The extraordinary productivity of the Chesapeake Bay makes it one of the most important sites for Bald Eagles. Individuals from eastern Canada and the northeastern US overwinter in the Bay, while Floridian birds spend their summers here. The visitors tend to concentrate in a half dozen locations around the Bay, but thousands of resident pairs nest around the Bay’s perimeter. The co-occurrence of breeding and non-breeding adult eagles throughout the year can introduce confusion to Atlas mapping efforts, so Maryland has traditionally only used Confirmed codes to represent blocks where eagles are nesting. However, other behaviors, such as courtship or territoriality, can indicate where an undiscovered nest may be, so for BBA3 we are encouraging the collection of Probable codes for Bald Eagles in addition to Confirmed codes.
Eagles breeding in the Chesapeake Bay likely stay in the general area of their nest year-round, but they associate closer to the nest during the breeding season. Bald Eagles appear to mate for life, and extra-pair copulations have never been documented. In Maryland and DC, Bald Eagles generally begin exhibiting courtship behaviors in December. As described by poet Walt Whitman, their famous cartwheel ‘dalliance’, is tremendously impressive—the pair flies high together, locks talons, and tumbles toward the ground. They release each other at the last moment and fly off, unharmed. Pairs will also engage in a chase display that sometimes involves airborne rolls and interlocked talons, as well as a J-shaped stoop.
Pair-bond behaviors coincide with their most intensive nest maintenance efforts, although nest maintenance often begins earlier in the fall and, indeed, can occur at any time of year. In Maryland, most nests are built in loblolly pines. Oaks and sycamores are also frequently used, and transmission towers have become less unusual nest sites. Bryan Watts, of the Center for Conservation Biology (CCB), has reported the extraordinary behavior of five ground-nesting pairs. Traditionally, preferred nesting habitat is mature forest that is undisturbed and within a mile or so of a large waterbody. The nest must be accessible to the eagles and capable of supporting their massive nests. This means that the nest tree is typically the largest tree in the area and extends well above the canopy.
Both sexes cooperate to construct their nest near the top of a live tree’s trunk along thick, horizontal branches. A typical nest is 5–6 feet across and 2–4 feet high, but because nest material is continually added, nests can become truly gigantic. The largest Bald Eagle nest on record was a Florida nest that was 9.5 feet in diameter and a whopping 20 feet tall. Bald Eagles use sticks found on the ground or broken off of nearby trees, and have been documented carrying nest material for a mile. Grass, moss, fine woody material, and feathers are used to line the nest cup. Depending on the tree shape, the nest may be flat, cup-, or cone-shaped. Across their range, Bald Eagles have an average of 1.5 nests per pair in their territory; if a nest attempt fails, they may switch to an alternate nest the following year. But as local eagle populations continue to grow and competition for nest sites increases, the average is likely to be lower in the Chesapeake.
Bald Eagles nest in every Maryland county and DC, but they are most abundant along the Chesapeake’s shoreline. Their territories (the area they defend against other eagles) average 0.4–0.8 mi2, but their home ranges (the total area they use) are much larger. Home range size can vary wildly, and studies of tagged breeding birds have documented a range of average sizes from 2.7–8.5 mi2. Both sexes will defend their territory from other eagles, and this aggression can extend to other species such as crows or Ospreys.
Fish are the preferred dietary choice of Bald Eagles, but they’ll eat whatever is available, including carrion. Bald Eagles aren’t particularly accomplished hunters and regularly resort to stealing from other birds, including Ospreys, herons, and other eagles. This kleptoparasitism occurs year-round, but is less frequent during the breeding season. Some individuals will become skilled at exploiting a particular local food source; one Florida pair learned to cooperatively hunt the Cattle Egrets that formed 85% of their diet, and Audubon reported eagle pairs cooperatively hunting waterfowl. On the Chesapeake Bay though, the most important food source is easily-captured dead fish. The scaly corpses, mostly gizzard shad and catfish, form as much as a quarter to two-thirds of their diet.
The propensity of Bald Eagles to chase other birds that have successfully captured food means that observers must be cautious when applying code T (territorial) or code A (agitated) to the eagles’ behavior. Similarly, an eagle carrying food is not necessarily carrying it to a nest site. Small food items may be consumed on the wing, but most prey is carried to a perch before being eaten. When code CF (carrying food) is used for eagles, it should always be accompanied by comments that indicate the food is being carried to the nest.
