{"id":1112,"date":"2017-11-10T11:00:04","date_gmt":"2017-11-10T11:00:04","guid":{"rendered":"http:\/\/wordpress.viu.ca\/biol325\/?p=1112"},"modified":"2017-11-12T22:56:01","modified_gmt":"2017-11-12T22:56:01","slug":"the-movement-and-migration-of-golden-eagles-aquila-chrysaetos-canadensis","status":"publish","type":"post","link":"https:\/\/wordpress.viu.ca\/biol325\/2017\/11\/10\/the-movement-and-migration-of-golden-eagles-aquila-chrysaetos-canadensis\/","title":{"rendered":"The Movement and Migration of Golden Eagles (Aquila chrysaetos canadensis)"},"content":{"rendered":"

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Figure 1.) Adult golden eagle taking off from nesting site on the side of a shear cliff (Retrieved from The Garden of Eadon<\/a>)<\/p><\/div>\n

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\u00a0tend 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.<\/p>\n

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Figure 2.) GPS tracked movement of golden eagles (Katzner et al, 2012)<\/p><\/div>\n

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).<\/p>\n

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Figure 3.) Death of juvenile golden eagles due to collision with wind turbines (Retrieve from\u00a0Green Building Law Update<\/a>)<\/p><\/div>\n

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).<\/p>\n

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 \u00a0Appalachian 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).<\/p>\n

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.<\/p>\n

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Figure 4.) Rehabilitated adult golden eagle from Pacific Northwest Raptors (Photo by Evan Hessels)<\/p><\/div>\n

1) Bohrer et al. Estimating updraft velocity components over large spatial scales: Contrasting migration strategies of golden eagles and turkey vultures.\u00a0Ecology Letters.\u00a0<\/i>2012.<\/strong>\u00a0<\/i>15<\/i>(2), 96-103. doi:10.1111\/j.1461-0248.2011.01713.x<\/p>\n

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. <\/i>2015<\/strong>.\u00a0<\/i>184<\/i>, 68-78. doi:10.1016\/j.biocon.2015.01.003<\/p>\n

3) Katzner et al. Golden eagle fatalities and the continental-scale consequences of local wind-energy generation.\u00a0Conservation Biology. <\/i>2017<\/strong>.\u00a0<\/i>31<\/i>(2), 406-415. doi:10.1111\/cobi.12836<\/p>\n

4) Katzner, et al. Topography drives migratory flight altitude of golden eagles: Implications for on\u2010shore wind energy development.\u00a0Journal of Applied Ecology.\u00a0<\/i>2012<\/strong>.\u00a0<\/i>49<\/i>(5), 1178-1186. doi:10.1111\/j.1365-2664.2012.02185.x<\/p>\n

5) Lish, J. W., Domenech, R., Bedrosian, B. E., Ellis, D. H., & Payton, M. Wing loading in north american golden eagles (aquila chrysaetos).\u00a0Journal of Raptor Research.\u00a0<\/i>2016<\/strong>.\u00a0<\/i>50<\/i>(1), 70-75. doi:10.3356\/rapt-50-01-70-75.1<\/p>\n

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.\u00a0Journal of Raptor Research.\u00a0<\/i>2013<\/strong>.\u00a0<\/i>47<\/i>(3), 311-315. doi:10.3356\/JRR-12-00019.1<\/p>\n","protected":false},"excerpt":{"rendered":"

  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 […]<\/p>\n","protected":false},"author":619,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_crdt_document":"","footnotes":""},"categories":[1],"tags":[],"class_list":["post-1112","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/posts\/1112","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/users\/619"}],"replies":[{"embeddable":true,"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/comments?post=1112"}],"version-history":[{"count":25,"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/posts\/1112\/revisions"}],"predecessor-version":[{"id":1187,"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/posts\/1112\/revisions\/1187"}],"wp:attachment":[{"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/media?parent=1112"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/categories?post=1112"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wordpress.viu.ca\/biol325\/wp-json\/wp\/v2\/tags?post=1112"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}