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The consequences of additional mortality of birds from collisions with a rapidly increasing number of wind turbines are receiving much attention worldwide. Currently, threshold assessments for an acceptable impact on populations are commonly used to evaluate the expected effect of wind turbines on local bird populations. These provide a seemingly clear-cut method for establishing whether damage to the integrity of a population will or will not occur, but questions have recently been raised as to the validity of their use. In this paper we examine whether two widely used threshold methods for evaluating the impact of extra mortality on bird populations, the 1% mortality norm and Potential Biological Removal PBR, have general applicability, or whether they should be used more cautiously. The 1% mortality norm is based upon the assumption that any additional mortality lower than 1% of the natural mortality has a negligible impact on a population, while the Potential Biological Removal or PBR method is used to estimate the loss of individuals from which a population can still recover. To evaluate the impact of additional mortality resulting from wind turbine collision on bird populations, we model the consequence of an increase in mortality rates on populations assumed to be regulated by logistic growth. We use the logistic growth equation to test how the population persistence of a species may be affected by different levels of additional mortality; and use case studies of existing, declining populations of Common Tern and Marsh Harrier in the Netherlands to determine how the effect of collision mortality operates to influence their population persistence. To examine the impact of additional mortality, we introduce a novel measure, the “Population Persistence Index” or PPI to describe the population persistence and changes therein following increased mortality. The PPI integrates the population growth at various densities - it is determined by the maximum population growth rate at small population size - and the carrying capacity, the population size where recruitment equals mortality.Our results show that the PPI can be very sensitive to additional mortality, especially in populations in which the mortality approaches the maximum recruitment. We found that a 1% increase in mortality rates of all post-fledging age classes from wind turbine strike lead to a c. 7-15% decrease in population persistence for Dutch populations of Marsh Harrier and Common Tern, which should not be considered negligible. Strong effects of additional mortality on PPI are also evident using the PBR. We show that for the PBR, the proportional change in PPI is independent of r and K and depends only on the so-called recovery factor Fr. When this recovery factor Fr=1 (generally used for populations that show sustained growth near carrying capacity), additional mortality results in an 87.5% reduction of the PPI according to the PBR, and in a 58% reduction when Fr=0.5 (used for populations with protracted gradual decline). For the PBR of Common Tern a 58% reduction of the PPI was estimated, and for Marsh Harrier population a 14% reduction of the PPI at their estimated Fr values, compared to the population not exposed to additional mortality. We show that the use of the ‘1% mortality criterion’ and the Potential Biological Removal (PBR) method, currently often used in Appropriate Assessments across species in most E.U. countries with rapidly growing wind industry, strongly underestimate the impact of additional mortality on species for which at low population density the recruitment rate is almost equal to the mortality rate.

Do assessment thresholds underestimate the mortality impact of wind farms on bird populations?