Biology 363/563 Ornithology

Dr. David Swanson, Office: CL 180


Deals with the question of "How do birds partition the environment among themselves?"

- Assume: Resources (food, nest sites, etc.) are limiting.


- There are 3 potential types of spatial distribution during breeding:

  • 1) Random - no known examples

  • 2) Regular - indicates territoriality

  • 3) Clumped - indicates coloniality


1. Territoriality = aggressive establishment, maintenance and protection of spatial relationships; Defended space = territory. Function = maintains proper spacing within the habitat.

  • A) Types of Territories:

    • (1) Breeding = ranges from a small space immediately surrounding the nest (colonial species) to large areas where all activities (breeding, roosting, feeding, etc.) are carried out. This latter type of territory is termed an All Purpose Territory.

    • (2) Roosting = many socially roosting birds defend sleeping positions.

    • (3) Nonbreeding = defense of feeding territories in winter or during the breeding season away from the nesting area.

  • B) Benefits and Costs

    • (1) Benefits:

    • a) Decreased competition for limited resources; allows "owners" to have better access to resources than they would otherwise have.

    • b) Increased reproductive success by providing resources for mate(s) and young, or by attracting more mates.

  • (2) Costs: Fighting and threatening displays are energy and time expensive, and they may also involve the risk of injury or death.

  • (3) Balance:

    • a) Expect territory defense only when benefits outweigh costs; territory size should maximize benefits relative to costs. (SEE HANDOUT).

EXPERIMENTAL: Crook (1965) - comparative study of Weaverbirds

1) When food sources were predictable through time (e.g., for species feeding on insects), birds defended all-purpose territories.

2) When food sources were patchy and/or unpredictable (e.g., for species feeding on seeds), birds defended small areas around the nest and tended to become colonial.

  • b) In birds defending all-purpose territories, territory size is proportional to body mass ---> Larger species defend larger territories. (SEE HANDOUT).

  • c) Territory size can also be related to food abundance - high food abundance leads to decreased territory size.

2. Coloniality = large numbers of birds nesting in a relatively small area; defend a territory that just surrounds the nest.

- About 13% of all birds are colonial nesters. Coloniality frequently occurs in birds whose food supply is unpredictable in space and time (e.g., pelagic birds feeding on shoaling oceanic fish and plankton are virtually all colonial-nesting species), or in birds where nesting space is limited (e.g., sandbars, river banks).

- Size of colonies = a few pairs to millions of pairs.

  • (1) Guanay Cormorant - Peru coast = 12,000 nests per acre ---> 4.5 million per colony.

  • (2) Common Murre - 3 Arch Rocks NWR (Oregon coast) = 22,000 nests on 17 acre rock ---> 750,000 per colony.


  • (1) Benefits

    • a) Anti-predator Advantages

        (i) Vigilance = "many pairs of eyes are better than one pair," birds are more likely to detect an approaching predator.

      • (ii) Central Position = individuals who nest in the center of the colony are less vulnerable to predation.

      • (iii) Predator Swamping = if colonial birds breed synchronously, the number of individuals swamp predators' food needs, so a reduced proportion of nests suffer predation (SEE HANDOUT).

    • b) Feeding Advantages

      • (i) Information Center Hypothesis = colonies serve as information centers to direct members to profitable food patches ---> some birds find a good food source, others follow the successful birds. This hypothesis appears valid in birds tested so far.

      • (ii) Synchronous Breeding - also may be important to finding food for young, as late nestlings are more likely to experience retarded development or starvation. (SEE HANDOUT).

  • (2) Costs

    • a) Increased competition for nest sites and materials, mates, and food around the colony.

    • b) Risks of rearing young other than their own. Individual recognition devices evolved to minimize this risk.

    • c) Increased infanticide and cannibalism - killing chicks that wander from the nest site.

    • d) Increased transmission of ectoparasites and disease.


1. Flocking = aggregation of conspecifics or several species into groups for certain purposes; flock composition varies from loose temporary aggregations to organized foraging associations of diverse species. Most birds flock outside of the breeding season.

- Conditions similar to those stimulating coloniality (unstable food resources and indefensible areas) also promote flocking.

