Contrasting ecological conditions may affect the distribution, abundance and impact of parasites and predators throughout the ranges of hosts and prey. Such patterns are evident on the archipelagos of Hawaii and the Galapagos, which vary in their distribution and abundance of avian parasites within and across islands. Previous research has documented higher intensity of parasitic fly larvae (Philornis downsi) in nests of Darwin's finches on elevated islands of the Galapagos. Here we examine P. downsi intensity and predation in 71 nests of Darwin's small ground finch (Geospiza fuliginosa) on Floreana Island. We found significant differences in parasite intensity, nest predation and clutch size between the lowland (0–100 m) and highland (300–400 m) habitats. Lowland finch nests had few P. downsi parasites (mean of 8 per nest), high nest predation (44% of nests) and large clutch size (3.4). Highland finch nests showed the opposite pattern, with many P. downsi parasites (40 per nest), low nest predation (17%) and small clutch size (2.5). This study suggests that the impacts of an introduced parasite are limited by its niche requirements and resource availability within and across islands. Our findings also imply that the vulnerability of bird populations to introduced parasites and predators is linked with variation in life history strategies across habitats.

The Critically Endangered mangrove finch Cactospiza (=Camarhynchus) heliobates is now confined to Isabela Island in the Galápagos Islands and is exclusively found in mangrove forests. Formerly it occurred also on neighbouring Fernandina Island, but is apparently extinct there. The population size and ecology of the species was relatively unknown until 1994. We conducted surveys, habitat assessments and behavioural observations of the species between 1996 and 2000. Although Isabela Island has approximately 760 ha of mangrove forests, breeding was confirmed at only two sites, comprising 32 ha in total, on the north-western coast. Our estimate of the population in these two areas is 100 individuals. Additionally, 3–5 territories (which probably contained breeding individuals) were discovered on the south-eastern coast. A comparison of habitat parameters showed that tree height and amount of dead wood were significantly higher within than outside territories, and these are therefore likely to be important habitat components for this species. As considerable structural differences were detected between the two sites holding the main populations and all other mangrove stands on Isabela, it seems possible that the latter are sub-optimal habitat. We therefore conclude that one of the reasons for the very limited distribution of the species is habitat degradation caused by hitherto unknown factors.