Catastrophic years for the White Stork: analysis of three cases in Ukraine. - V.N. Grishchenko. - Berkut. 18 (1-2). 2009. - P. 22-40.
Normal number dynamics of the White Stork are sometimes interrupted by sharp declines. They are accompanied by the delayed arrival and the reduction of breeding success. This is an interesting phenomenon called ‘catastrophic’ or ‘disturbance’ year (German – ‘Störungsjahr’). In catastrophic years the influence of adverse factors has complex and global character. It reveals itself on large territories including many countries. During monitoring of the White Stork population in Ukraine in 1992–2009, this phenomenon was registered three times: in 1997, 2005 and 2009. Peculiarities of these years are analysed. Data were obtained from the net of constant study plots in all parts of the country. This net is representative and covers the main breeding grounds of the White Stork in Ukraine. Breeding success, number dynamics and migration timing have been investigating. For the analysis of number dynamics the obtained data are sufficient since 1994. We used the average increment of numbers on study plots in percents. In catastrophic years number decreasing was considerable: from –11,3% in 2009 г. to –16,3% in 2005 г. (Table 1, Fig. 1). In separate regions this decline was even greater. For example, in 2005 in North-East Ukraine the stork population decreased almost in a third. It is important that such number declines do not concern with tendencies of its changes in former years. In 1997 the depression fell on the peak of wave of number increasing, in 2005 and 2009 drops happened during the period of linear increasing. In all the cases the evolution of numbers did not predict following events. These jumps are indeed a catastrophic appearance nowise connected with long-term trends. There were regional differences in values of number fluctuations. In 1997 the average increasing rate had no significant differences in various parts of the country, but in 2005 and 2009 it rose from the west to the east (Table 1). The renewal of the population after catastrophic years happened differently. The drop in 1997 did not change the situation: the wave of number increasing continued. The very high increasing in 1998 (maximal during the period of investigation) fully compensated the decline and gave further growth. Moreover, it occurred synchronously in all parts of the country. Only on some study plots the renewal of number lasted 2–3 years. After 2005 the population was not restored so fast in any region of Ukraine. Furthermore, in West Ukraine continued the small number decreasing. Reproductive parameters also were appreciably decreased in catastrophic years. They had the minimal values during 18 years of observations in 1997 (Fig. 2, Tables 2, 3). Catastrophic years have started with the delay of migration of storks. Mean dates of the first arrival distinctly overstep the limits of usual fluctuations (Fig. 3). But in separate points the birds can appear in normal times. In catastrophic years timing of migration was prolonged. If for 15 normal years the standard deviation of first arrival dates made on average 7,59 ± 0,34 (5,2–9,9), then for three unfavourable seasons it rose to 10,47 ± 0,37 (9,8–11,0). The difference is significant (t = 3,68, p = 0,002). Features and reasons of catastrophic years are discussed. A year can be considered as catastrophic if 4 conditions meet: 1) delay of the arrival, 2) number decreasing, 3) reduction of the reproductive success, 4) these phenomena take place in a large territory. Reasons of catastrophic years are now well known. First of all these are the adverse conditions of wintering and to lesser extent problems during the spring migration. Already there are many data evidencing that population dynamics of long-distance migrants to a great extent are determined by conditions in wintering grounds. Survival of adult birds and for many species also result of breeding depends on them. The White Stork was one of the first birds for which these regularities were studied. Close correlation between the number dynamics and the breeding success of the White Stork in Ukraine was found (Table 4). It means that fluctuations in both cases are determined by the same factors. First of all, these are the conditions in wintering grounds. Therefore, the wintering area serves as a peculiar regulating system extensively determining the state of the population. This connection can be shown not only in extremal years but also in quite normal breeding seasons. In population dynamics of the White Stork the influence of global factors predominates. It can be good shown comparing number dynamics in Ukraine and in Germany (Fig. 1). These countries are located at the opposite ends of the breeding range of the Eastern White Stork population, but number fluctuations correlate very close (r = 0,91, p < 0,001). For the comparison of years with each other two indexes based on normalized deviates of three main parameters are proposed (t_a – n. d. of mean arrival date, t_n – n. d. of mean increasing rate, t_f – n. d. of mean number of fledged youngs per breeding pair, JZa). Index of disturbance I_d is a mean absolute value of normalized deviates in a catastrophic year: I_d = (|t_a|+ |t_n| + |t_f|)/3. Index of favourability I_f is the sum of normalized deviates with the corresponding sign: I_f = t_n + t_f – t_a. 1997 was the most unfavourable year during the study period. Ukrainian storks belong to the Eastern core population by H. Schulz (1999). But it is not a single whole and can be separated in two parts: the core one and the peripheral one. The peripheral part has many differences from the core part, it is more reactive and unstable. [Russian].
Key words: White Stork, Ciconia ciconia, breeding success, number dynamics, arrival, population.
Address: V.N. Grishchenko, Kaniv Nature Reserve, 19000 Kaniv, Ukraine; e-mail: vgrishchenko@mail.ru.