Biology of the Grey heron Ardea cinerea Linnaeus, 1758 and its role in biocenoses in the south of the taiga zone of Western Siberia (Tomsk region, Russia)

In recent years, the Grey heron Ardea cinerea has been expanding its range in the south of the taiga zone of Western Siberia. In the Tomsk region during the XX-XXI centuries, it went from being a rare vagrant to a nesting species. The Grey heron is included in the regional Red Book as a rare species with insufficiently studied biology. Interest in it is also associated with its biocenotic role - the impact on soils and vegetation. It can also be an indicator of climate change, since its expansion to the north coincides with the trend of climate warming in Western Siberia. The aim of this work was to study the biology of the Grey heron in the south of the taiga zone of Western Siberia - migration, seasonal dynamics, nesting biology, biocenotic role in ecosystems. Observations of migration and seasonal dynamics were carried out from 1998 to 2023. In spring we used the method of daily 2-hour morning observations in April-May. In summer and autumn (from July to October) we carried out route-area counting of birds in the same place with a frequency of once every 5-7 days. A total we have 15 seasons in spring and 8 in summer-autumn. The main observation area was located in the vicinity of the village of Kolarovo (56°19'47.29" N, 84°56'49.61" E) on the Tom River (right tributary of the Ob River) 15 km south of Tomsk. Additionally, in 2002-2003, observations were conducted on the Ob River in the southern (Kozhevnikovo District), middle (Kolpashevo District) and northern (Aleksandrovo District) parts of Tomsk region. The southern points (Tomsk and Kozhevnikovo districts) corresponded to the subtaiga zone, the middle and northern points corresponded to the southern and middle taiga of Western Siberia, respectively. Nesting biology was studied at a Heron colony discovered on an island near the village of Kolarovo. We counted nests, colony structure, and clutch size (based on the eggs shells under the nests). Since some of the trees in the colony were dried out and dead, which is associated with herons, we assessed and compared the pH and chemical composition of the soils (phosphorus content, exchangeable bases etc.) under different trees in the colony (See Fig. 1) - long-used (dead), still alive and outside the colony (control area). The study showed that the arrival dates over the past 25 years have shifted by about 2 weeks towards an earlier appearance - from April 25 in the 2000s to April 5 in the 2020s (See Table 1). The trend is confirmed by the Kendall correlation (0.61;p < 0.05). One of the reasons for this is climate warming, another is the formation of a large colony, to which the birds tend to return as quickly as possible, whereas at the beginning of the observations the colony was just appearing. It was also shown that the density of birds on the lakes during feeding in spring increased by 3-10 times compared to the early 2000s (See Table 1). The number of nests in the identified colony was 81. They were located on 54 trees, mainly on bird cherry trees. There was 1 nest on 38 trees, 2 on 9 trees, 3 trees had 3 nests, and 4 on 4 trees. The average distance between nests was 4.28 ± 0.48 meter (from 0.5 to 9.7 m). The height of the nests above the ground was 7,26 ± 0.2 m. Chicks hatched in late May - early June. Clutch size was 3,91 ± 0.13 (n = 23). We found a badger Meles leucurus settlement under the colony on the island. The conditions here were favorable for it due to the possibility of eating fish that fell during feeding and probably chicks (we found heron bones). During the inspection of the colony, we found dace Leuciscus baicalensis and perch Perca fluviatilis. It was also shown that the excretory activity of herons negatively affects the properties of the soil, leading to its acidification and the death of the trees on which the nests are located. Compared to the control area (out of colony), the phosphorus content in the upper horizons of the soil under the nests is 3.5 times higher. The acidity indicators differed between the background and the control by 1.5 times. Under the nests 4.3 and in the control -6.1 units, respectively (See Table 2). Despite the negative impact, the problem is local in nature and does not cause significant damage. The article contains 2 Figures, 2 Tables and 35 References. The Authors declare no conflict of interest.

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