Acta Univ Latv (2007) 723: 71–97

Estimated population dynamics of the Corncrake Crex crex in Latvia and Europe in the 20^th century by ringing data analysis

Abstract

We collected data on 2916 Corncrakes ringed in 17 Bird Ringing Schemes in Europe 1908 - 1995 and analyzed the data (n = 643) of former Czechoslovakia, Croatia, Estonia, Hungary, Latvia, Lithuania, Norway, Poland and Switzerland to obtain quantitative estimate (TRIM index) of the Corncrake population decline in Europe during the last century. Our analyses is based on the assumption that Corncrakes were captured randomly: bird ringers used every opportunity to ring any bird, encountered, but no specific search for Corncrakes were made. We also analysed the ringing data collected in Latvia in detail to suggest on the behaviour of the ringers in order to test our assumption. We also analysed data on six passerine farmland bird species in Latvia to evaluate if bird ringers were actively ringing farmland birds later in our period of data analyses, when Corncrake ringing has declined. During the period of 1925 - 1995 there were 215 Corncrakes ringed in Latvia: 190 pulli and 25 adults. A decline of ringed pulli was observed in raw data as well as data adjusted to total number of birds ringed in Latvia in the respective year (rS = –0.46; p < 0.001) and data adjusted to active bird ringers in Latvia in the respective year (rS = –0.43; p < 0.001). A similar decline of ringed Corncrakes (n = 101) is observed also in data of the former Czechoslovakian ringing scheme in Prague. Index values of combined data for all ringed Corncrakes in Europe show significant decrease during the study period (r2 = 0.26; p < 0.00001). If we remove Latvia from data pool, since it contributes roughly one third of the data (n = 215) and therefore drives the index, the observed pattern remains unchanged (r2 = 0.11; p < 0.01). Correlation between index of ringed Corncrakes in Europe except Latvia (former Czechoslovakia, Croatia, Estonia, Hungary, Lithuania, Norway, Poland and Switzerland) and number of ringed Corncrakes in Latvia is statistically significant (r2 = 0.12; p < 0.01). Similarly to Corncrake, the number of ringed Skylarks (rS = –0.50; p < 0.001) and Whinchats (rS = –0.23 p < 0.05) have also decreased significantly in Latvia during the period of 1925 - 1995, while number of ringed Meadow Pipits (rS = 0.57), Common Whitethroats (rS= 0.68) and Linnets (rS = 0.49) increased significantly (p < 0.001). The only analysed species showing no significant trend was Yellow Wagtail. Qualitatively there is no doubt that ringing data verify the decline of the Corncrake population both in Latvia and in Europe. The average index of ringed Corncrakes of all European countries shows a decrease of about 5.5 times comparing 1925 - 1930 versus 1981 - 1990. Approximation by calculating decrease of available habitat shows a smaller decrease in Latvia by 1.5 - 3 times, but recent population density data for these calculations have been used, which might have decreased as well.