Breeding Biology of the Crested Auklet (Aethia cristatella, Alcidae, Charadriiformes) in the Northern Part of the Sea of Okhotsk

Abstract

The breeding biology of the crested auklet, Aethia cristatella, was studied over the period 1989–2019 on Talan Island, Tauiskaya Bay, Sea of Okhotsk. In the early 1990s, the breeding population numbered about 950 000 individuals. In subsequent years, their numbers decreased to 35 000–70 000 individuals in 2016–2017. The onset of egg laying varied from May 18 to June 2 depending on ice conditions, the long-term average being May 22nd. Hatching of chicks lasted from June 19th to July 7th, the average date being June 26th. The weight of hatchlings was 20–40 g (average 31.4). The maximum weight of chicks in the nest averaged between years from 156 to 296 g. By the time of departure to the sea, it amounted to 134–289 g, while the average chick growth rates among the nests varied from 3.5 to 7.4 g/day. Young birds left the island at the end of July to August, at an average age of 30–36 days. Some of them reached the sea, successfully flying off the slope, while the others descended to the ground and significantly lost weight. The average weight of the chicks captured on the ground varied from 109 to 177 g, and in some individuals from 80 to 270 g. There were flying birds among them, but probably those that failed to reach the sea, with a wing length of no less than 120 mm and a weight of no less than 180 g. Their share in favorable years exceeded 60%. Among the others, there were depressive and unhealthy individuals, often still covered with fluff, with a small weight and short wings. In unfavorable years, the vast majority were such, while the total number of the young birds encountered increased multifold. The reproductive success varied from 0 to 90.6% (on average, 35.6), and the egg hatching success ranged from 0 to 95% (on average, 66.1), while the fledging success ranged from 0 to 96.7% (on average, 44.5). The breeding success rates declined markedly beginning in 2007, and since 2014 zero breeding success has become regular. The main limiting factor of the reproductive success of crested auklets was the availability and abundance of their food in Tauiskaya Bay. Over the last few years, the food supply has noticeably worsened due to climatic and hydrological changes in the northern part of the Sea of Okhotsk. Among the land and avian predators that inhabit the Talan Island and contribute to the death rate of crested auklet adults, chicks, egg clutches, and fledging are the peregrine falcon, Steller’s sea eagle, the raven, the slate-baked gull, the ermine, the red fox, and three species of voles. The most noticeable damage to the population is caused by the peregrine falcon, with crested auklets amounting to 30–60% of its bird diet obtained on the island during the entire season.