Galápagos Mockingbirds

http://oikos.villanova.edu/nesomimus/

Cooperative breeding in Galápagos mockingbirds

What we know, and references that go with these conclusions

• Galápagos mockingbirds have a complex social organization, reflecting the ecological and evolutionary influences of the climatically variable environment that they inhabit. Unusual features of the social system include:
  • Mating system that shifts from socially monogamous, during times when males out number females in the population, to polygynous, during years when females outnumber males.
  • Occasional joint nesting, especially in social groups where two or more females is socially paired polygynously with a single male.
  • Plural territorial cooperative breeding: social groups often include two or three breeding females, usually with each having their own separate nest but with all females and their social mates and helpers belonging to a social group that defends a single coillective territory.
  • Flexible reproductive behavior: breeders can simultaneously serve as helpers at the nests of other pairs; helpers can attend more than one nest in the same breeding period; and non-breeders may or may not serve as helpers (some birds neither breed nor help but are members of the stable social group).

  Reference:
  Curry, R. L., and P. R. Grant. 1990. Galápagos mockingbirds: territorial cooperative breeding in a climatically variable environment. Pps. 290-331 in P. B. Stacey & W. D. Koenig, Eds. Cooperative Breeding in Birds: Long Term Studies of Ecology and Behavior. Cambridge University Press, Cambridge.

    Back to Top

• Galápagos mockingbirds have a complex social organization, reflecting the ecological and evolutionary influences of the climatically variable environment that they inhabit.
  • Alternation between wet El Niño conditions (when mockingbirds breed like crazy) and drought years (when mockingbird do not breed at all) produces drastic variation over time in population age structure
  • Variation in survival patterns, including especially sharp increases in mortality associated with wet El Niño conditions and caused by outbreaks of avian pox, cause changes in population structure and adult sex ratio that have repercussions for breeding strategies and the mating system

  Reference:
  Curry, R. L., and P. R. Grant. 1989. Demography of the cooperative breeding Galápagos mockingbird (Nesomimus parvulus) in a climatically variable environment. Journal of Animal Ecology 58:441-464. (Abstract) (Reprint, PDF format)

    Back to Top

• Galápagos mockingbirds live in large territorial social groups including helpers on most, but not all, islands
  • Groups on Genovesa (N. parvulus) range from 2 to 24 birds and can contain just one breeding female (singular breeding) or several (plural breeding)
  • Groups on Española (N. macdonaldi) are all large, with plural breeding probably the usual situation there
  • Group sizes and breeding structure on Champion (N. trifasciatus) are similar to those on Genovesa
  • Groups on San Cristóbal (N. melanotis) are small--just pairs or pairs plus an extra bird--and may not breed cooperatively

  Reference:
  Curry, R. L. 1989. Geographic variation in social organization of Galápagos mockingbirds: ecological correlates of group territoriality and cooperative breeding. Behavioral Ecology and Sociobiology 25:147-160. (Abstract) (Reprint, PDF format)

    Back to Top

• Galápagos Mockingbirds on Genovesa (N. parvulus) breed cooperatively, with helpers at roughly half of all nests. Helping behavior is flexible and reveals the influence of kinship:
  • Non-breeding males may or may not serve as helpers. Non-breeding females rarely serve as helpers
  • Breeders may serve as helpers at nests other than their own in the same plurally breeding social group
  • Both adults and juveniles are more likely to serve as helpers when nests available to be helped contain close relatives than when nests available to be helped contain distant realtives or unrelated birds
  • The incidence of helping is matches--even more than with kinship--a pattern predicted by associative learning: birds tend to be more likely to help at nests belonging to breeders that previously fed the helper when it was a nestling, whether as a parent or as an older helper

  Reference:
  Curry, R. L. 1988. Influence of kinship on helping behavior in Galápagos mockingbirds. Behavioral Ecology and Sociobiology 22:141-152. (Abstract)

    Back to Top

• Galápagos Mockingbirds on Genovesa (N. parvulus) exhibit plural cooperative breeding, with up to three reproductive females in each territorial social group.
  • Each female usually has its own nest (separate plural breeding) but they sometimes nest jointly, especially if polgynously paired to the same male
  • A pronounced dominance hierarchy exists among males in each social group, with all males in the group deferring to a single alpha male, whose movements define the bonmdaries of the groups total tetrritory
  • Conflict with social groups is pronounced, especially in drier years when resources for breeding are probably limiting. Dominant pairs frequently disrupt the nesting of subordinate pairs, but stop short of infanticidal destruction of eggs.
  • Subordinate males whose breeding has been disrupted often rediect their feeding, becoming helpers at the nest of the pair that carried out the disruption (often his parents)
  • Dominant males sometimes court females that are socially paired to subordinate males, sometimes with apparent copulation success, but subordinate males appear to be rebuffed if they attempt to copulate witgh females socially paired with dominant males

  Reference:
  Curry, R. L. 1988. Group structure, within-group conflict and reproductive tactics in cooperatively breeding Galápagos mockingbirds, Nesomimus parvulus. Animal Behaviour 36:1708-1728. (Abstract)

    Back to Top