During the voyage of the Beagle, Darwin collected mockingbirds on four different islands in the Galápagos. He determined that his specimens represented three different varieties, all unique to the archipelago, and that no two of these occurred together. Finding different mockingbirds on islands within sight of each other contributed to Darwin's realization that similar species could replace each other geographically, an idea that sparked his thinking about evolutionary processes. Four forms are now recognized: Nesomimus trifasciatus, the Floreana (Charles Island) Mockingbird, N. melanotis, the San Cristóbal (Chatham Island) Mockingbird, N. macdonaldi the Espanola (Hood Island) Mockingbird, and N. parvulus, the Galápagos Mockingbird.

For the past 11 years, we have investigated population ecology and social organization in this endemic genus. Most Galápagos mockingbirds, like many other cooperatively breeding species, live in groups holding collective territories. Mockingbirds in the Galápagos also experience a climate that varies widely and unpredictably. Conditions range from severe droughts to extraordinarily wet years associated with El Nino-Southern Oscillation (ENSO) events; both extremes occur on the Galápagos on average once every four years (Grant 1985). A major focus of our study has been to investigate how territorial behavior and climatic variation interact to produce a complex form of cooperative social organization. This complexity has provided challenges for deciphering and explaining patterns of social behavior amid large environmental and demographic variation, as well as excellent opportunities for testing hypotheses about the evolution of cooperative breeding.