Numerical trends, habitat use, and activity of Greylag Geese wintering in southwestern Spain

Abstract

The size of the Greylag Goose Anser anser population wintering in the Marismas of the Guadalquivir (SW Spain) has increased from 15,000-25,000 birds in the early 1970s to more than 75,000 birds in the middle of the 1980s. Under normal flooding conditions, most Greylags are found in Scirpus-dominated zones adjacent to the roosting places; under drought conditions the geese also use other habitat types, such as cultivated fields (autumn sown cereals and ricefields) and grasslands, 1-5 km from the roosting places. Probably as a consequence of the population increase, the use of grasslands has become more prominent in recent years, even when these are rainy. Feeding mostly occurs in the morning and late afternoon, and resting mainly takes place around midday. Under drought conditions, the geese spend more time feeding than under normal flooding conditions; yet their foraging return would be lower because of the lower profitability of the food available under such conditions. Throughout the winter, there is an increase in feeding at the expense of resting and vigilance; this may be in relation to the need for fat accumulation to face the spring migration. For adult birds this would be facilitated by the relaxation of their parental duties in the late winter. The recent increasing use of grasslands may indicate that the carrying capacity of Scirpus habitats has been reached, and therefore that the population would be seriously stressed under extreme drought conditions, such as those of the winter 1980-1, when 10,000 Greylags died mainly from starvation.