COOPERATIVE BREEDING AMONG BALD EAGLES

The streaming video cameras reveal more intimate detail of cooperative breeding than do ground reports and aerial surveys, while at the same time raising more questions and causing more uncertainty about the behavior and its causes. This page describes all of the reported instances of cooperative breeding among Bald Eagles, followed at the end by some of my observations and thoughts.

The 2 cases of cooperative breeding among Bald Eagles documented on video cameras are on Catalina Island off the coast of southern California, and near Lock & Dam 13 on the Illinois side of the Mississippi River:

CA, Catalina Island, 1992-2007

In 1980 the Institute for Wildlife Studies (IWS) began a partnership with the U.S. Fish and Wildlife Service and other organizations and agencies to establish the Bald Eagle Restoration Project on Catalina Island. Bald Eagles had ceased successful breeding there by 1950 at least partially as a result of DDE contamination that damaged eggshell production. The program has expanded, and IWS continues to monitor a number of Bald Eagle nests as well as those of Peregrine Falcons on the Channel Islands.
In the early years of the Restoration Project, researchers collected eggs laid at nests and artificially incubated them, replacing them in the nests with artificial eggs for the parents to “incubate.” Chicks hatched in the incubation facility were fostered into nests, either those of the parents or other nests, where parents brooded the eaglets until they fledged.
At the “West End” nest on Catalina Island in 1991, the female-male pair (she had no patagial tags and in 1998 was named “Wray”; he was K-77 or “Shenandoah”) produced 2 eggs which were removed and replaced with 2 artificial eggs. During incubation, a third adult eagle entered the territory several times but was “driven off by a member of the nesting pair” (Garcelon et al. 1995). The resident male and female successfully raised a foster eaglet to fledge.
In 1992 IWS installed a video camera on the nest (not yet streaming online). The pair laid 2 eggs which were removed and replaced with artificial eggs. A foster eaglet was placed in the nest, and the third eagle was observed incubating, brooding, delivering food and feeding the nestling, and defending the territory against human researchers who “invaded” the nest to band the eaglet. This helper was determined to be a female (K-69, later named “Dianna”) based on her size, and she brought twice as much food for the nestling as Wray did. The 2 females were hatched the same year in different nests on Vancouver Island, captured, and released from the same hack tower on Catalina Island in 1986, leading IWS ecologist Peter Sharpe to remark that “they may think they are sisters,” although genetically they were unrelated. (IWS). Researchers speculated that Dianna’s “helping behavior may have been driven by a lack of available mates” (Garelon et al. 1995).
The same trio persisted from 1992-2001. All eggs were removed and replaced with artificial eggs, and in some years foster chicks were placed in the nest. The 2 females shared parental duties but not always equally. In 1992 2 eggs were laid, 1 hatched, and 1 foster chick was raised to fledge. In 1993 and 1994 apparently no eggs were laid; 1 foster chick was raised to fledge in both years. In both 1995 and 1996 2 eggs were laid but none hatched; 2 foster chicks were raised to fledge in 1996. The eagles did not use the nest in 1997. In 1998 2 eggs were laid, 1 hatched, and 2 foster chicks were raised to fledge. In 1999 3 eggs were laid and none hatched; 1 foster chick was raised to fledge. In both 2000 and 2001 2 eggs were laid, none hatched, and 2 foster chicks were raised to fledge.
In 2002, 4 eggs were found in the nest. 1 of those eggs hatched, and for the first time the chick was placed back into the parental nest and raised to fledge, along with 1 foster chick.
2 eggs were laid and removed in 2003 but neither hatched. A foster chick was placed in the nest and raised to fledge.
In 2004, 3 eggs were laid, 2 of them within a day of each other (February 22-23), 1 each by Wray and Dianna, establishing this case as the rare system of cooperative polygyny. The eggs were removed, and 1 hatched. A foster chick was raised to fledge.
IWS began streaming the video camera online in 2005 although it malfunctioned during the incubation period; 4 eggs were removed, and 1 hatched chick was returned to the nest and raised to fledge.
K-77 disappeared in 2006 and was replaced by a new male (K-01 or “Superman”). 2 eggs were removed from the nest and neither hatched. A foster chick in the nest suffered a broken leg but was rehabbed and later fledged.
2007 was a banner year, when 5 eggs were laid, at least 1 and possibly 2 of them by Dianna; 1 of the eggs was found in a separate egg cup a few inches away from the principal cup (photo on p. 11 of this IWS report). 3 of the eggs hatched and the chicks were returned to the nest and raised to fledge.
Dianna disappeared in March 2008, just prior to the egg-laying period, and was not seen again.
References: Garcelon et al. 1995; Garcelon & Sharpe 1994-2007.

