Increasing numbers of a threatened insular population of the Common Raven Corvus corax

Abstract

Until the middle of the 20th century, the Common Raven Corvus corax was an abundant bird in the Canary Islands. However, during recent decades, populations have decreased dramatically in the most densely populated islands, apparently because of changes in land use and other anthropic activities. In the current study, we estimated raven population size and described its breeding distribution on Tenerife, the largest and most populated island. Non-breeding birds were roughly estimated in autumn 2015, then breeding territories were mapped using intensive field research in 2016. The non-breeding population was composed of a minimum of 50 birds, but the real figure could be approximately 65. In total, 28 breeding pairs were located (1.4 pairs 100 km−2), mostly (82.1%) in the Teno massif (28.5 pairs 100 km−2). Non-breeding and breeding populations increased from 2009 to 2016 by 7.6% and 12.8%, respectively. Because no specific conservation actions had been carried out, nor apparent change in threat regime detected, the reasons for these increasing numbers are unclear. Although casual observations suggested that by 2019 it had continued growing to approximately 37 breeding pairs, quantitative studies on human-related mortality factors are urgently required to guarantee the conservation of this still fragile population.