The role of the Hooded Crow (Corvus corone) in the nesting success of the Common Eider (Somateria mollissima) at two colonies in Troms county, Northern Norway

Abstract

There is evidence of a negative population trend for the two neighbouring eider colonies of Håkøya and Grindøya in Troms county, northern Norway. Casual observations suggest that nest predation may be an important factor, and that the hooded crow in particular could be the main egg predator. On this basis, a two year pilot study was conducted to investigate the role of the hooded crow in the nest predation. Eider nesting success was monitored on both colonies in 2006 and 2007, whilst in 2007, crows were removed from Håkøya. The number of nesting pairs of crows was monitored on both islands in both years, whilst in 2007 a crow activity index was estimated to assess the effectiveness of trapping. Cause of eider nest loss was recorded on Håkøya in both years. In addition we assessed if the nest habitat variables habitat (open, wood, or thicket), distance to the open and distance to the nearest crow nest were related to eider nest success. A logistic exposure model was used to analyse nest success, whilst a log-linear regression with a Quasi Poisson distribution was used to analyse crow activity. Crow removal in 2007 on Håkøya was successful in terms of removing established territorial and visiting crows and this most likely resulted in a large reduction in crow activity on Håkøya compared to the control area Grindøya. Modelling of daily nesting success probabilities revealed that eider nesting success on Håkøya increased from 61% of nests in 2006 to 80% of nests in 2007, while in contrast, nesting success on Grindøya stayed constant over the same period (38% – 39% of nests in 2006 and 2007 respectively). In addition, there was a strong season effect on the nesting success of eiders on both islands and in both years of this study, with nests found at the start of the season having a much lower probability of success than nests found later on in the season. The habitat variables did not improve the prediction of nest success.

Since the crow removal in this study was not replicated in space or time, and moreover the cause of nest failure was often undetermined, the causal link could not be verified by this pilot study. Nevertheless, the results indicate that the hooded crow could be an important factor in the decline of the breeding colonies of common eiders in Tromsø, and that a more long-term study would be valuable.