Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180


Population = a group of individuals belonging to the same species living within a certain specified area.

Population Density = the number of individuals per area for a given population. Population Density is controlled by 3 primary factors:

1) Natality = reproductive rate of the population.

- Potential Reproductive Output = rate of growth of the population if all eggs laid survived to breeding; results in very rapid exponential population growth.

- Almost never does a population grow exponentially as several factors regulate population size. However, when a species is introduced into a new and favorable habitat, its numbers may increase very rapidly initially before leveling off at a population far below the theoretical maximum. This initial rapid rate of population growth approaches the exponential growth curve. (SEE HANDOUT).

2) Mortality = death rate of a population.

- Longevity = lifespan of a bird; Potential Longevity = maximal age that a bird can attain (usually occurs in captive birds held under favorable conditions; Actual Longevity = average age attained by birds in the wild.

- Large birds tend to live longer than small birds. The average age of most small birds ranges from about 2 to 5 years. Average ages for larger birds can range upwards of 20 years. Maximum ages for wild birds range from 6 - 20 years in passerines (SEE HANDOUT), and up to 42 years for the Laysan Albatross among nonpasserines. Some captive parrots have lived to be 80 years old.

- Survivorship Curves = age-specific summary of death rates in a population. There are 3 main types of survivorship curves.

  • 1) Type 1 = high mortality initially, followed by low mortality until old age. This type is typical of humans.

  • 2) Type 2 = constant mortality with respect to age. This type is true of most birds.

  • 3) Type 3 = very high mortality of juveniles, low mortality of adults.


- In birds, death from old age is relatively rare, because predators or starvation or adverse climatic conditions take a heavy toll.

3) Dispersal = moving in to a population from outside (immigration) or moving out of the population (emigration).

- Recruitment = immigration of young into a population. This can account for an important fraction of a population (e.g., 12-42% of a population of Great Tits in Holland was composed of recruited young.


- Population Densities change relatively little through time, although annual fluctuations in density occur in response to various factors. (SEE HANDOUT).

- Factors regulating population size are responsible for temporal fluctuations. These include the following.

I. Density-Independent Factors = factors acting randomly, with reference to the population density, to regulate population size.

1) Weather - unfavorable weather can limit breeding success or cause mortality during the non- breeding season (e.g., winter mortality).

2) Food Supply - abundance of food is often dependent on climatic factors, and in poor years food supply may limit population size.

3) Habitat Limitation - seasonal, annual, or anthropogenic changes can affect resource levels such as nesting sites or availability of nesting materials.

- (2) and (3) contribute to setting a limit on the Carrying Capacity of the habitat. These latter two become more important at high densities, so they can be considered density-independent or density-dependent factors. Carrying Capacity = the maximum number of individuals that a habitat can sustain. This may change with time to some degree.

II. Density-Dependent Factors = regulate populations according to the population density.

1) Predation = focuses most intensely on eggs and young, but also may be important for adults. Acts more strongly at high densities. Keeps populations below the theoretical carrying capacity in some cases.

2) Competition = if resources are limiting, only a given number of individuals may persist in a given habitat (the number that can persist is the carrying capacity). At low densities competition is not an important factor in regulating population size, but at high densities (near the carrying capacity) it becomes very important.

3) Parasites and Disease = have the greatest effect at high population densities. Can be important factors regulating population sizes in some cases. Example: Avian cholera can kill very large numbers of wintering or migrating waterfowl that are concentrated in relatively small areas.

- However, the frequency of diseases and their impact on long-term regulation of bird populations is not well-understood.