IL, Upper Mississippi River Refuge, near Lock & Dam 13, 2014-present

The Mississippi River is a well-known flyway for Bald Eagles and many other migrating birds. Floater juvenile, sub-adult, and adult Bald Eagles roost in dramatically increased numbers during the winter at sites around the locks and dams where tailwaters remain open and the eagles can take fish.
The Stewards of the Upper Mississippi River Refuge have operated a camera on a nest in the refuge since 2011 (not streamed online until 2016). The eagles have relocated within the refuge several times.
In 2012 the established female, named “Hope”, was joined by what appeared to be a new male, named “Valor”. They laid 2 eggs but Valor was erratic in his incubation behavior, suggesting this was his first year breeding. Although both eggs hatched the chicks died probably for lack of consistent care.
Hope and Valor built a new nest in 2013, but a new adult male entered the territory and appeared to replace Valor as the breeding male. He and Hope had 2 eggs, both of which hatched.
A new nest location made observation difficult in 2014, although 3 adults were seen in the nest area. In 2015 the eagles relocated to the 2013 location where no camera was available, but again 3 adults were seen in the area, and 3 eaglets fledged that year.
In 2016 the camera finally was in position to record nest activities. Hope was joined by the original Valor (now “Valor I”), and the second male (now “Valor II”) remained. Hope was seen copulating with both males, so this was documented as a system of cooperative polyandry. The trio produced 3 eggs (although it was impossible to determine if both males were fathers), all of which hatched and the chicks fledged. All 3 adults participated in parental care.
In 2017 the same trio produced 3 eggs, and again both males engaged in copulation. 2 of the eggs hatched. Five weeks after hatch 2, intruder adults attacked the nest and after the ensuing battle with 1 or more of the trio, Hope disappeared and never returned to the territory. The 2 males continued to care for the eaglets, both of whom fledged. Intruder eagles continued to harass the nest.
A new female appeared at the nest in September 2017. Her plumage suggested that she was a young adult, and she was named “Starr.” Both males copulated with her, and in spring 2018 she laid 2 eggs, both of which hatched, although 1 died before fledging. All 3 adults contributed parental care.
In the 2018-2019 season the trio of Starr, Valor I, and Valor II raised 3 eaglets to fledge. In 2019-2020 they produced 2 eggs and 2 fledglings. The nest tree was blown down by strong winds on 8 August 2020, and the trio eventually built a new nest, inaccessible to the videocamera. As of the end of April 2021 ground observers have confirmed 3 eaglets being cared for by all 3 adults.
References: Stewards website; Stewards FAQs.

Other than these 2 instances of cooperative breeding among Bald Eagles that have been observed via nest videocam, 7 other documented cases of more than 2 eagles at a nest or of 1 eagle attending 2 different nests during the breeding season have been reported in published literature, and an 8^th possible case is discussed on a Facebook page:

AK, Amchitka Island, 1971, 1972, 1974, 1980

Amchitka Island is in the chain of Aleutian islands off the southwest coast of the state. It has a large Bald Eagle population, with frequent movement about the island as well as to and from other islands in the chain.
Helicopter surveys of the island revealed 2 nests in 1971 and 1972 and 3 nests in 1974 at which 3 adults were in attendance. The birds perched together and were observed in the nest together. Sexes were undetermined. Subadults were seen in the area, which observers speculated might be “the young of that nest from a previous year.”
Ground observers in 1980 located 3 nests with 3 adults each, sexes undetermined. One of these nests was later found abandoned with 1 unhatched egg, 1 deceased hatchling, and 1 deceased fledgling.
Clear-cut alloparental behavior by a helper eagle was not observed in any of these cases.
References: Sherrod et al. 1976; Heglund & Reiswig 1980.

