3. Results
3.1. Number of nesting eiders
In the 28 study colonies situated in the southern and central Baltic Proper, i.e. in the colonies on Ertholmene, Utklippan and along the coast of Gotland, a dramatic decline of the number of nesting eiders was observed from 2007 to 2008 (Fig. 2). In the 22 study colonies on Gotland, the 76% decrease (from approx. 6650 nests to approx. 1620 nests) was followed by increases in 2009 and 2010, although not up to numbers observed in 2007. Despite the study colonies on Gotland were situated along a 200 km coastline the changes in nest numbers was highly synchronous. All but one of the 22 study colonies declined from 2007 to 2008 and all but two increased from 2008 to 2009 (Fig. 2). At Utklippan and Ertholmene the observed decreases of 55%, (from 144 to 65 nests), and 36% (from 1660 to 1060 nests), respectively, between 2007 and 2008, were followed by increases in 2009 and 2010 up to the level observed in 2007. By contrast, no general decline of the number of nesting eiders was observed from 2007 to 2008 in the 75 study colonies situated in the northeastern Baltic Proper, i.e. in the colonies on islands along the coasts of Saaremaa and Hiiumaa in western Estonia and at Kolga and Tvärminne in the Gulf of Finland (Fig. 2). Actually, the total number of eider nests increased slightly from 2007 to 2008 in the study colonies along the coast of Saaremaa and Hiiumaa and at Tvärminne.