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The opinion regarding morphological changes in S. sylvaticus depending on being in syntopy with the competing sibling species S. tauricus is ambiguous. Most authors claim that allotopic and syntopic populations do not differ. Some of them indicate a tendency that the size of S. sylvaticus increases in allotopic populations, while others claim that such increase occurs in syntopic populations. There is an assumption that the nature of morphological changes in S. sylvaticus undergoes geographic variability and manifests differently in different parts of the range. Overall, the authors suggest that character change is an adaptive response of a subordinate species to the presence of a dominant competitor, so character change as a result of competitive interaction is more likely to occur in S. sylvaticus than in S. tauricus. The research was conducted in the territory of Kharkiv Oblast, Ukraine. During the observation period, from spring 2017 to autumn 2022, 666 specimens of the genus Sylvaemus were caught in 10 selected biotopes. Different types of oak forests, dry and floodplain meadows, riparian vegetation, steppe areas, chalk slopes, field protection forest strips, fields, as well as various ecotones were studied. According to the results of the research, being in syntopy with S. tauricus, S. sylvaticus has statistically significantly higher average values of parameters of exterior characters, which largely complicates the identification of these two species in the field. For the most part, confusion in identification occurs in forest biotopes, namely in dry and fresh maple–linden forests. Only two species are found in the studied oak forests in the territory of Kharkiv Oblast—S. sylvaticus and S. tauricus, whereas S. uralensis occurs only on forest edges. And it is in the oak forests that large specimens of S. sylvaticus are found, which in habitus closely resemble S. tauricus. In open biotopes, S. sylvaticus is closer in size to S. uralensis, often young or moulting individuals are very similar even in fur colour. Individuals of S. sylvaticus with a weakly expressed chest spot, and sometimes barely noticeable, are found in the steppe areas of Kharkiv Oblast. No differences in external characters were found in mice of the genus Sylvaemus that inhabit different biotopes, the morphospaces of the samples overlapped in all three species.

The trapping of small mammals was carried out in 2017 to 2023 in the territory of the Kharkiv Oblast, Ukraine. In total, 19 biotopes were studied: different types of oak forest, dry and floodplain meadows, riparian vegetation, areas of the steppe, field protection forest strips, fields, as well as various ecotones. The trapping of small mammals was carried out using the standard trap-line method on 93 trap lines. With a trapping effort of 17 310 trap-nights, 1295 individuals of small mammals were caught, 689 of which belonged to the genus Sylvaemus. Three species of mice of the genus Sylvaemus were found in the territory of Kharkiv Oblast during the study period: Sylvaemus uralensis, Sylvaemus sylvaticus, and Sylvaemus tauricus. According to the relative abundance, wood mice can be arranged in the following order: uralensis > sylvaticus > tauricus. The cyclicity of annual fluctuations in relative abundance is not noticeable. The similarity of the abundance fluctuation in S. sylvaticus and S. tauricus was revealed, taking into account that these are competing species. S. uralensis and S. sylvaticus lead the top three dominant species in the overall dominance structure of small mammals. The highest relative abundance of wood mice during the study period was recorded on dry meadows, on the border of riparian vegetation with fields and floodplain meadows, on the border of field protective forest strips with fields, and in dry maple-linden-oak forest. S. uralensis belongs to the eurytopic species, is found in almost all studied biotopes, avoids oak forests, occurs on the edges of forests, dominates in abundance in open biotopes, in particular in the steppe, on dry meadows, riparian vegetation, and field protection forest strips. S. sylvaticus is also prone to eurytopy, it is recorded in significant abundance in both open and forest biotopes, mostly prefers riparian vegetation, field protection forest strips, dry coniferous forest, fresh maple-linden-oak forest and forest edges. S. tauricus is found mostly in forest biotopes, in particular in various types of oak forest and on forest edges, it prefers field protection forest strips with an old stand of oak, linden, and robinia. All three species of mice of the genus Sylvaemus are common species.

The reliable identification of representatives of the genus Sylvaemus is compli-cated by the huge coincidence of external and metrical characters, as well as by the fact that in most of their range, mice are in conditions of sympatry or even syntopy Moreover, identification is complicated by opposite clinal variability in size and geographic variability in fur colouration. The species are also similar in karyotypes, so their reliable identification is usually possible only based on biochemical or molecular analysis. The craniological collection of small mam-mals of O. V. Zorya and collection specimens of the Museum of Nature of V. N. Karazin Kharkiv National University were studied. In total, 436 speci-mens of three species of the genus Sylvaemus were analysed: Sylvaemus ura-lensis, Sylvaemus sylvaticus, and Sylvaemus tauricus. Analysed were 14 odon-tometric and 23 craniological characters. Among the metric characters, three odontometrical—width of first upper molar (WM1), length of second lower molar (LM2), length of lower molar row (LM123)—and seven craniometrical characters—length of first upper molar (LM1), length of foramen incisivum (LFI), length of upper molar row (LUM), width of choana (WCH), height of skull including bulla tympanica (HBCB), length of lower molar row (LLM), length of mandible (LM)—made the greatest contribution to the differentiation of mice of the genus Sylvaemus. Width of first upper molar (WM1), length of first lower molar (LM1), length of second lower molar (LM2), length of brain-case (LBC), least interorbital constriction (LIOC), width of braincase (WBC), distance between incisor and M3 (LIM3), length of upper molar row (LUM), condylobasal length of skull (CBL), palatal length (LPP), greatest length of skull (GLS), and length of lower molar row (LLM) are the least variable metric char-acters. Scatterplots linking LLM/CBL, LUM/CBL, and LUM/GLS appeared to be the most suitable for differentiating the specimens, with minimal overlap of species in the morphospace. In north-eastern Ukraine, the best distribution of mice of the genus Sylvaemus was obtained as a result of the analysis based on both odontometrical and craniological characters, when the correctness of the classification was 100%.