CN, Barkhamsted Reservoir, 1993-1995, 1998

Bald Eagles have nested in the area since 1990, and ground observers made numerous observations of 1 nest in particular.
In 1993 a pair of eagles (both with a single band on the right leg and probably from MA) produced 1 chick and reared a second chick fostered into the nest by the CN Dept. of Environmental Protection. A third adult with bands on both legs was observed bringing food to the nest and feeding the chicks a few days before they fledged, but the next day it was chased from the nest tree by one of the resident pair. Later the helper brought food to the fledglings several times without challenge by the resident pair. At the time the helper was believed to be male, based on its size, but later was judged to be a female, probably from Maine.
In 1994 another extra eagle, likely a female, with only one band and thus presumably not the same as the year before, was chased away by the male parent. Later another extra adult, this one with 2 bands, was seen feeding the chicks in the nest and later feeding the fledglings. She was judged to be a female based on size and may have been the extra from the year before.
In 1995 the resident pair were seen incubating. A third adult, likely the same female eagle from 1993 and 1994, was seen feeding the chicks both before and after they fledged. The original female this season was unseen after the incubation period and was replaced by the extra female.
The resident male and new female began incubating in 1996 but the nest failed, and in 1997 the pair produced 1 fledgling.
In 1998 two eggs were laid, and a new extra adult female eagle appeared in the nest area about 3 weeks after they hatched. She later brought food to the nest tree for the fledglings.
There was no reporting after the 1998 season.
Researchers speculated that helpers’ assistance late in the breeding suggests the parents were “relaxing their guard as the young grew large enough to fledge.”
References: Hopkins et al. 1993, 1995, and 1999.

MN, Chippewa National Forest, 1976-1978

Small-plane surveys located a nest with 3 eagles present in 1976 and 1977. One of the eagles had “an off-white head,” thus probably a sub-adult about 4½ years old. It was observed perched next to the nest with one of the resident pair. The following year an eagle “with a nearly white head,” possibly the same extra as the year before, was seen in the nest with 2 nestlings. In 1978, 3 eagles in adult plumage were seen, 2 in the nest with a nestling and the third perched 200-300 meters from the nest tree; it seems probable that the third eagle was the same as in previous years.
Sex of the extra eagle was not determined, and it is unknown whether alloparental behavior by the extra eagle was involved.
Reference: Fraser & Frenzel 1983.

NY State, 1986-1993

An abstract of a report made at a meeting of the Raptor Research Foundation in 1993 described 2 instances of cooperative breeding in New York state.
At one nest a trio of 1 female and 2 males fledged 9 eaglets in 6 out of 8 breeding seasons. The researcher reported this as a “polyandrous” group although the report does not mention EPC.
In the other instance, a trio of 1 male and 2 females maintained 2 nests less than a kilometer apart and fledged 11 eagles in 6 out of 8 breeding seasons. Even though EPC is not mentioned, the description by the researcher of this group as polygynous seems plausible since each female probably had her own nest with her own eggs.
In both cases all adults at both nests “participated in nest building, incubation and rearing (food provisioning).”
Reference: Nye 1994.

TX, Llano River, 2004-2006

Three adults formed a trio over 3 breeding seasons. The extra appeared to be a mature female, based on plumage and size. She engaged in nest building and maintenance, food delivery, feeding eaglets, and nest defense. She was last observed in January 2007.
References: Orrega et al. 2006 and 2009.

BC, Vancouver Island, 2004-?, 2014-2015

A nest near the mouth of Parksville Bay, in an area of high Bald Eagle population, was built in 2008.
In 2014 an extra female adult perched nearby while the resident pair incubated and brooded 2 eaglets. During incubation and brooding all 3 adults defended the nest and territory, delivered food, and fed the eaglets.
In 2015 the same behavior was observed. The helper female was seen less as the nestling period progressed. Later in October the tree and nest fell, and all 3 adults began building a new nest in an adjacent tree.
Ground reports submitted to The Bald Eagle and Osprey Nest Record Registry of the Wildlife Tree Stewardship Program suggest that a trio occupied the territory from as early as 2004, when the extra was not yet a mature adult.
References: Dawe 2016; Wildlife Tree Stewardship Atlas.

