Biology 363/563 Ornithology

Dr. David Swanson, Office: CL 180


III. BROOD PARASITISM = involves laying eggs in nests of another species (= host), then abandoning them to the care of the foster parents.

  • A) Relieves parasites from many of the costs of reproduction (e.g., nest building, incubation, feeding young). Allows them to lay more eggs per breeding season. This increases their potential fitness.

  • B) Practiced by Old World Cuckoos, New World Cuckoos, Cowbirds, Honeyguides (Africa), Whydahs (Africa), Black-headed Duck (South America), and Parasitic Weaver (tropical Africa), for a total of about 80 species. This makes up about 1% of all birds. SEE PAGE 459, GILL.

  • C) How did brood parasitism evolve?

    • 1) Selective Factor = enhance female reproductive output.

    • 2) Two scenarios for the evolution of brood parasitism have been proposed.

      • (a) Start with active takeover of another species nest (occurs in Cliff Swallows, Chestnut Sparrow-Kenya, Bay-winged Cowbird-South America). Brood parasitism evolves from this situation to avoid fighting over the nest.

      • (b) Start with Intraspecific Brood Parasitism = laying eggs in the nest of other members of the same species (occurs in European Starling, Cliff Swallow, several ducks, etc.). From there Facultative Parasitism evolves (= laying eggs in the nests of related species with similar eggs; occurs in Redheads, Ruddy Ducks), and then this leads to an expanded repertoire of hosts (= brood parasitism).

  • D) Effects on Host:

    • 1) Rarely fledge young other than that of the parasite, as the parasite young usually hatches first and grows faster. The parasite young may actually remove host eggs or kill host young. SEE HANDOUT AND PAGE 461, GILL.

    • 2) Exhaustion of host parent providing abilities, so renesting attempts are decreased in host parents.

  • E) Host-Parasite Coevolution

    • 1) Parasite Adaptations

      • (a) Produces small eggs (relative to body size) with short incubation times; small eggs may also allow the female brood parasite to produce more eggs.

      • (b) Protrusible Cloaca = allows laying in nests or cavities too small for the adult parasite to enter. This also contributes to egg-laying being very rapid, and the parasite egg has an extra-thick shell.

      • (c) Egg Mimicry = small egg size approaches host egg size in most cases. Coloration of parasite eggs may also resemble that of the specific host (Eurasian Cuckoos). Egg mimicry undoubtedly arose in response to the host evolving egg-recognition abilities.

      • SEE PAGE 462, GILL.

      • (d) Chick Mimicry = plumage pattern and gape coloration matches that of the host species closely. This has undoubtedly evolved in response to species-specific recognition abilities in the host. Example: Whydahs parasitize estrildine finches and the pattern of the Whydah plumage and gape matches that of the host finch closely. SEE PAGE 462, GILL.

    • 2) Host Adaptations

      • (a) Egg and chick recognition devices

      • (b) Nest desertion or burying clutch under new nest floor (e.g., Yellow Warbler - Brown- headed Cowbird interaction -- record is a 6-story Yellow Warbler nest.

      • (c) Toleration if parasite helps host young to survive. Example: In Panama, the Giant Cowbird parasitizes the Oropendola, but the parasite young pick botfly larvae from the host young, thereby improving their survival.

IV. COOPERATIVE BREEDING = non-breeding individuals help to raise the young. Occurs in

> 150 species in 10 different Orders.

  • 1) Characteristics = low breeding rates, high adult survival, low dispersal (this may limit breeding opportunities), and deferred maturity.

  • 2) Organization of System - considerable diversity exists.

    • A) Breeding Pair + Helpers

    • B) Defend all-purpose territory

    • C) Helpers are genetically related to breeding pair (generally they are offspring from a previous brood) and are predominantly male

  • 3) Is this altruistic behavior contrary to natural selection, which acts to increase fitness of individuals?

  • - Cooperative breeding increases the survival of young of related birds. Because relatives share many of the helper's genes, the helpers indirectly pass their genes (or a good portion of them) on to the next generation through the breeding pair. It is likely that this strategy increases the reproductive fitness of the helpers (reproductive fitness = portion of genome passed on to the next generation) over trying to breed on their own with a small probability of success.