Breeding biology of the common redstart Phoenicurus phoenicurus (L., 1758) in the south-east of Western Siberia

We analyzed the results of long-term studies (1983-2019) of breeding biology of the common redstart (Phoenicurus phoenicurus) 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°22N, 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 516 nests. The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of Institute of Biology of Tomsk State University (protocol code № 23 of 21.03.2022). 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 10.3 ± 1.4 % and population average density was 11.4 ± 1.6 pairs/10 ha. Nest-box occupation rate in deciduous forests was 4.4 ± 0.5%; in mixed - 15.3 ± 1.3%; in coniferous - 10.4 ± 2.2%, and population density was 4.9 ± 0.6, 17.0 ± 1.5 and 11.6 ± 2.4 pairs/10 ha, respectively. In spring, redstarts appear in last decade of April-first decade of May (21 April -7 May), on average, April 28. The mean long-term date of the earliest egg laying was May 19 (13 May - 27 May); the median laying date was May 29 (23 May - 3 June). No certain long-term trend in the timing of reproduction was detected. There were no cases of two breeding cycles in the study area confirmed by ringing. However, once we observed how 8 days after the departure of fledglings a new clutch was laid in the same nest-box from which fledglings flew out. The mean clutch size is 6.84 ± 0.04 eggs, the mean length of eggs -18.60 ± 0.02 mm, the breadth of eggs -13.83 ± 0.01 mm and the mean volume of eggs - 1818 ± 3 mm3 (n = 2734 eggs). The share (proportion) of successful nests where, at least, one young fledged is 55.0%. From 3081 eggs, 1875 chicks hatched and 1667 fledglings flew out. The success of incubation is 60.9% (hatched chicks / eggs in nests where, at least, one egg was laid), feeding (fledglings / hatched chicks) -88.9%, breeding (fledglings / eggs in nests where at least one egg was laid) - 54.1%, reproductive success in successful nests - 86.0%. Embryonic mortality is 8.3%, partial brood mortality - 5.6% of the total number of hatched nestlings. The mean number of fledglings per successful attempt and breeding attempt are 5.83 ± 0.07 and 3.21 ± 0.13, respectively. Predators ravaged 41.7% of nests, in which 28.3% of eggs in incomplete and completed clutches and 4.2% of nestlings were destroyed. In total, 3.3% of nests were abandoned, and losses amounted to 1.7% of eggs in incomplete and completed clutches and 1.3% of nestlings. Reproductive success, numbers of fledglings per breeding attempt in the group of deciduous and mixed forests are significantly higher than those in coniferous forests, respectively 58.0%, 3.55 ± 0.16 and 42.8%, 2.33 ± 0.25. This is due to higher predation in coniferous forests. Clutch size, volume of eggs and number of fledglings per successful attempt in the group of deciduous and mixed forests and coniferous forests does not significantly differ, 6.88 ± 0.05, 1820 ± 4 мм3, 5.85 ± 0.08 and 6.69 ± 0.09, 1813 ± 6 мм3, 5.73 ± 0.18, respectively. In the European part of the redstart's range, in the latitude range of 48-69° N, clutch size (first breeding cycle) varies between 6.13 - 6.86 eggs (on average, 6.51 ± 0.04 eggs) (See Table 2), which is significantly less than in Western Siberia (6.84 ± 0.04 eggs) (p < 0.001). The average clutch size in the European latitude range (55-60° N) close to our study area is 6.56 ± 0.05 (6.34-6.68 eggs), also significantly less compared to our data (p < 0,001) (See Table 2). Our calculations based on the literature data given in Table 2 showed a significant tendency of increasing the clutch size in latitude range from 48 to 69° N. The dependence of the clutch size (y) on latitude (x) is described by linear regression equation: y = 5.73+0.013x (R2 = 0.24, p = 0.03, n = 20). The egg volume in Europe varied in the limits 1 700 - 1 899 мм3 (in average 1791 ± 16 mm3) and was not connected with latitude (r = 0.18, our calculations on the data are given in Table 2). The mean egg volume in Europe and Western Siberia are not significantly different. Breeding success (fledglings / eggs in nests where, at least, one egg was laid) in Europe varies in limits 41.5 - 82.8%, fledglings per successful attempt and breeding attempt are 5.73 ± 0.08 and 4.31 ± 0.16, respectively (our calculations on the literature data are given in Table 3). Вreeding success of the redstart in Western Siberia is at the lower limit of the indicator known for Europe. The number of fledglings per attempt in Western Siberia was significantly less than that in the European part of the range (p < 0.001). We explain it by a higher level of predation in Western Siberia compared to most sites in the European part of the redstart's range. The number fledglings per successful attempt in Europe and Western Siberia is not significantly different.