Ardea
Official journal of the Netherlands Ornithologists' Union

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Millington S.J. & Grant P.R. (1984) The breeding ecology of the Cactus Finch Geospiza scandens on Isla Daphne major, Galapagos. ARDEA 72 (2): 177-188
This study documents the breeding characteristics of a small, isolated population of the Cactus Finch Geospiza scandens on Isla Daphne Major, Galapagos from 1979 to 1981. The numbers of birds in the population that were at least one year old changed very little. Recruitment was very low, except from the 1978 cohort. Females paired predominantly with older males: only one female paired with a male younger than herself. Cactus Finches breed in apparent response to two sets of environmental cues; 1) the cactus flowering in October-December, and 2) rainfall and the associated plant/insect response in January to May. The onset of breeding was less synchronised in the pre-rains spell than during the rains. Clutch size and fledging success were positively correlated with the amount of rainfall. There was some evidence of a density-dependent effect on clutch size independent of rainfall. Higher clutch sizes resulted in higher numbers of fledglings, but these survived less well to independence, with the end result being no significant difference in number of surviving young from different clutch sizes. Breeding output and success were much more variable between bouts than within bouts, and did not vary systematically with the age of the breeding birds, territory quality or position on the island. Finally, the breeding of G. scandens was compared to that of mainland Ecuadorian birds studied by Marchant (1959, 1960) and to the long-term studies of temperate passerine species. Breeding in G. scandens is unusually risky in the face of the unpredictability of future rainfall and food supply. Birds breed when there is sufficient Opuntia pollen and nectar, or plant and animal foods produced in response to rainfall, for them to raise young; in addition they have larger clutches in response to apparently higher food levels associated with higher rainfall, and this is more likely to result in the survival of fledglings to independence. The need is stressed for comparative data under different conditions of population density, sex ratio, rainfall and food supply.


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