Distribution and abundance of Rufous-tailed Robin Luscinia sibilans and Siberian Blue Robin Luscinia cyane in the south-east of Western Siberia

Rufous-tailed Robin Luscinia sibilans (Swinhoe, 1863) and Siberian Blue Robin Luscinia cyane (Pallas, 1776) are taiga species of Siberian-Chinese origin that expanded westward into the Altai-Sayan Ecoregion and the West Siberian Plain during the first half of the 20th century. However, they remain poorly studied to date. This paper aims to analyze the dispersal routes, current distribution, habitat preferences, and population dynamics of these nightingales at the westernmost edge of their ranges in southeastern Western Siberia over a 50-year period. The materials were collected between 1979 and 1993 in the northern low mountains of the Kuznetsk Alatau (55°01'-55°29'N, 88°07'-88°29'E) and from 1974 to 2024 in 14 districts of Tomsk Oblast (56°07'-59°42'N, 80°46'-89°03'E). These areas are grouped into five natural-geographical zones along the dispersal gradient of the nightingale, extending from east to west and from south to north: Prichulymye, Tom-Chulym interfluve, Priobye, Ob-Vasyugan interfluve, and Ket-Tym interfluve. Bird counts were conducted using the method developed by Yu.S. Ravkin, as well as the absolute mapping method on model sites. The total length of the routes was approximately 1.600 km in the Kuznetsk Alatau and more than 3.800 km in Tomsk Oblast. In the Altai-Sayan mountain region, since the known distribution boundary in the Northeastern Altai, only the Siberian Blue Robin has expanded westward to the Northern and Northwestern Altai, reaching as far as the Tigirek and West Altai reserves over the past half-century. In the Kuznetsk Alatau, which lies entirely within the breeding range, both nightingale species actively colonize dark coniferous, aspen-fir (chernevaya) taiga, and mixed forests in low- and mid-mountain areas. Additionally, the Siberian Blue Robin nests in the pre-goltsy crooked forests of the highlands at altitudes up to 1000 meters. Nightingales expanded eastward from the West Siberian Plain into the southern taiga of Yenisei Siberia at the beginning of the 20th century. Between the 1940s and 1970s, they advanced far north into the middle taiga, reaching the Turukhansk region (62°20'N, 89°00'E) from the northern foothills of the Eastern Sayan along the Yenisei River valley, including the right bank and spurs of the Yenisei Ridge. Meanwhile, the Rufous-tailed Robin extended its range to the borders of the northern taiga (64°26'N, 87°32'E). Long-term observations revealed that their dispersal eastward into the southern taiga of Western Siberia (Tomsk Oblast) follows two main 'corridors' from Krasnoyarsk Krai, originating in the taiga of the northern foothills of the Eastern Sayan and the Kemchug Upland along the Chulym River valley; and another northward from Kemerovo Oblast along the northern spurs of the Kuznetsk Alatau and the Tom-Chulym interfluve. A more northern colonization route westward along the Ob-Yenisei watershed and the Ket River valley appears unlikely. In Tomsk Oblast during the 1960s to 1990s, the Rufous-tailed Robin and Siberian Blue Robin inhabited only the southeastern part of the region, within the zone of subtaiga forests and southern taiga, extending from the Middle Prichulymye and the Tom River basin to the Ob River valley. The stable northern limit of their distribution reached 57°N, with only isolated sightings recorded as far north as 58°N (See Fig. 1). To the west, their range extended in a narrow “tongue” along the left bank of the Ob River up to 82°E. They were not observed south or west of the borders of Tomsk Oblast on the West Siberian Plain. Since the early 1990s, the Rufous-tailed Robin has exhibited noticeable fluctuations and a contraction in its range. Its current distribution remains stable only in the Prichulymye, extending northward to 58°N. Westward, sightings are not recorded annually, are confined to the Tom River basin (84°-85°E), and are nearly absent in the Priobye. In contrast, the Siberian Blue Robin’s nesting range has expanded significantly. In the Priobye and on the right bank of the Ob River, it has reached the boundary of the middle