The Breeding and Feeding Characteristics of Darwin's Finches on Isla Genovesa, Galapagos

The breeding and feeding of four species of finches were studied on Isla Genovesa, Galapagos from January to May 1978. Food supply and finch diets were studied in the nonbreeding season (November) as well. The species in order of decreasing body size were Geospiza magnirostris, G. conirostris, G. difficilis, and Certhidea olivacea. Initiation of breeding was not a simple function of body size; the medium—sized cactus finch, G. conirostris, started breeding first, followed by G. difficilis and G. magnirostris in that order. Eggs of G. conirostris were first laid within °1 wk of the first rains. Certhidea laid a maximum of three clutches, G. magnirostris and G. conirostris laid up to four, and one pair of G. difficilis laid five. Similarities among all four species were found in incubation periods (10—14 d, mode 12 d), nestling periods (11—17 d, modes 13—15 d), and uniform success of clutches of different sizes. Similarities among the three Geospiza species were found in the interval between successive clutches (°30 d); the modal size of clutches (four eggs); seasonal variation in clutch sizes, in which average size was largest among second clutches and smallest among fourth clutches; egg and nestling production; and the average interval between fledging from one nest and egg laying by the parents in the next nest. This average interval was 1—4 d, but some pairs of all three species started laying 5—6 d before the nestlings of the previous brood had fledged. The species differed in the number of fledglings produced per pair; from the first three clutches G. difficilis produced 9.5 ± 0.5 per pair, G. conirostris produced 7.9 ± 0.7, G. magnirostris produced 5.1 ± 0.9, and Certhidea produced 3.2 ± 1.5. The relatively low success of G. magnirostris was attributed to predation of nestlings by owls (Asio flammeus), which occurred most frequently within 1 wk of expected fledging. The low success of Certhidea is attributed to disturbance at the nest by G. difficilis and mockingbirds (Nesomimus parvulus), and possible predation by mockingbirds. Compared with finch species in a climatically similar region of the Ecuadorian mainland, Darwin's Finches have large clutches, a much longer nestling period, higher nesting and breeding success, but similar, and low, incidences of egg infertility. Certhidea fed nestlings on an exclusively arthropod diet, whereas the three Geospiza species fed nestlings on a mixed diet of arthropods, fruits, and seeds. All Geospiza species fed size selectively on spiders in proportion to their average body and beak size. Diets of the three Geospiza species were largely overlapping throughout the breeding season. Differences reflected the differences in beak sizes. Seeds of Bursera graveolens, which are difficult to crack, were absent from nestling diets of (the smallest) G. difficilis, and most frequent in nestling diets of (the largest) G. magnirostris. Only G. conirostris fed nestlings on parts of Opuntia helleri (cactus). Diets of the three species diverged markedly in the nonbreeding season, at which time they clearly reflected the adaptations of beak sizes and shapes. The results are discussed in terms of the strong seasonality in food production and availability, coupled with interannual variation in amount, onset, and pattern of rainfall, and hence food production. It is concluded that adaptations for food gathering in the nonbreeding season, when food is scarce, have indirect influences upon reproductive traits, resulting in small but significant demographic differences between species.