  • A) Advantages - similar to those for coloniality

    • (1) Feeding Advantages:

      • a) Numerous individuals searching for food are more likely to find good food sources.

      • b) Groups may be more effective at capturing prey than individuals (e.g., pelicans encircle and trap schooling fish, Cattle Egret flocks flush prey - if prey is missed by one member it is usually captured by another.

      • c) Flocks help ensure that all individuals find at least some food on a regular basis. This is especially important in stressful winter conditions. Support for this idea comes from the fact that small birds (higher mass-specific metabolic and thermoregulatory costs) tend to flock more than larger birds.

    • (2) Predator Avoidance:

      • a) Flocks may confuse predator when they can't isolate an individual to prey upon.

      • b) Safety in numbers - the larger the flock, the less likely a given individual will be preyed upon.

      • c) Increased predatory detection occurs with more individuals watching. This decreases the amount of time that an individual must spend watching for predators, so it increases the amount of time available for foraging.

    • - Optimal Flock Size = theoretically maximizes feeding time relative to time spent in vigilance and in fighting competitors (SEE HANDOUT).

  • B) Disadvantages - The main disadvantage is increased competition for available food.

  • C) Dominance Hierarchies and Dominant - Subordinate Relations

    • - Dominance and aggressive reinforcement of status are normal parts of the lives of birds. In territorial birds, territory "owners" are usually dominant within their territory and drive out intruders.

    • - Dominance Hierarchies = organization of the flock by social rank or status. This type of organization exists in many colonial or flocking birds.

    • - The classical studies were performed with chickens (Allee 1936), and demonstrated a "peck order," which is an example of a dominance hierarchy.

    • - Dominance is established by winning aggressive encounters, once "winners" have been established, subordinates generally cease challenging. Dominance is maintained by ritualistic threat displays.

    • - Dominants have the advantage in gaining access to food, nest sites, mates, etc. and tend to survive longer and produce more offspring than subordinates.

    • - Subordinates are usually first to emigrate if environmental conditions deteriorate, as they have the least access to food and shelter ---> This has important implications for the evolution of migration systems in birds (e.g., differential migration of age or sex classes).

    • Correlates of Dominance (What makes a winner?)

      • 1) Age and Sex - generally males dominate females and older birds dominate younger birds.

      • 2) Body Size - generally larger birds dominate smaller birds.

      • 3) Location - prior-residency enhances dominance status in territory disputes (e.g., the ability of territorial male Stellar's Jays to win fights decreased with distance from their home territory).

      • 4) Hormones - increased aggressive behavior resulting from higher testosterone levels may lead to higher dominance status (e.g., in several species, males injected with testosterone showed increases in aggressive behavior and dominance rank).

    • Advantage to Flock = Dominance hierarchies act to decrease competition among members and reduces fighting (and the resultant potential decline in fitness due to fighting).

2. Roosting = aggregation of birds (conspecifics or heterospecifics) spending the night in the same area. The size of roosting flocks varies from a few individuals in a single tree to millions of individuals in a small area.

- Examples: (a) Wintering Bramblings in Switzerland ---> approx. 72 million in two small (16 acres total) pine woods.

(b) An estimated 19,000 Broad-winged Hawks (during migration) spent the night in a 12 km2 forest in Panama.

- Roosts generally occur in thick foliage, thus protecting birds from wind, precipitation, etc. (= favorable microclimate). A few species will roost huddled together in a small mass to conserve heat (e.g., Brown Creepers, Northern Bobwhite).

3. Niche Partitioning to Reduce Interspecific Competition

- Competition among species for limited resources is the single most important force shaping community structure.

- Competitive Exclusion Principle = no two species can occupy the same ecological niche (= sum total of all environmental factors important to survival and reproduction).

- Natural selection, over time, has resulted in niche-partitioning = species divide up the habitat so that each species specializes on a certain portion of the habitat. This allows coexistence of several species within the same habitat.

- Example: MacArthur (1958)

1) 5 species of warblers were present in a conifer forest in Maine and had similar food requirements.

2) Each species specialized on a different portion of the tree, and each foraged in a slightly different manner.

3) This division of the habitat allowed the stable coexistence of all 5 species. (SEE HANDOUT).

- Niche-partitioning of a habitat may also occur among sex classes or age classes of a single species.