CO, Weld County, 2018-2019

Since 2015 nest observers from the Front Range Nesting Bald Eagle Studies group have monitored 11 Bald Eagle and Golden Eagle nests on the front range of the Rockies, including 2 BAEA nests that are fewer than 2 miles apart:
- - The Erie nest was occupied from at least 2013, but the male was electrocuted in November 2015. A developer obtained a permit to fell the nest tree in December. A new male and the resident female built a new nest about 2 miles away in early 2016 and were observed incubating, but no eggs hatched. They were incubating again in late February 2017 and had 2 hatchlings in April, but the nest fell on May 22 as a result of bad weather and the eaglets were lost. The pair built another nest about ¾ mi. away in September and fledged 3 eaglets in June 2018.
  - A pair of subadults bonded in the CR16 territory in February 2016 and worked on several nest sites. Incubation was first observed in February 2018, with 2 hatchlings in March, but the nest tree was destroyed by wind and the eaglets perished. The pair built a new nest in fall 2018 on an old heron’s nest.
In fall 2018 monitors noted that the male at the Erie nest regularly flew away from the nest in the direction of the CR16 nest, and they also saw that the females at the two nests incubated for long periods of time without relief. In March 2019 watchers observed and photographed the CR16 male flying back and forth between the two nests and spending time at both of them. The observers deduced that the resident Erie male had disappeared sometime in the fall and that the CR16 male had taken up duties at the Erie nest in addition to those at his home CR16 nest. They dubbed him the “shared male.”
One eaglet hatched at the CR16 nest and was cared for by both parents until late April, when the male became less attentive and was seen perching away from the nest. The eaglet was not seen after May 3 when it would have been nearly 40 days old, and its fate was never discovered. The male afterward stayed with the CR16 female for several days, but he then began spending more time at the Erie nest and delivering food for the two eaglets there, although he continued to spend nights with the CR16 female.
The Erie eaglets fledged in June 2019. In one incident they visited the CR16 nest tree, where the shared male offered them food, but the CR16 female chased them away from her territory.
The observers assumed that the one male was the father of both the Erie eaglets and the CR16 eaglet, a case of extra-pair copulation, although they did not report seeing copulation between him and the Erie female. While this is falls in the category of “cooperative breeding,” there is no evidence that either female was aware of the polygynous behavior on the part of the male.
The Front Range Eagle Studies group has not reported on breeding during the 2019-2020 or 2020-2021 seasons at either nest as of 4/28/21.
References: Front Range Eagle Studies – The Erie Nest, and Front Range Eagle Studies – The CR16 Nest.

PA, Seigarts Island Wildlife Management Area, 2016-2017

A Facebook page called Gertie’s Notch Nest reports a trio of adults at a Bald Eagle’s nest in central PA, along the Susquehanna River in the Sweigarts Island Wildlife Management Area. The Facebook page name is presumably an alternate spelling of the town of Girtys Notch near Liverpool, where the reported nesting first took place.
Details about the history of the trio are unclear, but the Facebook page reports an intruder in the area in 2015 and the trio together in 2016 when 1 eaglet fledged. The current nest was built in 2017 on a small island in the river, and 2 eaglets fledged.
There is little information on nesting activities of the 3 adults, including whether the third eagle provides alloparental care, and no determination of the sex of the extra.

Several observations arise from these cases:

More female helpers were reported than male (6 female, 3 male, 10 unknown), contrary to the usual arrangement among cooperatively breeding raptors and birds in general.
In only 1 instance was it possible to determine the genetic relationships within the group, the CA, Catalina Island nest, where the helper was an adult female not kin to the dominant pair.
18 of 19 helpers were mature adults, as is typical of raptor cooperative breeders but not of cooperative breeders among birds in general.
Cooperative polygamy was confirmed in only 4 cases, 3 of polygyny and 1 of polyandry. Since cooperative polygamy is reported to be more common than non-polygamous pair nesting among raptors in general, the few reported cases among Bald Eagles seem likely to be a result of inconsistent observations at most of the nests.
All instances involved only 1 helper, unlike in most raptor species. Perhaps this has to do with the large size of Bald Eagles whose nests would have to be even larger than they already are to accommodate more than one additional full-grown eagle.
Most (9 of 12) helpers were observed engaging in alloparental care of nestlings and/or fledglings. Those where alloparental care was not seen may be the result of inconsistent nest monitoring by observers.
The longevity of the helping behavior ranged from 1 to 16 years (the latter at the CA, Catalina Island nest), with an average of about 5 years. In one case (MN, Chippewa National Forest) an older subadult eagle likely became a helper through at least its first 2 fully mature years.
In 2 instances birds that were initially chased away by the nesting pair later became helpers. It is impossible to know why an established pair would treat another Bald Eagle as an intruder at first but later accept its help, or whether an intruder from the beginning was either attempting to challenge for the nest or offering its help. Click here for discussion of intrusions at Bald Eagle nests.

