle larger prey and a wider range of sizes.
Now let’s look at community structure which has evolved from the various feeding habits of birds. As a habitat gets larger and/or more complex, we can add more species because we create more potential niches. What does this tell us? It says that as the total number of bird species increase, the number of species in each guild increases at a different rate. Frugivores increase the fastest, followed by gleaners, flycatchers, and nectarivores. Thus you should be able to predict the number of each of these guilds in a tropical forest if you know the number of species. Or, conversely, if you know the number of flycatchers, you would be able to predict the number of species in the other guilds as well as the total number of species.
But when will a new species enter (or leave) a habitat based on food? When is there enough food. Crossbills, found in pine forests of the western US and Canada, will breed all year around if they have a good food supply, and show “irruptions”- irregular population fluctuations depending on the food supply. They are non-territorial but will stay in one place if the food supply is good and leave if it is not.
So bird composition of an area is at least partly dependent on the food supply. See The structure of avian communities along elevational gradients in the northwestern United States for more information.