Carrying capacity of staging areas and facultative migration extension in common cranes

Abstract

1. Between 1981 and 1991, the number of cranes Grus grus at Gallocanta, NE Spain, showed peaks during Autumn and spring migrations and troughs in midwinter. Autumn and spring counts increased throughout the study period, while the number of wintering cranes varied between years without a clear trend. 2. Large amounts of waste seeds were usually available on cereal and sunflower stubble fields in autumn. However, food availability decreased throughout the winter as a consequence of both the ploughing of stubble fields and the consumption of waste grain by cranes. This decrease was not compensated by newly sown grounds. 3. The number of cranes staging at Gallocanta during autumn and spring did not correlate with food availability. However, during midwinter, the minimum number of cranes was significantly correlated with food availability. 4. Midwinter crane numbers correlated better with food availability in the previous fortnight, suggesting that there was some delay between actual food decrease and the cranes' decision to leave the area. 5. A decrease in crane numbers was observed when the expected carrying capacity (= the number of birds which could be supported for the rest of the season) was less than the number of cranes present. Furthermore, the population size adjusted exactly to the level corresponding to the expected carrying capacity. 6. The results of this study suggest that the midwinter crane population in Gallocanta is limited by the carrying capacity of the area. This influences how many birds migrate further south in autumn. Historical census data indicate that there has been a northward shift in the winter range of the western population of common cranes associated with increasing agricultural food resources at this and other staging areas.