Here are a few more thoughts:

CA, Catalina Island

Researchers speculated that Dianna’s “helping behavior may have been driven by a lack of available mates” (Garcelon et al. 1995). This seems plausible, since at the time the trio began there were few eagles on the island. But it doesn’t answer the question of why Wray was willing to accept Dianna, especially if Dianna was the eagle who was “driven off” from the territory the year before a third eagle was accepted as a helper (Garcelon et al. 1995).
Speculation that the 2 females “may think they are sisters” (IWS) because they spent their pre-fledging weeks together in a hack tower would explain some familiarity between them, but related helpers are usually offspring of the nesting pair, not siblings.
In 2007, 1 egg was laid in slightly apart from the nest cup where the other 4 eggs were laid, as if the 2 females were staking claim to their own “nests.” This is unlike what happened in 2004, when the females each laid an egg on the same day but the eggs were not observed to be in separate areas. So what brought about the egg sequestration in 2007? Some conflict between them was observed on camera, and perhaps this increased as Dianna’s cobreeding continued, leading her to establish her own “nest.”
Could Dianna’s departure before the 2008 egg-laying season have had something to do with the original male’s replacement by Superman in 2006? The eggs laid that year did not hatch, and although 2007 was successful for both females, perhaps the new male somehow communicated that one mate was enough for him.

IL, Upper Mississippi River Refuge

The countless Bald Eagles roosting along the Mississippi River during the winter might be constricting the breeding capacity of the area, leading even mature adults to offer to and accept help from other adults.
What induced Hope to accept first Valor I, then Valor II, then both of them? Which of them should be considered the dominant male, or are they equal cobreeders? Could the 2 Valors have been siblings and thus shared some mutual familiarity? But even if so, it begs the question of why siblings would become cobreeders, in contrast to the more typical offspring.
It is remarkable that after Hope disappeared during the 2017 season, the males retained the “group” structure, staying at the nest and jointly caring for the eaglets. This alloparental partnership between the 2 males may have solidified an unusual bond between eagles of the same sex, which continued through the following season when they together accepted a new female, Starr.

AK, Amchitka Island

This region has a large population of Bald Eagles, and several instances of cooperative breeding reported as far back as the early 1970s are not surprising. If other observations of cooperative breeding throughout the breeding territories in Alaska reveal a higher than typical number of instances, this would lend credence to habitat saturation as a significant factor in the occurrence of cooperative breeding.

CN, Barkhamsted Reservoir

In 1993 an extra female (2 leg bands) was chased from the nest but later allowed to help. The following year a different extra female (1 leg band) was driven off, then the earlier helper (2 leg bands) resumed alloparental care, suggesting that she had established a relationship with the nesting pair that the other extra was not allowed to usurp. This is reinforced by the fact that she replaced the original female in the third season.
Researchers speculated that the fact that feedings by helpers occurred close to and after fledge may have been because the parents were becoming less attentive. But this might seem counterproductive. Parents typically taper off their feeding after the eaglets fledge so that the fledglings can learn to fend for themselves, and it might defeat the purpose for the parents to allow a helper to provide food during this critical stage.
That the helpers found an opening late in the season to provide food despite earlier parental hostility speaks to their determination to assist, which suggests the helping behavior was driven more by the helpers than the parents.

MN, Chippewa National Forest

This is the only case where a subadult was seen in the nest with nestlings, and this eagle appears to have continued to be present at the nest for at least 2 more seasons.
Observers did not report specific alloparental care in their infrequent aerial surveys, but it seems likely to have occurred.

NY State

This is the only reported instance in which a polygynous trio had 2 nests in different trees, presumably with each female having her own and the male dividing his time between them. It seems likely that such an arrangement might be more common among Bald Eagles than has been observed or reported.

BC, Vancouver Island

The Bald Eagle population is high in this area and habitat saturation should be considered as a possible catalyst for cooperative breeding. As in Alaska, more observations of cooperative breeding in and around Vancouver Island might shed light on whether habitat saturation is an important cause.

More surveillance of Bald Eagles via streaming video cameras seems likely to reveal additional information about cooperative breeding, even while on-going monitoring of nests from the ground continues to be essential. Observations by the numerous online viewers might someday show up on the radar of researchers and perhaps lead to better understanding and more answers to the numerous unresolved questions.