Water birds

Water birds can be distinguished by hibernating region:

1.  Water birds hibernating predominantly to the northeast of the Netherlands, e.g. eider ducks and wild swans,
2.  Water birds hibernating predominantly in the Netherlands, e.g. oyster eaters and widgeons,
3.  Water birds hibernating predominantly to the southeast of the Netherlands, e.g. grebes and Brent geese.

The index of all bird species hibernating in the Netherlands is compiled on an annual basis. Subsequently, the geometric mean of these indices per category is calculated.