The Erie Nest

The Erie nesting Bald Eagles are emblematic of the approximately 200 pairs remaining that live in Colorado, a state that now ranks fifth in human population growth (photo 1). The pace of human development is highest in the northern Front Range (photo 2), home to the Erie Bald Eagle nest and the rest of our study area. Unfortunately, many of the counties in the northern Front Range—including Weld County, home of the Erie nest—lack land conservation programs, and unchecked development is diminishing wildlife habitat at a blistering pace.

Photo 1: U.S Census data from 2017, which shows Colorado to be the 5th fastest growing state in population.

Photo 2 U.S. Census data showing five of the fastest growing counties in Colorado, all of which surround the FRNBES study area.

Everything about Bald Eagles is large, and that includes the territories that they gather resources from and actively defend. In the ten-nest area that FRNBES studies in the northern Front Range, near-nest territories average 1.5 square miles (roughly 720 football fields). Scientific studies conducted by Colorado Parks and Wildlife (CPW), our state wildlife experts, recommend ½ mile buffer restrictions around eagle nests for nine months out of the year to avoid disturbance. While many Colorado developers have long abided by CPW’s nest buffers, these rules are currently not legally enforceable.   Furthermore, federal wildlife authorities have chosen not to enforce federal statutes that protect eagle nests and communal roosts from disturbance, even when violations are scientifically documented.

So what makes the Erie nesting Bald Eagles the “canaries” in the proverbial Colorado Bald Eagle coal mine? The answer lies in their recent history, which documents ongoing struggles with human encroachment over the past four years. These disturbances began with the legal removal of their original nest tree in 2015, paving the way for the construction of 2,200 homes on an undeveloped square-mile tract.  Since that time, the eagles have twice moved their nest location, largely due to the difficulty of finding a suitable nest tree and sustainable environment, and their productivity suffered as a result.

Recent History of the Erie Nesting Bald Eagles

Permitted Destruction of the Erie Nest

On December 15, 2015, a wildlife photographer contacted FRNBES and inquired about an old growth cottonwood near Erie, Colorado that had previously housed a Bald Eagle nest (photo 3). He informed us that the big cottonwood was lying on the ground along with several neighboring trees (photo 4). After a number of phone conversations with state and federal regulatory agencies, we were surprised to learn that eagle nests could be legally removed or “taken” via the permitting system of the US Fish and Wildlife Service. The Erie nest “take” was permitted in order to build 2,200 homes on this undeveloped tract.

Photo 3: The Erie Bald Eagle nest in 2013. Photo by Bryce Bradford.

Photo 4: The scene taken about 1 week after the Erie nest tree and adjacent cottonwoods were destroyed according to legal permitting.

Several days after the initial call about the Erie nest “take”, we visited the site on a cold and snowy day. We spotted the nesting Bald Eagle pair perched on an oil pumper, just behind the hulk of their former nest tree, which was lying in a tangle on its side (photo 5).  Determined to document the fate of these two pair-bonded Bald Eagles after their nest has been removed, we continued to visit the old nest over the following weeks.

Photo 5: Erie Bald Eagle pair on oil and gas pumper behind the toppled remains of their nest tree.

Following the Erie Pair to a New Nest Location (Season 1)

During that time frame, we spotted the pair flying northwest and followed them to a site about 2 miles from their former nest. We were delighted to find both Bald Eagles the next day near where they disappeared the previous evening.  The female was perched near the top of a weathered old growth cottonwood (photo 6), while the male was in an adjacent cottonwood. A new nest began to take shape in that old cottonwood during the next month (photo 7). There are many possibilities why the eggs laid in that nest failed to hatch after 55 days of faithful incubation (35 to 38 days is the average). First, this was a new pair, as the former male had been electrocuted near the old nest site only a month prior to the nest “take”. Other factors could have been the stress of setting up shop at a new nest; inexperience of the new, young male eagle (3.5 to 4 years old); or perhaps due to temporary egg abandonment on a frigid March day as a ditch excavator operated near the base of their nest. More of the Erie Bald Eagles in this new nest year can be seen in this short slide show with original music.

Photo 6: Erie adult female perched upon the site of their future nest in early January 2016.

Photo 7: Erie female launching from the new nest tree.

Season 2 at the “New” Erie Nest

Like all nesting Bald Eagles in the Colorado Front Range, the Erie pair continued to maintain their nest site through the following season, and began incubating eggs in late February 2017. A sense of this often-peaceful setting during early incubation is captured in a slide presentation, once again with original music.

After well over a year since the tragic loss of their former nest, and the previous year’s nest failure, it was heartwarming to witness two hatchlings appear in late April (photos 8 and 9).  The female Bald Eagle demonstrated a ferocious determination to protect and cover her babies through the many soaking rain and snowstorms during April and May (photo 10). On the morning of May 23, we received an early call from a farmer friend that lives adjacent to the nest—the nest had fallen during the night (photo 11). The old cottonwood supporting the nest could withstand no more of the incessant moisture, and both 7 week old eaglets fell to their death, along with the nest that evening.

