New data on the distribution of the Red-throated thrush (Turdus ruficollis Pallas, 1776) in the northeastern border of its range

The report presents new data on the distribution and biology of the Red-throated thrush Turdus ruficollis Pallas, 1776 in Western Yakutia, where now lies the northeastern boundary of the species range. This species was not observed within the Sakha Republic (Yakutia) territory until the early 70-ies of the twentieth century. The first Red-throated thrush in Yakutia was discovered in 1974 near the town of Olekminsk (an adult female was caught on 13 Jul with a breeding spot at the vascularization stage). For the first time, breeding of the Red-throated thrush in the Sakha Republic (Yakutia) territory was confirmed by AP Isaev in 2002, in the middle reaches of the Milyuk River (SouthWestern Yakutia). To date, new data on this species distribution have been accumulated. We obtained information on the density of Turdus ruficollis Pallas population during bird counting according to the method of YS Ravkin (Ravkin, Livanov, 2008) in the summer of 2009-2012 in Western Yakutia (Fig. 1). On the registration route, all encountered birds were recorded. We identified their species, the number of individuals and approximate distance from the recorder to the recorded birds, and estimated the length of the route in kilometers. The population density of birds (N) of each species in individuals per 1 square kilometer of the area was determined according to the formula: N of the species = ((n1 x40) + (n2 x 10) + (n3 x 3) + n4)/L, where n1-n4 - number of individuals encountered in the detection bands, respectively, 0-25 meters (close), 25100 (not far), 100-300 (far) and 300-1000 meters (very far); 40, 10, 3 and 1 - conversion factors; L - the length of the registration route in kilometers. In 2009, we discovered the Red-throated thrush in the Western part of the Lena plateau during bird counts in the Djerba River middle reaches (the left tributary of the Lena River). Broods with insecurely flying juvenile birds were noted in this area, from 10 to 13 Jul, 2009. Also, we encountered 2 Red-throated thrushes in the Chayanda River upper reaches from 16 to 17 Jul, 2009. A nomadic flock of these birds was seen on 13 August, 2010, in the vicinity of Mirny. Two Red-throated thrush pairs showing breeding behavior were sighted at the same site in 2011 (the first decade of July). We observed birds of this species near the Pilka River mouth (the right tributary of the Lena River) in late July, 2011. The Red-throated thrush as a breeding species was found in Chamcha village suburbs (the middle Nuya River) during routing bird counts in the western part of the Lena-Vilyui interfluve on the highway Lensk-Udachny. The population amounted to approximately 9 individuals/km² on the forest slopes of the river valley (the Lena left tributary) in mid-June. The Red-throated thrush nest was found in a mixed pine-larch forest on 14 June, 2012. Red-throated thrushes were not rare birds (about 4 individuals/km²) during the counts on 24 June, 2012 in larch forests with admixture of pine trees on the valley slope of the Vilyui river right bank (the Svetlinskoye Reservoir), where its nest was found 8 m from the gravel road on the right bank of the Reservoir (the middle Vilyui River) on 24 June, 2012. By the moment of finding, the female had been incubating 5 eggs. In recent years, the Red-throated thrush has been also noted in the mountains of southern Yakutia, the Aldan plateau and on the Eastern spurs of the Zverev Ridge, where the male with a brood of insecurely flying juveniles were sighted on 29 June, 2015. The Red-throated thrush was not reported as a breeding bird before the early twenty-first century in the territory of Yakutia. Currently, this species partially inhabits the southwestern and southern part of the Sakha Republic (Yakutia), but birds are encountered sporadically. Apparently, the dispersal of the Red-throated thrush occurred from West to East. Most likely, this distribution process has not finished yet and is continuing. The paper contains 1 Figure and 19 References.

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