Various methods are used to identify representatives of the genus Sylvaemus, most of which are metric. In order to exclude anomalies in further calculations and to minimize allometric variations associated with the growth of animals, it is necessary to take into account age as an important factor of variability. Usually, a number of criteria are used to determine the age and describe the growth of mice: measurements of body and skull parameters, body weight, the degree molar wear, and the weight of the lens of the eye. Length is a more consistent guide for determining the age of mice than body weight, which usually ceases to be directly related to age after reaching adult size. Age can also be determined by the fusion of the epiphysis with the diaphysis in the bones of the limbs and by the degree of development of the thymus. Most often, teeth are used to determine age. Among such age criteria, we distinguish eruption and replacement of teeth sets, tooth growth in length, overgrowth of the tooth pulp cavity, tooth wear, and annual layers in the tooth tissues. Usually, the age of mice is determined by the degree of molar wear. Some authors do not consider this method universal due to individual feeding habits of animals and other environmental factors. The order of wear of tooth rows can sometimes change, and the degree of wear of the right and left tooth rows can also differ. The craniological collection of small mammals of O. V. Zorya, collected in the territory of Kharkiv Oblast, Ukraine, was studied. In total, 198 specimens of three species of the genus Sylvaemus were analysed: Sylvaemus uralensis, Sylvaemus sylvaticus, and Sylvaemus tauricus. Analysed were 14 odontometric and 23 craniological characters. Among the metric characters, four odontometric (LM2, WM1, LM2, LM123) and nine craniological characters (LIOC, LD, LFI, LPP, WCH, GLS, DI, LLM, LM) made the greatest contribution to the age group differentiation of mice of the genus Sylvaemus. Among the features that contributed the most to the differentiation, length measurements prevailed. LLM, LM123, LPP, LIOC, and GLS are the least variable metric characters. It is not possible to divide the sample of mice of the genus Sylvaemus into age groups based on odontometric and craniological character since the ranges of values of metric characters overlap.

The study of the species composition, biological and ecological features of small mammals was carried out for five years (2018–2022) in the surroundings of the village of Rozsohuvata (Kharkiv Oblast, Ukraine). During the study period, nine species of mouse-like rodents and three species of insectivorans were detected. The research area is represented by a ravine-beam system, agrocoenoses, field-protecting forest strips, riparian vegetation around artificial reservoirs and streams, dry meadows, and pastures. Trappings of small mammals were carried out on thirty lines of traps. During the entire period of trapping, the dominant species was Sylvaemus uralensis and in some seasons Sylvaemus sylvaticus dominated. Starting from 2018, the relative abundance increased in autumn, reached a peak in 2020 and declined in the following years. In general, 2019 was the year of maximum abundance and most species were recorded in the summer of this year. The greatest number of species was found in the ecotone between riparian vegetation and field (10 species), as well as in the area of the multi-grass steppe in the beam (8 species). The eurytopic species include Sylvaemus uralensis and Sylvaemus sylvaticus, whereas the stenotopic species is Mus spicilegus. According to the obtained abundance scores, only one species—Myodes glareolus—occurs randomly, whereas the rare species here are Mus spicilegus, Sorex minutus, and Crocidura suaveolens. Six species are non-abundant: Apodemus agrarius, Sylvaemus tauricus, Mus musculus, Microtus levis, Cricetulus migratorius, and Sorex araneus. Only Sylvaemus sylvaticus belongs to common species, and only Sylvaemus uralensis is abundant. It should be mentioned that Cricetulus migratorius, which usually belongs to the category of random or rare species, turned out to be non-abundant in the study region, and it was recorded regularly in almost half of the studied biotopes. Myodes glareolus, which usually prefers oak forest, was extremely rare. The abundance of Apodemus agrarius, which usually belongs to common species in river valleys, was also low. Sylvaemus showed greater ecological valence, dominated in abundance and inhabited most biotopes, except for Sylvaemus tauricus, which began to occur only in the last two years and mainly in forest strips.

