Population dynamics:

The populations of many birds will vary significantly from year to year.

Population limitation can be caused by a number of different things such as food, nest sites, competitors, predators, disease, or harsh weather. Two of these limiters determine a carrying capacity or ceiling. These are food and nest sites. The other factors can maintain the population below the carrying capacity.

Example: experiments have been conducted on cavity nesting birds whereby they have correlated the number of birds with the number of dead trees. (Raphael and White 1984). Under a certain threshold of dead trees the birds are limited by trees. After a certain level, other factors probably limit their density.

Another few things to consider in any population of birds are their niche requirements and their habitat selection. Certain birds will choose certain habitats. .

Population regulation deals with density-dependent and density-independent factors that prevent a population from exceeding carrying capacity.
• Density independent factor: unrelated to the number of individuals in the population.
• Density dependent factor: strength of regulating factor varies with the number of individuals.

Example: A study was done in Finland showing that temperature can regulate a population. When plotted on a graph the relationship between Great Tits (Parus major) breeding numbers and mean temperature of the preceding winters in an area of Finland can be seen.. In the winters 1969-1971. In the winters 1969-1971, extra food was provided. Filled symbols – pair numbers after winter of food provision, open symbols – pair numbers after other winters. From von Haartman 1973.

Changes in Ecological Factors as they effect Populations of birds.

Some examples:
• The more grass-seeds the more sparrows there are:
• More birds in the type of sediment (which is related to the prey).
• Dipper territories are larger in acidic stream conditions.

Nest-site limitation
• Examples
Brawn and Balda 1988: cavity nesting birds in Ponderosa Pine forest in Arizona. Shows that populations of birds can be nest-site limited but if they are in areas where there are lots of cavities, nest-sites are not limiting.
• Dunn 1977: Weasels, which each rodents, will eat bird nests. The more rodents, the less nests are destroyed because weasels have enough food to eat. Yet, the higher the density of nests, the greater percentage of nests that are destroyed.

Ectoparasitism and Disease: (Examples)

• Brown and Brown 1986: Cliff swallow study. The bugs per nest and bugs per nestling increased with colony size.

Ideal Free Distribution: This is very important in population dynamics. This model is the null model.

• Individuals have knowledge of all habitats
• Individuals are free to move between habitats and stay in one of preference.
• Loss of an individual from Habitat A increases the fitness of the rest of individuals in A.
• Gain of an individual in Habitat A decreases the fitness of the rest of individuals in habitat A.

By Rob Nelson


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