Quantifying the demographic cost of human-related mortality to a raptor population

<div><p>Raptors are exposed to a wide variety of human-related mortality agents, and yet population-level effects are rarely quantified. Doing so requires modeling vital rates in the context of species life-history, behavior, and population dynamics theory. In this paper, we explore the details of such an analysis by focusing on the demography of a resident, tree-nesting population of golden eagles (<i>Aquila chrysaetos</i>) in the vicinity of an extensive (142 km²) windfarm in California. During 1994–2000, we tracked the fates of >250 radio-marked individuals of four life-stages and conducted five annual surveys of territory occupancy and reproduction. Collisions with wind turbines accounted for 41% of 88 uncensored fatalities, most of which were subadults and nonbreeding adults (floaters). A consistent overall male preponderance in the population meant that females were the limiting sex in this territorial, monogamous species. Estimates of potential population growth rate and associated variance indicated a stable breeding population, but one for which any further decrease in vital rates would require immigrant floaters to fill territory vacancies. Occupancy surveys 5 and 13 years later (2005 and 2013) showed that the nesting population remained intact, and no upward trend was apparent in the proportion of subadult eagles as pair members, a condition that would have suggested a deficit of adult replacements. However, the number of golden eagle pairs required to support windfarm mortality was large. We estimated that the entire annual reproductive output of 216–255 breeding pairs would have been necessary to support published estimates of 55–65 turbine blade-strike fatalities per year. Although the vital rates forming the basis for these calculations may have changed since the data were collected, our approach should be useful for gaining a clearer understanding of how anthropogenic mortality affects the health of raptor populations, particularly those species with delayed maturity and naturally low reproductive rates.</p></div

Golden eagle life cycle.

<p>Golden eagle life cycle.</p

The Altamont Pass windfarm as viewed from its southern boundary in April 1995.

<p>The dense configurations of small turbines in this photo are currently being replaced by fewer, larger, and more widely spaced machines.</p

High quality nesting habitat for golden eagles in the Diablo Range, California.

<p>High quality nesting habitat for golden eagles in the Diablo Range, California.</p

Probability of annual survival () for four stage-classes of golden eagles radio-marked in the vicinity of the Altamont Pass Wind Resource Area, California, 1994–2000.

<p>We show survival estimates with and without turbine-related and known human-caused deaths included in the analysis. All undiagnosed deaths were treated as "natural" so as to obtain a maximum estimate of natural mortality (see text). No juveniles were killed by turbine blade-strikes.</p

Ages of breeding golden eagles at territories within 30 km of the Altamont windfarm.

<p>The asterisk indicates that the calculation included two individuals of uncertain age and therefore gave the maximum possible representation of subadults for that year. Note that yearly variation in the number of aged eagles reflects differences in sampling effort rather than population.</p

Causes of death among 88 uncensored golden eagle fatalities during 1994–2000.

<p>Juveniles include only those radio-tagged as fledglings. Undiagnosed fatalities included four unrecovered individuals (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0172232#sec002" target="_blank">Methods</a>).</p

Monthly variation in the proportion of individual golden eagles detected in the windfarm at least once per month in aerial surveys of 101 juveniles (radio-tagged as fledglings), 155 subadults, 51 floaters, and 47 breeders in the Diablo Range study area, 1994–2000.

<p>The increase in percentage of juvenile occurrence in the windfarm resulted as eagles radio-tagged as fledglings in May and June dispersed from their natal territories during September–November of each year.</p