State of Populations and Breeding Biology of the Horned Puffin (Fratercula corniculata Alcidae, Charadriiformes) in the Northern Part of the Sea of Okhotsk

The breeding biology of the horned puffin, Fratercula corniculata, was studied in the years 1989–2019 on Talan Island, Tauiskaya Bay, Sea of Okhotsk. Even though the estimates of the population size of the horned puffin made in the early 1990s and in 2007–2008 (90 000–100 000 and 75 000–112 000 individuals, respectively) failed to reveal the dynamics due to the fact that different study methods were applied, data on the nesting density in these periods indicated an increase by more than 30%. In subsequent years, higher bird densities were recorded on the surface of model screes. Taking into account these data, the maximum numbers of horned puffins on the island could reach 166 000 individuals. Depending on the ice cover conditions, the dates of the onset of egg laying varied between June 6 in 1990 and June 17–18 in 1987, 1999, and 2001, the average long-term date being June 11. Chicks started hatching in different years between July 17 to 29, the average date being July 22. The weight of hatchlings varied from 35 to 53 g (mean 47.2). The average weight of monthly chicks in 1999–2006 ranged from 145 to 329 g, and in some individuals from 80 to 410 g. The average age of the young birds captured on the ground was 44.7 days. The first individuals were recorded in different years at the end of August (August 27–30) and at the beginning of September (September 2). According to indirect estimates, fledglings left nest burrows between September 10 and 20, ending in late September to early October. The average weight of fledglings heading to the sea in 1987, 1988, 1989, and 1991 was 349, 338, 347, and 318 g, respectively. The wing length varied from 153 to 156 mm. The reproductive success of horned puffins varied from 10.3 to 90.5% (on average, 56.4%); the hatching success, from 19.8 to 95.2% (on average, 75.5%); and the fledging success, from 26.7 to 100% (on average, 74.6%). In the long-term aspect, the breeding success was most often quite high, with the exception of 2001, 2009–2012, and 2014–2015, when this indicator was lower than the long-term average. Among terrestrial mammals, red foxes cause the main threat to adult horned puffins living on the island, as well as to their clutches and chicks descending to the sea, but foxes do not render a significant impact on the state of the horned puffin population. Due to their small numbers, these predators control only certain parts of the island, while horned puffins are dispersed almost throughout its territory, where in most cases they are safe from fox predation. Voles act as utilizers of already abandoned eggs. Slaty-backed gulls and ravens do not attack adult horned puffins, but willingly eat adult birds found dead, as well as dead or still living chicks during their descent to the sea. Adult birds are also taken, albeit rarely, by Steller’s sea eagles.