A 20-year study of wintering common crane fluctuations using time series analysis

Abstract

The numbers of common cranes (Grus grus) staging and wintering at Gallocanta lake, northeastern Spain, have increased throughout the period 1970-90. Consequently, we modelled the use of this area by cranes using Box-Jenkins Time Series Analysis to evaluate the influence of local food availability, duck hunting, and habitat and food conditions at other more traditional crane wintering areas in southwestern Spain on Gallocanta crane numbers. Food availability was not correlated with crane numbers at Gallocanta except during mid-winter. However, restrictions on duck hunting in 1973 and its total prohibition in 1981 correlated with increased crane numbers, and probably provided the most immediate reason for this increase. Habitat and food conditions at other wintering areas had no apparent influence upon crane numbers at Gallocanta. Because annual productivity figures were not correlated with the observed increase, we suggest that a northward shift in the wintering range associated with food provided by agriculture, together with a possible decline in mortality due to legal protection along the species' migratory route, probably have been major long-term factors contributing to the increase in cranes wintering at Gallocanta lake.