Some 900 stork are spending the winter in the Netherlands this year, this weekend’s annual stork count has shown.

Most storks travel to warmer climes in Africa, but some 35% stay put, Dutch bird protection organisation Vogelbescherming Nederlands says.

Storks probably base their decision to stay or go partly on the presence of food, the organisation said, but other factors are in play which the count may help clarify.

Although the Netherlands now some 1500 breeding pairs, the number of storks that remain in the winter has stayed more or less constant, which means more are leaving.

Count organisers STORK , which reported some 300 volunteers had been out and about, said the cold had led the storks to gather in bigger groups in places where food is plentiful, such as the vicinity of waste processing plants. A group of 20 storks were spotted roosting at Pernis, in the middle of the Rotterdam port area.

Storks were nearly extinct in the Netherlands in the 1970s because of the widespread use of pesticides, such as DDT.

Vogelbescherming reintroduced the bird and encouraged it to stay by building roosting stations for the birds. Since then their numbers have grown and they are no longer on the red list of endangered birds.