The small size of many island populations of endangered birds, together with their high density, low genetic diversity and naivety to introduced pathogens, may render them vulnerable to disease epidemics. Such an outbreak occurred in an isolated population of threatened South Island Saddleback Philesturnus c. carunculatus on Motuara Island, New Zealand. In 2002, the population dropped from ~130 birds to ~60 birds, apparently from an outbreak of systemic coccidiosis. However, within three years the population had recovered to > 130 individuals. This appears to be the carrying capacity of the island as areas left vacant after the crash were re-occupied by the end of the third year. Reproductive success of saddlebacks was similar across the three years following the crash and there were no significant differences in clutch size, nesting success or fledgling mass as the population increased. In contrast, feather mite loads increased and leucocyte counts changed significantly in both adult and nestling saddlebacks as the population recovered, suggesting higher population densities may reduce immunocompetence. Although reproductive success did not appear directly affected by population size, our observations indicate increased levels of stress at high densities could render saddlebacks susceptible to recurring disease outbreaks.