The Bald Eagle: Descriptive Essay

Nesting and Habitat

The Bald Eagle (Haliaeetus leucocephalus) is the only Eagle that is confined to North America, and it builds one of the largest nests out of all birds on the continent. They fill out their nests with branches and sticks and line them with grasses, mosses, and other softer materials, which help them last for years (Guinn et al. 2004). These nests are called eyries and can be found near large bodies of water with an abundance of resources for feeding and nesting (Travsky et al. 2004). In general, they choose nesting sites that are further from human interaction and near bodies of water with open vegetation. However, a study shows that greater nest success occurs in dense forest areas farther away from water (Livingston et al. 1990). More research into nesting shows that Bald Eagles prefer softwood or forests that are softwood dominant when choosing a nesting site (Gerrard et al. 1975). While it is ideal to be farther away from human activity, evidence has shown that eagles near more developed areas have adapted to more urbanized areas. In Saalfeld et al. 2010, a study in Texas showed Bald Eagle nests were located in trees surrounded by shorter and smaller trees rather than taller ones. This was consistent with the predicted hypothesis from the study that eagles select nest trees with an unobstructed view and an easy flight path from the nest (Saalfeld et al. 2010). Eagles in the Vancouver area have adapted to human-altered landscapes; management strategies should focus on maintaining edge habitat, monitoring population expansion in urban areas, and protecting nest sites (Goulet et al. 2021).

Migration

Research into Bald Eagles' migration has shown that eagles generally migrate south along the Appalachian Mountains and the Atlantic Coast in the fall then travel north to breeding grounds in the spring (Mojica et al. 2011). Lithium et al 2007 further justified this migration path by researching Bald Eagle usage of these two flyways within their study showing these same two corridors are used by the eagles. In another study, it showed four-movement strategies used by Bald Eagles in southeastern Alaska and western Canada: birds breeding stayed year-round, non-breeding birds migrated to the south for winter, non-breeding birds that stayed for their feeding sites, and birds that were nomadic in the area (Wheat et al. 2017). When comparing this to the framework of other migratory patterns, some similarities arose. For example, Jenkins et al 1992 showed that Bald Eagles departed within a narrow time frame, traveled alone in the same direction, displayed a determination to stay on course, and did not rest or stop to consume food (Jenkins et al. 1992). Bald Eagles also migrate in order to find food sources that are abundant, such as salmon (McClelland et al. 1994). While migrating, eagles often momentarily stop at interior reservoirs and lakes, open ranchland, and the Pacific marine coast in order to rest or find food (Hunt et al. 2009).

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Diet

The Bald Eagle is an opportunistic and generalist predator, and its diet mostly consists of prey in the area from which they reside (Buehler 2000, Thompson et al. 2005). Bald Eagles consume a large amount of prey and obtain it from hunting, scavenging, and stealing. Newsom et al. studied scat from eyries, which showed that diets contained on average 86.0% fish, 9.7% birds, and 3.7% mammals (Newsom et al. 2010). To expand this information Hanson et al 2017, look into the relative abundance of prey and further justified that Bald Eagles eat what is available to them in their environment (Hanson et al. 2017). To add further evidence, Harvey et al 2012 suggest that Bald Eagles use top-down control based on looking at the food webs of the ecosystem. In this study, the high consumption of Bald Eagles and their prey decreased the trophic effect of the eagle population throughout the rest of the food web. (Harvey et al. 2012). According to a study in California, Bald Eagles fed largely on waterfowl by scavenging ducks and geese as well as rodents that were in their environment (Frenzel et al. 1998). The birds specifically had migrated to the area and were abundant. Through these studies, it is shown Bald Eagles tend to eat a diet that consists mostly of fish and whatever prey is abundant in their area.

Reproduction

Research into Bald Eagle reproduction found that they can produce from zero to thirty offspring within their lifetime (Jenkins et al. 2006). This study further correlated that larger males with larger body mass produce more offspring throughout their life. In Mougeot et al. 2013, the density of the population was the cause for most of the variation in breeding performance. They concluded that milder springs are rarely in correlated with higher nesting success. When females arrive at sites with poorer conditions due to food shortages, breeding declines. Elliot et al. 2011 furthered this study by showing it can also be due to territory size and reduced food sources from the intraspecific competition (Elliot et al. 2011). While reproduction can be very high, there can also be a failure. In a study looking at Bald Eagles in Alaska, most nesting failures (92%) occurred during incubation when weather conditions were most severe (Steidi et al. 1997). Incubation is the most critical time and is the phase most determining if a nest will be successful or not. To further validate nesting failures during incubation, most active nests that failed in a study done in Alaska were abandoned during incubation (Gende et al. 1997).

Endangered

Bald Eagles were once on the endangered species list due to the chemical DDT and lead poisoning. In a study over the four flyways from 1997 to 2013, Trauma and poisonings throughout the flyways were the leading causes of death for Bald Eagles throughout the study period (Russell et al. 2014). Another study showed the ingestion of bullet fragments that resulted in lead poisoning. This was because of large game hunting and carcasses not being found and left for the eagles and other animals to prey on. This peaked in December due to elk and deer hunting: 91 % Of the Bald Eagles were admitted to the hospital with toxic lead levels (Stauber et al. 2010). January to April are the worst months for Bald Eagle lead poisoning due to the hunting season-ending. While there is not a ban on using these led bullets, there are education efforts being used to try and persuade hunters to use solid copper deer slugs as opposed to lead (Neumann et al. 2009). DDT was another big issue that was causing the Bald Eagle to go on the brink of extinction. DDT is a pesticide that thinned the eagles' eggshells, causing them to break during incubation (American Forests et al. 2014). This was causing a major decline in the entire population of eagles everywhere. The pesticide was polluting most of North America. In California, between 1947 and 1961 an estimated 37 to 53 million liters of DDT-contaminated acid sludge, containing 348–696 metric tons of DDT, was disposed of at an ocean dump site 16 km northwest of Santa Catalina Island (Sharp et al. 2005). Issues such as this caused a drastic decrease in Bald Eagles (Sharp et al. 2005). As a result of actions like these, in 1963, fewer than 500 pairs of Bald Eagles remained. Most birds were drastically affected by DDT and a nationwide ban of the pesticide went into effect on June 14, 1972 (Amsterdam News et al. 1997). After the banning, The Fish and Wildlife Service established five recovery plans with different goals for each of the different geographic regions. Over the course of 20 years, the number of breeding pairs in the wild increased from 100s to more than 5,000. (Wood et al. 1993). Augustin 8, 2007, the USFWS removed the bald eagle from the endangered species list (Martin et al. 2008). While Bald Eagles are off of the endangered list, there are still conservation efforts being put into place to assure they do not go endangered again.