Population status and nest success of the Critically Endangered Mariana Crow Corvus kubaryi on Rota, Northern Mariana Islands

The Mariana Crow Corvus kubaryi is a Critically Endangered species found only on the island of Rota, Northern Mariana Islands. It was extirpated from the neighbouring island of Guam by the introduced brown tree snake Boiga irregularis and the Rota population has been in decline since at least 1995. We identified only 60 pairs present on Rota in 2007 compared with an estimated 117 pairs in 1998, a decline of nearly 50% in nine years. The decline may be linked to proximity to human activities, though more data are needed. We monitored 204 crow nests between the 1996 and 2009 breeding seasons. Crows initiate clutches between August and April. The overall estimate of nest success was 25.7% (n = 204). On average 49% of pairs produced at least one fledgling per season. The mean number of fledglings per pair per year is 0.66. Mean clutch size was 2.6 (n = 82), mean number of nestlings was 1.4 (n = 106), and mean number of fledglings per nest was 1.3 (n = 68). Daily survival rates declined in later years, and increased during the nest cycle. The number of pairs with successful nests did not change during the study period, nor did the number of fledglings per pair. Predation appeared to be the primary cause of nest failure. The breeding season lasted around nine months and pairs re-nested after failure. Predation of adults and juveniles by cats, combined with possible inbreeding depression, habitat disturbance and human persecution appear to be the cause of decline of the Mariana Crow. We strongly recommend a programme of invasive predator control, habitat maintenance, and captive rearing to ensure the species' survival.