taiga at 59°N. In the Ob-Vasyugan interfluve, the Siberian Blue Robin has recently extended its range westward to 80°E. Furthermore, in 2018-2019, it was recorded nesting much farther south in the subtaiga mixed forest zone near Novosibirsk, and in July 2024, it was found on the Turtas River in the Uvatsky District of Tyumen Oblast - 450 km west of its previously known sites in Tomsk Oblast. A consistently high nesting density of Rufous-tailed Robin (ranging from 10 to 45, with a mean of 28 individuals/km2) and Siberian Blue Robin (ranging from 11 to 40, with a mean of 30 individuals/km2) has been observed over many years, along with the greatest diversity of inhabited locations, in the northern low mountains of the Kuznetsk Alatau. These species prefer various habitats within dark coniferous and aspen-fir (chernevaya) taiga, as well as fir, coniferous-small-leaved, and aspen forests on mountain slopes and along river valleys. In some seasons, the abundance of Rufous-tailed Robin reaches 54-65 individuals/km2, while that of Siberian Blue Robin reaches 50-60 individuals/km2. In the southern taiga of Tomsk Oblast, the Rufous-tailed Robin was abundant (815 individuals) only during the 1960s to 1980s in the Prichulymye and the Tom-Chulym interfluve. Over the past 50 years, significant fluctuations in population numbers have been observed, including complete absences of sightings for several years near the western range around Tomsk and in the Priobye. Population density nowhere exceeds 1-8 individuals per km2 (See Table 1), and from the Prichulymye westward to the Tom River valley, it steadily decreases by a factor of 2 to 3. The Siberian Blue Robin is distributed more evenly. From the 1960s to the 1990s, its abundance in the Prichulymye ranged from 13 to 30 individuals per square kilometer, while in the rest of the territory up to the left bank of the Ob River, it ranged from 2 to 12 individuals per square kilometer. During a 2- to 3-fold decline in population between 2000 and the 2010s, it remained common in the Prichulymye (2-9 individuals/km2) and in the Tom and Ob basins (1-4 individuals/km). Over the past 5 to 10 years, nesting density throughout the southern taiga of Tomsk Oblast has increased three- to sixfold, reaching 15 to 35 individuals per square kilometer (See Table 2). A significant expansion in the diversity of habitats occupied by the species has also been observed, including floodplain landscapes along the Ob River and the river valleys of the Ob-Vasyugan interfluve. During periods of high abundance, the Rufous-tailed Robin forms dense nesting groups, while the Siberian Blue Robin establishes linear vocal-nest groupings (parcels) consisting of 2-5 pairs with highly synchronized breeding periods. These birds persist in the same habitats for several consecutive years. This behavior represents an adaptation of nightingales - late-arriving long-distance migrants - to successful nesting in the peripheral parts of their range. When abundance decreases, the distances between nightingale nesting sites increase, and parcel nesting becomes less pronounced, with single pairs becoming more common; this pattern is more typical of the southern taiga in Tomsk Oblast. In the Kuznetsk Alatau, all Rufous-tailed Robin nests (n = 45) were located 20-300 m apart (mean 128 ± 180 m), with nests within dense groups spaced 20-140 m apart (mean 75 ± 92 m). Distances between groups ranged from 400 to 600 m. In linear groups of Siberian Blue Robins, nests (n = 16) and territorial singing males (n = 11) were observed 70-200 m apart (mean 114 ± 106 m), while single pairs were located 200-400 m from the linear parcel groups. Currently, favorable nesting conditions with a high population density of Rufoustailed Robin and Siberian Blue Robin - comparable to the optimal densities observed in their southern Far East ranges - are fully established in the mountain-taiga landscapes of the low mountains of the Kuznetsk Alatau. These conditions are still developing at the edge of their distribution in the flatland taiga of southeastern Western Siberia. The article contains 1 Figures, 2 Tables and 81 References. The Authors declare no conflict of interest.

Keywords

Authors

References