Incubation begins after the first of two or three oval, white eggs are laid in late January to early March. Both sexes incubate, although the female does most of it, and the chicks hatch asynchronously 35 days after they were laid. Asynchronous hatching, and the accompanying size difference between chicks, is a common strategy among species that have fluctuating food availability, like raptors. In years when prey is abundant, each chick has sufficient food and will likely survive. Conversely, if the parents are unable to provide enough food, the larger chicks will outcompete smaller siblings, who quickly die. This helps ensure the survival of at least one healthy chick, rather than more, weaker chicks—or no offspring at all.
For the first 2–3 weeks after the downy chicks hatch, the female spends 90% of her time at the nest; the male is there about half the time. Both sexes will bring food for the chicks, and after 3–4 weeks their contributions are about equal. The parents tear small pieces off and feed the pieces to the chicks. After five or six weeks, the chicks can feed themselves and the parents begin to roost away from the nest, typically in a nearby tree. In Maryland and DC, chicks normally leave the nest in May or June.
Recently fledged eagles are uniformly dark brown with white underwing coverts and armpits, as well as a dark beak and cere. Bald Eagles do not attain their distinctive adult plumage for 4–5 years; the intervening plumages contain increasingly more white. The parents dutifully respond to their chicks’ begging calls, feeding them for up to six weeks. The family will associate with each other for as long as ten weeks before the juveniles disperse. In one Florida study, young eagles stayed within a few hundred yards of the nest throughout the post-fledging period, but like other wide-ranging species, code FL (recently fledged) should be used only when it is likely the young birds were hatched in that block. When code FL is used, comments should be included that indicate why it is an appropriate choice.
Bald Eagles have rebounded tremendously since their despondently low population numbers of the 70s. The impact of organochlorides like DDT on eagles is well-known, but gratuitous shooting also played a major role in supressing populations. Despite a crude carrying capacity estimate for the Chesapeake Bay of 1,500 nests prior to European colonization, the first local survey of Bald Eagle nests in 1936 estimated only 600–800 in the same region. At the time, the Maryland human population was 1.7 million, indicating that there was likely much more suitable nesting habitat available than there is today, with a statewide human population 3.5 times larger. The Bald Eagle Protection Act (now the Bald and Golden Eagle Protection Act) wasn’t enacted until 1940, but even after that, wanton shooting of eagles was still widespread across the country—a bounty on Bald Eagles even existed in Alaska until 1952. The National Wildlife Health Center autopsied nearly 1,500 Bald Eagles between 1962 and 1984, and found that 22% had been shot.
The population of Bald Eagles breeding in the Chesapeake Bay is now estimated by Bryan Watts at CCB to be over 3,000, of which 1,500 are likely in Maryland. A decade ago, the number of chicks fledged per nest was one of the highest in the country. But as the eagles come up against habitat-imposed limits, Watts says that competition between adults has increased and fewer chicks are fledging.
Historically, remote nest sites have been critical for a successful breeding attempt. In fact, Audubon described eagles as being easily approached by humans—unless that person was carrying a gun. Perhaps not surprisingly for a long-lived species, Bald Eagles learned to associate humans with mortality and to avoid them wherever possible. However, as persecution of eagles has waned it has been conjectured that the eagles’ suspicion of humans may be decreasing. This possibility remains to be tested, but if true eagles might be expected to nest in more developed locations, which is certainly now the case.
The longevity of eagle nests, the value of the surrounding habitat to the species, and the role eagles play in monitoring ecosystem health means that reporting the location of each nest as precisely as possible is hugely beneficial. The easiest way to do this for the Atlas is by using the comment box. It’s not necessary to report the same nest’s location on every checklist, but every nest found should have the location documented.
Now a smashing example of conservation success, Bald Eagles were removed from the federal Endangered Species List in 2007 and from the Maryland list in 2010. Not even fifty years from the establishment of the Endangered Species Act—enacted because the persistence of so many species, including the Bald Eagle, was precariously uncertain—the Chesapeake Bay population of breeding Bald Eagles is at an astonishing ten times the initial recovery goal.
References: Audubon, J.J. 1840–1844. The Birds of America: from drawings made in the United States and their territories. New York: J.B. Chevalier.
Buehler, D.A. (2020). Bald Eagle (Haliaeetus leucocephalus), version 1.0. In Birds of the World (A.F. Poole and F.B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.bale