Breeding biology of the great tit Parus major (L., 1758) in the south-east of Western Siberia

We analyzed the results of long-term studies (1983-2019) of breeding biology of the great tit (Parus major) in the sub-taiga forest subzone of the south-eastern part of Western Siberia. Data were collected in the surroundings of Tomsk (56°28'N, 84°54'E, 90-100 m a.s.l.), the village Kireyevsk (Tomsk oblast, 56°22'N. 84°05'E, 90 m a.s.l.) and the village Lomachevka (Kemerovo oblast, 56°08N, 86°50'E, 190 m a.s.l.). In total we examined 297 nests, 2194 eggs were measured. Nest-boxes were placed in deciduous (aspen-birch and birch), mixed (pine-birch-aspen) and coniferous (pine and Siberian pine) forests. The density of nest-boxes in all types of forests was 107/10 ha. Nest-box occupation average rate was 4.3% and population average density was 4.7 pairs /10 ha. In deciduous forests tits occupied nest-boxes more often (6.3%) than in mixed (3.6%). and coniferous forests (4.0%). The mean long-term date of the earliest egg laying was May 3 (20 April-14 May); the median laying date was May 12. A long-term advancement of laying dates was revealed (r = - 0.54, p < 0.05). That was consistent with a significant trend to warming in April (r = 0.63, p < 0.05). The dependence of date of earliest egg laying (y) on year (x) is described by linear regression equation: y = 574. 6 - 0,27x (R2 = 0.27, p < 0.01, n = 27). Thus, the annual shift in the beginning of egg laying averaged 0.3 days. Full clutches started no later than 30 days from the date of appearance of the first egg in the season contained 7-16 eggs. The mean clutch size is 11.92 ± 0.12 eggs (n = 202) (See Table 1). The clutch size decreases in the row (line): mixed (12.59 ± 0.15) - coniferous (11.77 ± 0.22) - deciduous forests (11.15±0.23). In all comparison pairs the differences are statistically significant (See Table 2). The clutch size decreased along breeding season and amounts to 0.62 eggs per decade. The dependence of the clutch size (y) on the date of appearance of the first egg (x) is described by the linear regression equation: y = 12.41 - 0.062 • x (R2 = 0.28,p < 0.001). The mean length of eggs (L) 18.60 ± 0.02 mm, the maximum diameter of eggs (B) 13.83 ± 0.01 mm. The volume of eggs calculated from the formula: V = 0.51 TB2 [18] was 1747 ± 3 (1234-2151) mm3. The egg sphericity index (shape index) calculated from the formula: S = (B/L) 100 [19] is 76.21 ± 0.02 (65.50-84.42) %, CV=3.83%. The volume of eggs and the index are negatively correlated (r = - 0.15, p < 0.05), so there is a weak but statistically significant tendency to increase the sphericity index of eggs with a decrease in their volume. The volume of eggs in contrast to the size of the clutch is maximum in deciduous forests and decreases in the row (line): deciduous (1761 ± 6 mm3) -coniferous (1750 ± 7 mm3) - mixed forests (1740 ± 4 mm3). In pair deciduous - mixed forests the differences are statistically significant (p < 0.01) (See Table 2). 12.2% females laid second clutches after a successful first breeding cycle. This is noticeably less compared to European populations of the species located at latitude similar to the study area: 28.1% at Ryazan region [6] and 63.4% at Moscow region [7]. The share (proportion) of successful nests where, at least, one young fledged in first, replacement and second clutches is 67.7%. From 2407 eggs, 1773 chicks (nestlings) hatched and 1519 fledglings flew out. The success of incubation is 73.7%, (hatched chicks /eggs in nests where at least one egg was laid), feeding (fledglings / hatched chicks) - 85.7%, breeding (fledglings / eggs in nests where at least one egg was laid) - 63.1%. Predators ravaged 16.0% of nests, in which they destroyed 10.3% of eggs and 4.2% of chicks. 12.1% of nests were abandoned; losses were 6.8% of eggs and 2.1% of nestlings. The wrynecks (Jynx torquilla) threw out 4.3% of the nests; losses were 3.0% of the eggs. Embryonic mortality in nests survived to hatching was 7.9%, partial mortality of nestlings was 8.0% of total number of hatched nestlings. Reproductive success in successful nests (first, replacement and second clutches) was 84.1%, the embryonic mortality -7.9%, and the partial mortality of chicks - 8,3%. The maximum breeding success is observed in mixed forests (66.9%), the minimum is in deciduous forests (55.9%), and the value of the index in coniferous forests is an intermediate values (62.2%). The mean number of fledglings in first, replacement and second clutches per successful attempt and breeding attempt are 10.03 ± 0.18 and 6.77 ± 0.35, respectively. The maximum brood size is observed in mixed forests (10.54 ± 0.22), the minimum is in a deciduous one (9.06 ± 0.36), and the value of the index in coniferous forests is an intermediate values (9.89 ± 0.40). A similar trend is observed in respect to the number of chicks per breeding attempt: mixed forests (7.85 ± 0.48), deciduous forests (5.09 ± 0.60), and coniferous forests (6.68 ± 0.79) (See Table 3, 4). In the European part of the great tit's range, in the latitude range of 55-60° N close to our study area clutch size (first breeding cycle) varies in limits 8.20-11.64 eggs [6, 25, 35, 37-44] and according to our calculations the average is 9.69 ± 0.18 eggs, which is significantly less than in Western Siberia (11.92 ± 0.12 eggs) (p < 0.001). The mean egg volume in European population in latitude range of 55-60° N varies in limits 16541712 mm3 [41, 43, 46-48] (n = 9), and according to our calculations the average is 1675 ± 7 mm3, which also is significantly less than in Western Siberia. The overall breeding success in first, replacement and second nests (fledglings / eggs in nests where, at least, one egg was laid) in the latitude range 50°-67° N in Europe varies in limits 41.6-73.9% [6, 8, 25, 43, 44, 50-56]. According to our calculations this index the average is 62. 0% (n = 20), which almost coincides with our data for Western Siberia (63. 1%). In Europe, in the latitude range of 55-60° N the brood size in first breeding cycle varies in limits 6.81-10.20 [6, 7, 37, 52, 54, 57] and according to our calculations the average is 7.72 ± 0.33 (n =10), which is significantly less than that of the species in our study area (10.03 ± 0.18) (p < 0.001). The observed differences in brood size (2.31 chicks) are close to those of the clutch size in the European part of the species' range and the southeastern part of Western Siberia (2.23 eggs). According to our calculations, the number of chicks per attempt in first breeding cycle in the European populations of the species in the specified latitude interval is 6.24 ± 0.38 (n=10) [6-8, 25, 39, 44, 52, 54, 55, 57] and does not differ significantly from our data for Western Siberia (6.77 ± 0.35). The paper contains 4 Tables and 57 References. We thank A.V. Artemyev for help in writing the paper. The Authors declare no conflict of interest.

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