The grass on Altamont Pass is already gold by midsummer, and the wind through the Diablo Range has been strong enough for turbines since the 1980s.
It has also been strong enough for golden eagles for far longer than that.
Surveyors working the California hills east of San Francisco Bay kept turning up the same thing.
Eagle pairs were holding territory inside the wind area, not off on some neighboring ridge.
They were building nests there, and fledging young.
So why didn’t the rotors push them out?
Why the rotors didn’t push the eagles out
Altamont Pass is a low saddle in the northern Diablo Range. Warmer air over the Central Valley and cooler marine air off San Francisco Bay create the temperature difference that drives steady wind across the pass, which is why the developers came. That same moving air builds the lift a large soaring bird rides for much of its day, so the eagles and the turbines both trace back to one feature of the terrain.
Golden eagles are long lived and strongly territorial. A pair that has learned a hillside, found the squirrel colonies and the updraft lines, and raised young there before is not easily moved. When the machines arrived, the birds already holding the best lift lines stayed put, and the farm went in around them rather than replacing the ground.
Staying is not the same as being unbothered. The survey work records where the pairs are and how they breed; it does not measure what the turbines cost them day to day. That gap matters more than it might look.
What the surveyors kept finding on the turbine ground
Inside the wind area, eagle pairs were about as thick on the ground as outside it. Surveyors detected roughly one pair per 6.9 square miles surveyed inside, against one pair per 7.8 square miles in the surrounding region. Nests on those territories showed evidence of use: construction activity, incubating adults, eggs laid, or young present.
Breeding output tracked the same way. Pairs monitored in the wind area fledged an average of 0.37 young per pair, similar to the 0.30 young per pair recorded outside it. On both measures a wind farm is usually judged on, the turbine ground read as ordinary eagle country.
One measure did not read as ordinary. A substantially higher share of territorial pair members inside the wind area were subadults, a mean of 29 percent, against 3 percent of pairs aged in the surrounding region. The report treats that gap as a sign of potentially higher adult mortality or displacement inside the farm.
The count that sits behind all of this
The surveys ran from 2014 to 2021, with no fieldwork in 2017. The design covered a roughly 2,000 square mile block of the northern Diablo Range and identified as many as 230 territorial pairs, up to 21 of which overlapped the wind resource area in any given year. That area was drawn as a 2 mile buffer around current and historical turbine locations, so “inside the farm” means within two miles of a turbine site. The age comparison rests on 16 pairs aged inside and 122 outside.
Region wide, the share of pairs fledging at least one young was 0.27 and 0.26 in the final two years, above the seven year average of 0.22. Those estimates describe the whole study area, not the wind farm alone.
The fatality record is older than the survey. Carcass searches at the original small turbines estimated about 67 golden eagles killed a year, and later comparisons found repowered high capacity turbines carried significantly lower collision risk per megawatt than older machines. One repowered facility still logged 0.14 golden eagle fatalities per megawatt per year against a county threshold of 0.08 in its three year report. Camera systems that spot an approaching eagle and stop a single turbine have been tested elsewhere, and at one Wyoming facility fatalities fell 82 percent at the treatment site relative to a control.
The subadult signal and what it means for the population
The 29 to 3 gap is the part of the result that does not settle. Inside the wind area, birds still in subadult plumage turn up in breeding slots at close to ten times the rate seen outside, and the straightforward reading is that adults there are being lost and replaced. A territory that keeps refilling can look stable in a survey while steadily consuming adults, which is why occupancy alone is a weak measure of how a population is really doing.
Researchers have also tried painting turbine blades to make them easier for birds to see, with results that have varied between sites and species. A visual cue that works for one bird in one landscape does not transfer automatically to another.
Fire adds another layer. A severe wildfire affected more than 60 historical breeding territories in the study region, and the report flags it as an emergent threat that may interact with existing ones. Whether the pairs on turbine ground hold up may depend on recruitment from outside.
Where the science points next
The report sets a bounded goal: map where eagles are, which sites they occupy, and whether they nest, so that possible population level impacts can be identified and reduced. You can read it in full from the geological survey that produced it. It does not yet tell managers which turbines carry the greatest risk to which nesting pairs, or whether curtailment timed to a known pair’s hunting hours would change anything at that territory.
Work on insect surveys at a Wyoming wind range has shown how hard it is to catch the full ecological picture around a large farm. The eagle record at Altamont carries the same lesson at a larger scale: the animal’s relationship to the machine is neither simple avoidance nor simple tolerance.
What the record does establish is that these eagles are staying. Pairs hold territory in the wind area, raise young at rates close to the surrounding range, and the vacancies fill again. The open question is who keeps filling them.
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