Researching golden eagles in the NCA over 50 years - The Golden Eagle Project

The Morley Nelson Snake River Birds of Prey National Conservation Area (NCA) is world-famous for supporting one of the highest concentrations of nesting birds of prey in North America. One of the prominent raptor species nesting along the cliffs of the Snake River Canyon is the golden eagle. A collaborative group of researchers from USGS, the BLM, Boise State University, and others have been monitoring and researching the breeding biology of eagles in the NCA for more than 50 years – making it one of the longest continuous running raptor projects in the world.

Each year we survey historical territories to monitor occupancy, breeding behavior, and nesting success. This involves long hours of searching to find occupied nests and then monitoring nests to determine whether nestlings are present, the age of the nestlings and if nestlings successfully fledged. Some years technicians also rappel into nests and install cameras to more accurately determine if and when nestlings fledged.

Golden eagle chick (C) Elizabeth Materna, USFWS

Eagle nests along the Snake River Canyon are quite large, averaging about 5-6 feet wide and 2 feet high, but the largest recorded nest was 20 feet tall, 8.5 feet wide. Eagles usually lay 1-2 eggs and incubate for 41-45 days. After hatching, nestlings remain in the nest for another 45-81 days before taking their first flight. Eagles do not typically attempt to breed until they are 4-5 years old. Golden eagles take longer to reach sexual maturity and lay fewer eggs than smaller species, like American kestrels that breed in their second year and can lay 4-6 eggs. This “slow pace” lifestyle and long generation times make it harder for eagle populations to recover from threats that affect their reproduction and survival.

Golden eagle nest (C) Caitlin Davis

With the wealth of knowledge that comes with this long-term monitoring dataset, researchers and graduate students have been able to conduct studies on eagle response to fire disturbance, diet, diseases and parasites, heat exposure, and general productivity and population trends. Because of this work, we have been able to see that golden eagle territory occupancy in the NCA has declined by 40% in the past 50 years with half of that decline occurring in the most recent 15 years. Our goal, along with continued long-term monitoring of occupancy, nest success, and productivity, is to continue to identify the potential stressors that could be driving this decline.

There have been extensive habitat changes in the NCA because of wildfires, livestock grazing, and drought. Specifically, fire has allowed for invasive annual grasses, like cheatgrass, to expand dramatically. By the mid-1980s, wildfires had converted over 50% of the shrub habitat in the NCA to non-native grasslands. This likely causes bottom-up ecosystem effects that can impact golden eagle nesting success and territory occupancy. Research conducted by the Golden Eagle Project showed that the primary preferred prey of golden eagles nesting in the NCA are black-tailed jackrabbits and that eagles are more likely to breed and raise more young when jackrabbit abundance is high. Jackrabbits are associated with shrub habitats and their populations are less dense in disturbed grasslands. Therefore, the degradation of native shrub habitat can decrease the amount of preferred prey species, which can lower the NCA’s carrying capacity for golden eagles.

Black-tailed jackrabbit (C) Jon Nelson

Additionally, studies done by the Golden Eagle Project provide evidence that decreasing jackrabbit abundance has led to golden eagles shifting their diets to more diverse prey sources such as ground squirrels, waterfowl, and rock pigeons. However, nestling survivorship was positively associated with the proportion of black-tailed jackrabbits and negatively associated with rock pigeons, meaning that diet shifts may come at a cost. Rock pigeons can carry avian trichomonosis, a protozoan disease that can be fatal for nestling golden eagles.

The Golden Eagle Project is continuing to work towards the identification of potential stressors that could be impacting eagles in the NCA and surrounding areas. Factors such as declining prey abundance, thermal stress, disease, parasites, and human impacts are all likely affecting eagles and may be further exacerbated by climate change in the future. Our goal is to identify and weigh the relative impacts of these stressors so that we can develop targeted strategies that will maximize conservation efforts.

(C) Emma Regnier

A huge thank you to Ashely Santiago, Caitlin Davis, Julie Heath for writing this fantastic blog, giving us a deeper insight into the golden eagle research happening in the NCA and for their ongoing dedication to this extremely special species.