The study of the species composition and biotope preferences of small mammals around Kolomak had been carried out for four years (2017–2020). During the study period, 9 species of mouse-like rodents and 3 species of insectivores were found. No Cricetulus migratorius, Terricola subterraneus or Microtus oeconomus were found from the theoretically expected species already known for this area. Around Kolomak, 11 biotopes were investigated, including maple-linden oak forest, agrocenoses, dry and flooded meadows, which are located along the banks of a pond and in a gully-ravine system. The first year of research was in a year of high abundance (2017), and then 9 species were immediately discovered, but species with small abundance, such as Crocidura suaveolens, Sorex minutus, and Micromys minutus, were found in years with a small relative abundance of small mammals. Myodes glareolus, Sylvaemus tauricus and Sylvaemus uralensis are dominant species in the captures. According to the trapping results, 2017 was the year of high relative abundance of small mammals, 2018 was the year of the lowest relative abundance, 2019 and 2020 were years with an average relative abundance. During the study period, 6 species were identified in forest biotopes (Apodemus agrarius, Sylvaemus tauricus, Sylvaemus uralensis, Myodes glareolus, Sorex araneus, and Dryomys nitedula). In ecotones with floodplain biotopes, 8 species were found (Apodemus agrarius, Sylvaemus sylvaticus, Sylvaemus uralensis, Mus musculus, Micromys minutus, Myodes glareolus, and Sorex araneus). Four species (Mus musculus, Sylvaemus sylvaticus, Sylvaemus uralensis, and Microtus levis) were discovered near human settlements. In general, biotopes with the greatest species diversity and number of caught individuals are ecotones of dry and floodplain meadows. In years of high abundance, both species diversity and the number of individuals caught in the oak forest and in ecotones near the pond increased. It should be noted that Myodes glareolus was caught in clear-cuts during the two years (2019-2020) only in the summer of 2020. Earlier, not a single specimen of this species was caught there, although there is a dense weed grass cover in this area and the shrub layer has also grown up in some places, and the clear-cut is surrounded by oak forest.

Since 1957, zoologists of the Kharkiv Sanitary and Epidemiological Station have been conducting annual monitoring of small mammals in Kharkiv Oblast. To assess the current state of populations of mouse-like rodents and shrews, we analysed a dataset for the period from 2000 to 2018. The small mammal fauna was studied at 117 sites in 27 districts (raions) of Kharkiv Oblast. Various types of oak forests, dry and floodplain meadows, riparian vegetation, steppe areas, forest stripes, gardens, fields, and haystacks were examined. Four species of insectivores and 16 species of mouse-like rodents were identified. The dominant and widespread species in the region are Sylvaemus uralensis, Apodemus agrarius, Sorex araneus, Microtus levis, Myodes glareolus, and Sylvaemus tauricus. Some species are less common and in some areas of the region have not yet been identified, including Sorex minutus, Sylvaemus sylvaticus, Crocidura suaveolens, Micromys minutus, Cricetulus migratorius, and Microtus oeconomus. The least common species with single records include Neomys fodiens, Mus spicilegus, Lagurus lagurus, Terricola subterraneus, Dryomys nitedula, and Arvicola amphibius. Indigenous species of the region include seven species, another seven species are those that increased their number and range, three species have recently appeared in the region due to dispersal from adjacent territories, while, on the contrary, the abundance and range of three species have declines. Microtus oeconomus is characterised by a current southward range expansion. Myodes glareolus, which was mentioned in the last century north of our region, is now widely distributed throughout the region and further south. Lagurus lagurus is an invasive species with periodic population outbreaks in the region and currently is rarely recorded.

For many years, research on the species composition of small mammals and monitoring of their abundance on the basis of Donetsk biostation (Zmiyiv district Kharkiv region) have been conducted by students of Kharkov University during summer practice and by employees of the Kharkiv Regional Sanitary-Epidemiological Station. Despite the large amount of accumulated data, recent research did not cover the whole range of habitats typical for this area and, accordingly, modern ideas about the small mammals of these places are extremely incomplete. During 2017–2018, 10 types of habitats were investigated including different types of oak forests, meadows, forest edges, and agricultural lands. Subsequently, 9 species of mouse-like rodents and 3 species of insectivores were recorded, among which Sylvaemus tauricus, Myodes glareolus, Apodemus agrarius, and S. uralensis dominated. Using the method of trapping by vertical cylinders, the species Neomys fodiens was first recorded for the studied territory and Terricola subterraneus, which has not happened here for the past 10 years. It is worth noting that the T. subterraneus avoided getting into the traps of Hero, which were repeatedly placed near its colonies. Sylvaemus uralensis was noted only in floodplain biotopes and did not occur in the oak forests. For some species, in particular, the Myodes glareolus and Sorex araneus, their migration to wetter habitats is indicated in the summer. The largest species of small mammals is noted on the meadows surrounded by the oak forests, if they have not been systematically disposed of. In the summer, the largest share of caught individuals was recorded in agrocenoses, in particular, especially in crops of early grain crops. A rather low number of caught specimens is noted on the edges of the oak forests, which border on the floodplain meadows — there are mainly two types of mouse — Sylvaemus tauricus and S. sylvaticus. Results suggest a significant decline in the abundance of small mammals in 2018 compared to previous year, which was manifested by the complete lack of caught individuals in the spring and a slight increase in numbers in the second half of 2018.