Nesting behaviour and factors affecting reproductive success of Velvet Scoter Melanitta fusca breeding at Lake Tabatskuri, Georgia

Abstract

The Velvet Scoter Melanitta fusca is declining globally and the species is classified as vulnerable (VU) on the International Union for Conservation of Nature (IUCN) Red List. Of particular concern is the tiny relict breeding population in the Caucasus, which has been classified as critically endangered. Information on this population remains scarce, and more knowledge of its ecology is required for its effective conservation at its last breeding site in the entire region, Lake Tabatskuri in Georgia. Here, we aim to determine the main drivers of nesting success for the scoters breeding at Lake Tabatskuri, which contribute to the birds’ overall productivity. From 2019–2021, data were collected from all nests on the one breeding island in the lake, during visits made prior to and during incubation, and from camera traps installed by some nests. Results suggested that predator abundance and the frequency of predation attempts had a major impact on nesting success despite the scoters’ defence of their nests. Deploying cameras close to (c. 1 m from) Velvet Scoter nests slightly affected their behaviour, but predation rates appeared to be lower at these nests. In the absence of mammalian predators, Armenian Gulls Larus armenicus (the most numerous breeding bird on the island) and Marsh Harriers Circus aeruginosus predated eggs, which had a significant impact on the outcome of the scoters’ breeding attempts. Predation of nesting females by Marsh Harriers also added to adult mortality and potentially limited production of offspring at Lake Tabatskuri. Continued monitoring of Velvet Scoter abundance and production of offspring at this model site, as well as studying the abundance and impact of the gulls, is essential for determining whether predator control measures are needed to maintain the population.