During nocturnal migration birds can navigate on celestial cues (moon and stars) and make use of magneto-reception. In the absence of celestial cues, on foggy and rainy nights, birds rely only on their magnetic compass. This compass can become distorted by artificial light, especially by the long wavelength component of the spectrum. This is specifically apparent in migratory birds which are trapped by light beams at sea and start circling clockwise around it.
The North Sea branch of the Dutch Ministry of Public Works (RWS) commissioned Altenburg & Wymenga to carry out a study on the impact of conventional lights in migratory bird populations in the North Sea. For this assignment we were faced with two challenges: How to estimate the number of victims per species? (since dead birds disappear in the sea or are eaten by scavengers) and, secondly, how to relate the victims to the population size of the species concerned?
We choose a worst case model approach, assuming that all birds that come into contact with an illuminated platform die at the spot. We calculated the (potential) collision risk assuming spheres around platforms of respectively 1, 2 and 3 km and we calculated the probability that adverse weather conditions occur. The species specific collision risk was then scaled in relation to 1% of the natural or background mortality (an EU guideline in the Bird Directive that is used to disentangle a negligible impact from a so-called significant impact). It appeared that potentially, in a worst case, large numbers of birds may be affected by conventional illumination. The study is presented in two reports, a model study and a background report concerning the assumptions in the model and a further elaboration of the results. The model results were communicated to and presented at the OSPAR Convention.