We evaluated the effect of the removal of hooded crows Corvus cornix on common eider Somateria mollissima nesting success using a partial Before and After Comparison of Impact (BACI) design over three years in two eider breeding colonies (Håkøya and Grindøya) in northern Norway. These breeding colonies had over the last decades been subject to severe declines in number of breeding birds and it was suspected that increasing nest predation by crows was contributing to the declines. Eider nesting success was monitored in both colonies during 2006-2008. Crows were removed by live-trapping from Håkøya in 2007 and from Grindøya in 2008. We monitored the number of nesting pairs of crows and general crow activity. Crow removal was generally successful in reducing the number of established territorial and visiting crows. Modelling of daily nesting success probabilities according to a logistic exposure model revealed that eider nests found at the start of the season had a much lower probability of success than nests found later on in the season. This is likely to be due in part to the increase in number of active nests during the first half of the season. The effect of crow removal appeared to differ between the two colonies. Eider nesting success on Håkøya increased from 61% in the pre-removal year 2006 to 80% during crow removal in 2007 and declined to 74% in the post-removal year 2008. In contrast, nesting success on Grindøya remained constantly low (38-40%) during the same period. This difference between the two colonies could be explained by a difference in predation pressure, or by a higher general disturbance level on Grindøya making unattended nests vulnerable to predation by a range of alternative predator species acting compensatory to the removal of crows. New investigations should be undertaken to clarify the interaction between crows and other nest predators in determining eider nesting success. Where compensation appears to occur, conditions for this process should be investigated. This will help to indicate when crow removal can be effective and which other actions can be employed to increase common eider nesting success.
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Vol. 16 • No. 2