False Spring in the Southeastern European Russia and Anomalies of the Phenology of Spawing Migrations of the Pallas’ Spadefoot Toad Pelobates vespertinus (Pelobatidae, Amphibia)

In March 2020, an anomalously early start of the spawning migrations of Pelobates vespertinus was established in the Medveditsa River valley. Males of this species, emerging from a state of hibernation, were observed in the shallow waters of the lake Troynoye on March 13, one month ahead of the long-term mean date over the past 30 years and almost two weeks before the earliest recorded date. From 1892 to 1990 the phenological scenario of a false spring was realized six times only, while seven times — over the past 30 years (1991 – 2020). A second feature of the development of spring processes by the type of false spring in 1990 – 2020 is repeated recurrence of return cooling. The emergence of discreteness (discontinuity) of the spawning migration period against the background of early development of spring processes and repeated periods of return cold is a relatively new feature of the phenology of anuran amphibians. The cumulative proportion of P. vespertinus males arriving at their spawning biotopes from the first to the fifth day from the date of the start of spawning migrations of the species increases on mean (median) from 4 to 38% of the total number of mature males. In 2014, an abrupt change in the proportion of males who came to spawning biotopes during the first five days was found: in 2009 – 2013 was less than a quarter of the total number of males (15 – 23%), and in 2014 – 2017 it was half or more (43 – 70%). Consequences of such phenological abnormalities can become an additional factor in reducing the reproductive success of P. vespertinus and degradation of their local populations. In addition, the abnormal development of spring processes requires correction of census technique and analysis of the sex structure of P. vespertinus populations.