Figure 1.) Adult golden eagle taking off from nesting site on the side of a shear cliff (Retrieved from The Garden of Eadon)
Migration in golden eagles is unlike many other species of birds, with a “clear” division between those that migrate and those that do not. It has been shown that northern golden eagles tend to overwinter in southern areas, whereas southern populations tend to stay put. For any bird, migration is very expensive, costing a large amount of energy. It is especially so for golden eagles as they are large predatory birds with a relatively high wind loading (body mass to wing size ratio). Due to this they must apply specific strategies for long distance travel when migrating. These strategies then allow for different dispersal and population studies of a species that is typically very hard to study, especially at a large scale. However along with human urban expansion, many things cause problems for golden eagles during their movement leading to conservation concerns for the species.
Figure 2.) GPS tracked movement of golden eagles (Katzner et al, 2012)
For soaring birds such as the golden eagle there are two main soaring methods exploited: thermal soaring and dynamic soaring. Thermal soaring exploits the differential heating of air leading to updrafts in the air, which lead to lift. Dynamic soaring uses the deflection of horizontal wind off different topography which will also create an updraft leading to lift (Bohrer et al, 2012). The deflected wind updrafts are also known as orographic updrafts. Testing of golden eagles wing loading showed relatively high values which led to the hypotheses that exploitation of relatively weak thermal updrafts was not enough for migration and stronger orographic updrafts needed to be used (Lish et al, 2016). GPS tracking of several golden eagles showed that during migration they followed mountainous terrain abundant in orographic updrafts, leading to the belief they do indeed require more orographic updrafts for long migrations (Fig 2) (Katzner et al 2012).
Figure 3.) Death of juvenile golden eagles due to collision with wind turbines (Retrieve from Green Building Law Update)
As with most things, upon finding any exploitable resource, humans will do so. Figure 2 shows a dead juvenile golden eagle which had collided with a large wind turbine. These turbines are built in areas with constant wind or drafts to generate energy, which is also the same “resource” exploited by large soaring birds such as the golden eagle. At the Altamont Pass Wind Resource Area (APWRA) in California alone an average of 70 golden eagles are killed each year, which they consider may be substantially lower than the real value (Pagel et al, 2013). This number compared to the number of golden eagles typically observed yearly is shockingly large. The consequences of this occurring may have much further reaching effects on golden eagles populations than just a few mortalities a year. A study in 2016 proposed that having such mortalities in key migration points may affect the gene flow of golden eagles during their large migrations leading to a loss of genetic diversity among populations. This study did genetic testing of 67 golden eagles killed by collision and confirmed that roughly 25% of the eagles were genetically different migrants. This may be a cause for concern due to how golden eagles are distributed in small abundances over very large areas, especially for resident birds which in turn may lead to possible inbreeding effects (Katzner et al, 2017).
On a brighter note the migratory behaviours of golden eagles allow for a relatively accurate estimation of continental population size, something which is very difficult to do for many bird species and can be crucial for their conservation. Similarly to what is done at Little Mountain on Vancouver Island, hawk watches are done all around the continent. Hawk watches are a form of citizen science data collection of migratory hawk abundances, which is instrumental in the research of migratory hawks. When compared with mark recapture studies, areas with numerous hawk watches such as the Appalachian Mountains in Pennsylvania, showed that roughly 24% of migrating golden eagles are observed at hawk watch sites, and 55% are observed by hawk-watchers overall. Using this data population estimates put North America’s golden eagle population in the range of 50,000-80,000 individuals, the majority of which are location in the west (Dennhardt et al, 2015).
Golden eagles being one of the largest predatory birds, must migrate using specific orographic updrafts resulting in relatively consistent migration patterns. However, using the specific terrain generating these currents results in a literal collision between human wind turbines and the golden eagles exploiting the same resource. These migratory patterns do however lead to a greater understanding of golden eagles, allowing for populations estimates through citizen science, something essential in their conservation.
Figure 4.) Rehabilitated adult golden eagle from Pacific Northwest Raptors (Photo by Evan Hessels)
1) Bohrer et al. Estimating updraft velocity components over large spatial scales: Contrasting migration strategies of golden eagles and turkey vultures. Ecology Letters. 2012. 15(2), 96-103. doi:10.1111/j.1461-0248.2011.01713.x
2) Dennhardt, A. J., Duerr, A. E., Brandes, D., & Katzner, T. E. Integrating citizen-science data with movement models to estimate the size of a migratory golden eagle population.Biological Conservation. 2015. 184, 68-78. doi:10.1016/j.biocon.2015.01.003
3) Katzner et al. Golden eagle fatalities and the continental-scale consequences of local wind-energy generation. Conservation Biology. 2017. 31(2), 406-415. doi:10.1111/cobi.12836
4) Katzner, et al. Topography drives migratory flight altitude of golden eagles: Implications for on‐shore wind energy development. Journal of Applied Ecology. 2012. 49(5), 1178-1186. doi:10.1111/j.1365-2664.2012.02185.x
5) Lish, J. W., Domenech, R., Bedrosian, B. E., Ellis, D. H., & Payton, M. Wing loading in north american golden eagles (aquila chrysaetos). Journal of Raptor Research. 2016. 50(1), 70-75. doi:10.3356/rapt-50-01-70-75.1
6) Pagel, J. E., Kritz, K. J., Millsap, B. A., Murphy, R. K., Kershner, E. L., & Covington, S. Bald eagle and golden eagle mortalities at wind energy facilities in the contiguous united states. Journal of Raptor Research. 2013. 47(3), 311-315. doi:10.3356/JRR-12-00019.1
Golden Eagles are really beautiful birds! It’s always upsetting to find out how human actions results in the loss of such brilliant birds. Since it is unlikely wind turbines will ever be removed, it seems like the most probable method to prevent birds colliding with them would be to defer the birds from them somehow. Do you know if any research or attempts to prevent birds colliding with wind turbines has been conducted?
Great work!
Thanks Kaitlin,
There is quite a bit of research being done by different energy companies to help avoid collisions without moving the turbines. The most prominent method seem to be using acoustics to deter the birds from coming nearby, but there is also new turbine designs which (supposedly) help prevent from coming near by preventing roosting etc. Here’s a website that gives a good summary of some of other ways companies may begin to mitigate the problem, http://grist.org/climate-energy/for-the-birds-and-the-bats-8-ways-wind-power-companies-are-trying-to-prevent-deadly-collisions/.
What a lovely photo of Ares (from The Raptors)! Do populations have a destination they can be seen returning to year after year? Or do they migrate just south generally, not as structured?
It seems to be a little of both. Although not a huge amount of research has been done to determine their migratory movement it thought that golden eagles will move to a typical wintering ground every year. Though they don’t typically stay in the exact region for long, seemingly roaming around in search of food over a large area before moving back up north.
Nice blog Evan. Has there been more GPS studies than those done out east? It seems like a species that would lend itself well for real-time GPS tracking, although the cost may be a bit prohibitive.