Offshore structures may kill 'hundreds of thousands' of migrating birds

Collisions with offshore structures in the North Sea could account for the mortality of hundreds of thousands of nocturnally migrating birds, a new study has suggested.

The research, published in Bird Study, the Journal of the British Trust for Ornithology, collected bird corpses from an unmanned tall offshore research platform in the south-eastern North Sea on 160 visiting days from October 2003 to December 2007.

Corpses were identified to species and kinds of injury, ages, and fat and muscle scores were determined. Nocturnal bird calls were recorded, identified to species and quantified. Local and large-scale weather parameters were also considered.

Migrant passerines often make use of man-made structures while migrating over the North Sea, but these structures are also suspected to kill hundreds of thousands of individual birds (Photo: Mark Ambler/BTO)

A total of 767 birds of 34 species — mainly thrushes, European Starlings and other passerines — were found on 45 visits. Most carcasses were in good body condition and young birds were not more affected than adults. Three quarters of the 563 examined individuals had collision induced injuries. Birds in poor body condition were less likely to be collision victims than those in good condition. Mass collision events at the illuminated offshore structure coincided with increasingly adverse weather conditions and an increasing call intensity of nocturnal birds.

Assuming an average of 150 dead birds per year at this single offshore structure and additionally assuming that a considerable proportion of the corpses were not found, the researchers estimated that mortality at the 1,000+ human structures in the North Sea could reach hundreds of thousands of birds.

Given that offshore industrialization will progress in the future, the researchers predicted that collision numbers at offshore structures will only increase considerably as the number of man-made structures increases. The reinforcement of measures to reduce bird strikes at such sites is therefore strongly urged, particularly in the light of the substantial declines noted in a significant proportion of migrant species.