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When ice forms on the river, mallards are concentrated in the holes in the ice, which in exceptionally cold weather may be reduced to two in number. Difficulties in counting do not appear until the number of mallards is approaching a thousand. The birds are exposed to some disturbances from wanderers along the river bank and also to the mallard counter. They react by disappearing only on particular occasions. During the summer there is quite a lot of traffic on the river. Especially the motor boats seem to have increased in number during recent years. Mallards living in town during summer seem to have adapted to this traffic and ignore it completely. The greatest disturbance factor consists of people wandering or skating on the ice in winter. When such a disturbance has occurred during a census, the count has been repeated later. Birds are in the habit of circling around above the river during a disturbance, and normally return soon after it is over.