Demography of the Cooperatively Breeding Galapagos Mockingbird, Nesomimus parvulus, in a Climatically Variable Environment

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Abstract
(1) The Galapagos mockingbird is a group territorial and cooperatively breeding species. We studied its demography on Isla Genovesa for 11 years (1978-88), a period that included 7 years with moderate rainfall, 2 extremely wet years when El Nino-Southern Oscillation (ENSO) event occurred, and 2 drought years. (2) Annual survival of adults (.hivin.x = 61%) was high during average conditions but lowere in both drought years and during an epizootic. Males survived slightly betternthan females. Survival did not vary with age among adults, but survival of juveniles was lower (.hivin.x = 35%) and twice as variable as adult survival. (3) Mockingbirds bred in response to rainfall, producing up to 5.2 fledglings per bredding female each season (.hivin.X = 2.3). Clutch size, number of clutches and total production of fledglings increased in wet years. Helpers attended 35% of all nests and increased production of fledglings by 19%. (4) Differential mortality produced variation in the adult sex ratio. Breeding by males was constrained by the relative scarcity of females, and the proportion of males acting as helpers varied correspondingly because non-breeding males most often helped. The proportion of females breeding did not vary with adult sex ratio, because polygyny occurred in 2 years when females predominated, and most of the few females the helped also bred. (5) Variation in natality and survival caused substantial variation in population density, but the density of breeders remained relatively constant (C.V. = 21%) compared to density on non-breeders (C.V.=93%). Group territories, which were defended year-round, filled all available habitat througout virtually the entire study. (6) Group size averaged 4.2 adults, with up to four breeding females in each group. Groups were larger and plural groups more frequent in years when density was high and when yearlings predominated in the population; 67% of yearlings remained in their natal territories, where 47% of females and 62% of males also first bred. (7) Climatic variation influences mockingbird social organization mainly through its effects on natality and survivorship, which indirectly influence the population''s density, age structure, and sex composition. Plural breeding is maintained in this species because climatically-induced demographic variation favours the tendency of young birds to breed without establishing independent territories.