Influence of climatic and weather factors on the breeding season of mallards (Anas platyrhynchos) in the south of Western Siberia

In recent decades, the climate is warming, which causes a significant change in the time of seasonal phenomena of the annual life cycle of birds, including reproduction. Information on waterfowl is sparse and contradictory. There is no such data for the Western Siberia region. The purpose of this research is to summarize the materials of half a century of observations and to give a conclusion about the influence of climatic warming on the reproductive period of mallards in Western Siberia. The research was carried out on Lake Krotovaya Lyaga (53°43' N, 77°53' E) in the North Kulunda steppe from 1970 to 2018 (with the exception of 1976, 1977, 2008, and 2012-2014). Mallard nests were counted from the beginning of May to the end of July. The total length of the nest-initiation period was 75 days (April 14 - June 27). The date of laying the first egg in 1879 mallard nests is determined by standard calculations (Mednis, 1968). To assess the weather conditions, the data of the nearest weather station (Karasuk) and the North Atlantic Oscillation Indices (NAO), which are available in the public domain, were used. Statistical parametric and non-parametric analyses of the results were carried out using the programs PAST_3.17 and Excel 2010. For the period 1970-2018, the shift of the date of transition of the average daily air temperature through 0 °C to earlier dates was not detected. However, over a shorter period (1979-2018) this indicator had a significant trend with a shift of 2.5 ± 0.8 days for every 10 years (Z = 2.7; p = 0.007). Against this background, the start date of the first clutch, the average date of the egg-laying period (up 30 days), and the start date of the last clutch do not have this trend. The listed dates varied widely in different years. A highly reliable positive relationship of breeding dates with the date of transition of average daily air temperatures through 0 °C and a negative one with the February NAO index was established. The combined effect of these factors (local weather and a large-scale climate index) accounted for 25 % of the variability of the nesting start date and the average annual date of the egg-laying period. The time interval between the date of establishment of positive air temperatures and the beginning of breeding increased in the years with an early, warm spring and decreased in the cold years.