Population status and nest success of the Critically Endangered Mariana CrowCorvus kubaryion Rota, Northern Mariana Islands

Abstract

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.

Invasive species and Pacific island bird conservation: a selective review of recent research featuring case studies of Swinhoe’s storm petrel and the Okinawa and Guam rail

“…When seed loads are too high, however, these birds may become victims of fatal entanglement, entrapped by an excess of seeds stuck in their feathers that inhibit their ability to fly (Walker 1991;Burger 2005).…”

“…A translocation program to re-introduce the species to Guam failed due to unsustainable predation by invasive predators (BirdLife International 2015; L. Zarones in litt.). Recent research has documented a steep (*50 %) decline in the remaining crow population, with only *60 pairs remaining in (Zarones et al 2014. Cat predation is a major cause of Mariana crow adult mortality and nest failure, contributing to low (*26 %) reproductive success (Zarones et al 2014).…”

“…Recent research has documented a steep (*50 %) decline in the remaining crow population, with only *60 pairs remaining in (Zarones et al 2014. Cat predation is a major cause of Mariana crow adult mortality and nest failure, contributing to low (*26 %) reproductive success (Zarones et al 2014). Cat and other predator control, habitat maintenance, and captive breeding are urgently needed to prevent the imminent extinction of the Mariana crow.…”

Reproductive success of Mariana Swiftlets (Aerodramus bartschi) on the Hawaiian island of O'ahu

“…Rota, one of the proposed reintroduction locations for Mariana Swiftlets, is currently free of brown treesnakes, but introduced rats (Rattus spp.) and feral cats (Felis catus) are suspected or known predators of eggs, nestlings, and adults of endangered birds on the island (U.S. Fish and Wildlife Service 2007, Berry and Taisacan 2008, Zarones et al 2015. Therefore, understanding the responses of swiftlets to potential predation by these invasive species is important when planning translocations.…”

“…Fish and Wildlife Service , Berry and Taisacan , Zarones et al. ). Therefore, understanding the responses of swiftlets to potential predation by these invasive species is important when planning translocations.…”