Photo 8: Erie male tending to his two eaglets during the second season in the nearly dead, old-growth cottonwood.

Photo 9 Erie female taking flight leaving her two eaglets during the second season nesting in the nearly dead cottonwood tree.

Photo 10: One of the many May 2017 snowstorms and the Erie female covering her two almost full-grown eaglets.

Photo 11: Remains of the nest that fell on May 22, 2017 after numerous wet snowstorms.

The Erie Pair Find Yet Another Nest Location (Fall of 2017)

Once more, the Erie Bald Eagles faced the challenge of finding a suitable nest tree replacement.  Not only are old-growth cottonwoods the only nest substrate used by Bald Eagles in the Colorado Front Range (photo 12), but our studies indicate that these massive trees need to meet several additional requirements: 1) significant distance from human activity (typically ¼ mile); 2) relatively open canopy for entry, exit, and field of view; 3) proper branch structure on which to build a nest; and 4) proximity to waterways and other food sources. Another limiting criterion is that when nesting Bald Eagles are uprooted or choose to relocate, they prefer to re-settle nearby. Our studies of ten Bald Eagle nest relocations in the northern Front Range indicate that the average nest move is only 0.6 miles (1km).

Photo 12: Three old-growth cottonwood nest trees in the FRNBES study area.

During a careful survey of old-growth cottonwoods within one mile of the recently fallen nest tree, we were unable to identify any trees that met all of the above-stated requirements. A decision was made—a team from FRNBES would build a new nest on a platform, just a couple of trees away from their old nest tree (photo 13 and 14). Although we built an awfully nice nest, the Erie eagles passed on this location. Perhaps it was the frequent haying on the adjacent farm, or really who knows what else. Instead, they began building a new nest in September, 2017, about ¾ miles to the south within their original near-nest territory.  We didn’t spot this new nest until mid-October, and were thrilled to discover both eagles up in their home.

Photo 13: Nearly completed nest constructed by the FRNBES team.

Photo 14: Finishing touches on the platform nest.

So how did we miss this old-growth cottonwood in our survey? The likely answer is that the Erie eagles were forced to compromise, this time choosing a well-suited tree, in their familiar territory, near abundant prey, but out of necessity closer to human activity.  Their new home is situated on a quiet agricultural horse property, with a dirt access driveway passing only a few meters from the base of the tree (photos 15a to 15c). This tree likely served as a perch prior to being selected as a nest tree, allowing the eagles to become accustomed to the low-key human activity over time. When left without other good nest site options, they settled for a location that they knew well.

Vertical Photo 15a: Resident of horse ranch walking up the two- track driveway that passes near the Erie Bald Eagle nest. Male adult is perched above nest and vocalizing as the walker approaches. Top photo on right (15b) – Resident walking along dirt drive near Erie nest tree. Bottom photo on right (15c) – Zoom of Erie male eagle (from photo 15a) vocalizing his displeasure as walker approaches.

From the history of the Erie nest and of several others, it can be argued that Bald Eagles are being forced into more marginal nesting situations in eastern Boulder and western Weld Counties, likely contributing to these nest failures—especially if eagles are compelled to nest in unsafe trees so they can distance themselves from oil wells, subdivisions, and other disturbances.

2018-Finally A Successful Season for the Erie Nesting Eagles

Following four consecutive failed seasons, the Erie nesting Bald Eagles finally fledged three juveniles in June, 2018 (photo 16). Likely one of the more important factors in the 2018 season’s success is the stability of the nest, which is buttressed in a well-supported and stable crotch in the upper reaches of a relatively healthy old-growth cottonwood (photo 17). In early April of 2018, intense windstorms with gusts up to 89 mph swept the FRNBES study area, destroying four Bald Eagle nests, yet the Erie nest survived. The Erie eaglets were well-protected, both by the vigilance of their parents and the stability and positioning of the nest.

Photo 16: Erie female and three eaglets in May 2018.

Photo 17: Erie nest tree and Erie Bald Eagle pair. The nest is held in a strong supporting crotch in the upper reaches of the old-growth cottonwood.

Comprehensive studies by FRNBES (photo 18) documented how these three juvenile eagles used the surrounding terrain during the 2018 post-fledge dependence period (June 11 to August 14), a time during which the juveniles can fly yet remain within their nest territory. As we have found during our other post-fledge dependence studies, the three Erie juveniles relied upon their parents for food and protection.

Photo 18: FRNBES map showing use area of the three juvenile Bald Eagles during post-fledge dependence in the summer of 2018. Note the CPW ½ mile recommended buffer in pink; perches (green) and differing size solid pink circles denoting relative time at perches; planned development in yellow and orange; and potential development in salmon.

