Socotra Cormorants (Phalacrocorax nigrogularis) are a regionally endemic, locally abundant species restricted primarily to the Arabian Gulf and coastal Oman. The species has declined since the 1980s and is currently categorized as Vulnerable by the International Union for Conservation of Nature. Breeding phenology, breeding performance and variation in breeding population size were studied on Siniya Island, the largest colony in the United Arab Emirates. Laying dates were between 13 September and 6 October during the 2011- 2015 breeding seasons. Incubation was estimated to be 24-27 days, and clutch size ranged from 2.21-2.79 eggs/ nest. Hatching success ranged from 58.71 ± 5.85 in 2011 to 81.76 ± 4.86% in 2012. The total population varied over the 5 years of study from 28,152 ± 3,780 pairs in 2011 to 41,568 ± 3,761 pairs in 2014. Population estimates using density-area calculations were closely aligned with ground counts. The use of a drone with a mounted camera greatly improved the counts in 2015. The Socotra Cormorant breeding population on Siniya Island appears to be stable over the short term with annual fluctuations comparable to other cormorant species. Thus, our data suggest the breeding population on Siniya Island could have surpassed that of other colonies in the Arabian Gulf, underscoring its global significance.