Data on spring phenology and reproductive parameters of the Ross's Gull were collected in 1982–2013 on the delta of Lena River, northern Yakutia (71°42′–74° N, 120°–129°30′ E). During this period, significant changes in weather occurred, the mean annual temperature (F1,27 = 7.74, p < 0.04), the mean June temperature (F1,27 = 11.75, p < 0.002), the summer temperature (F1,27 = 9.13, p < 0.005) increased, and the mean daily air temperatures crossing 0°C shifted to earlier dates (F1,26 = 14.73, p < 0.001). However, the duration of the period with positive temperatures slightly increased (F1,26 = 3.53, p < 0.07). The arrival of Ross's Gulls to the Lena Delta depended by mean June temperatures (r = –0.48, p < 0.01), minimal June daily temperatures (r =–0.70, p < 0.001), and dates of snow melting (r = 0.62, p < 0.05).
The maximum amplitude of fluctuations in eggs numbers among all arctic gulls was revealed for complete clutches of the Ross's Gull. In more northern regions, this amplitude was higher. The clutch size was negatively correlated with dates of clutch initiation, the females that began to nest earlier showing larger clutch sizes (r = –0.85, p < 0.002). The clutch size decreased in years with lower temperatures of the prenesting period, after gulls arrival and before the onset of breeding (F1,172 = 27.31, p < 0.00001), and after relatively colder June temperatures (F1,172 = 8.86, p < 0.003).
The reproductive effort of the Ross's Gull depended negatively on the temperature of the prenesting period. In cold years, the dates of the onset of breeding shifted to later times (r = –0.75, p < 0.001), the clutch size declined (F1,172 = 27.31, p < 0.00001), and the egg volume was reduced (F1,226 = 23.4, p < 0.00001). Despite an increase in the nesting season temperatures in the Lena Delta since 1982 and the earlier arrival of Ross's
Gull (F1,27 = 14.87, p < 0.001), no shift in the dates of nesting initiation was detected. In our opinion, this is because the amplitude of interannual changes has a wider range than the long-term trend. The patterns of variations in the dates of nesting initiation, in clutch sizes and eggs volume are due to the dynamics of environmental conditions, primarily air temperatures.