As shown in photo 18, the 2018 fledglings utilized an area that extends well outside Colorado Parks and Wildlife’s recommended ½ mile buffer, within which all human activity is to be restricted nine months out of the year to avoid unlawful disturbance of eagles. It is certainly discouraging to learn from land use documents that the entire eagle use area and much of the adjacent land is either already planned for near-term development, or will likely be developed in the future.

Polygamous Merging Between the Erie Nest and the CR16 Pair—Early Fall 2018

Verification of a 2-Nest Polygamous Triad

A momentous change occurred during early fall of 2018 when, initially unbeknownst to us, the adult male from the Erie nest disappeared. Our best conjecture is that the male from the CR16 nest, a nest less than two miles to the west, filled in for the missing Erie male sometime in September while retaining his bond with the CR16 female. However, this polygamous relationship was not confirmed until nearly six months later.

For many months, we puzzled over the daily late afternoon retreat of the male from the Erie nest, who would fly more than a mile to the northwest before disappearing low in the vicinity of Boulder Creek. Why would the Erie male roost at night so far from his mate? We soon discovered that this was not, in fact, the Erie male. Our suspicions of a polygamous triad were verified after setting up in the early morning with a good view of both the CR16 and Erie nests (photo 19). From this vantage point, we were able to observe what we now knew to be the CR16 male make several roundtrips to both nests, splitting his time between the CR16 and Erie females (photo 20). That male became known as the “shared male.”

Photo 19: CR16 nest to the left in the foreground, and Erie nest nearly 2 miles away in right background.

Photo 20: Shared male (blue circle) flying from the CR16 nest to the Erie nest in the background.

Spring 2019 in the Polygamous Triad—Success at Erie, Loss at CR16

Equipped with a new understanding of this unusual polygamous two-nest triad, we were better able to make sense of the remarkably long solo periods that both female eagles spent incubating and brooding at their respective nests. During this season, there was only one hatchling observed at the CR16 nest (photos 21 and 22). Up until April 24th, the growing CR16 eaglet remained well attended by the adult eagles (photo 23). However, this changed between April 25th through May 4th, with the eaglet being left alone at the nest for longer intervals, even going unattended for nearly a whole hour during one observation session. The shared male spent only brief periods at the CR16 nest during this nearly 10-day time period, and was often observed perching alone about a mile away in the original CR16 nest tree.

Photo 21: CR16 female and solo eaglet in late April 2019.

Photo 22: Shared male to left and CR16 female feeding lone eaglet in late April 2019.

Photo 23: CR16 female feeding her eaglet on April 2, 2019, just 3 days prior to finding the nest empty.

The last time we saw the nearly 40-day-old CR16 eaglet was on May 3rd; two days later we were devastated to find that the nest was empty (photo 24). While the fate of the young eaglet was never determined, it undoubtedly remained vulnerable to other raptors as it was at times left unprotected during the period leading up to its disappearance. In fact, two days prior the disappearance, a red-tailed hawk was observed lingering within 20 feet of the nest. After being absent for nearly one hour from the nest, the CR16 female suddenly appeared and immediately covered her young eaglet as the hawk retreated. It is possible that the eaglet was knocked out of the nest by a hawk, or perhaps one of the great-horned owls that nest in this same area along Boulder Creek. With the shared male spending little time by the nest, the CR16 female may have been forced to spend more time away than she otherwise would have, leaving the young eaglet unprotected from predators.

Photo 24: CR16 female perched at the top of the tall pole (perch L) to the left of the pumpkin (with a fake eagle on top). Photo taken on the same day the CR16 eaglet was first observed missing from the nest.

Following the mysterious loss of the CR16 eaglet on May 5th, the shared male eagle spent the next several days in close company with the CR16 female (photos 25 and 26). It is clear that, like humans, they also mourn.  With a nestling no longer requiring their careful attention in the evenings, the CR16 pair once again began their spring night-roosting ritual in the same dilapidated cottonwood that stands nearly ½ mile from their nest (photo 27). By about May 18th, nearly two weeks after the loss of the CR16 eaglet, the shared male shifted his attention to the Erie nest, and his prey contribution increased notably (photo 28). Yet toward the end of each of these days, the shared male would rejoin his original mate, the CR16 female. During the spring and summer, it was a pleasure to see the CR16 pair side by side at dusk, sometimes in the glow of sunset at their scenic night perch tree (photo 29).

Photo 25: The CR16 pair perched side by side (female on left) just two days after their eaglet went missing.

Photo 26: The CR16 pair once again perched side by side at one of their favorite perches just days after they lost their young eaglet.

Photo 27: Back to their old routine with no eaglet to attend to, the CR16 pair at their scenic night perch.

Photo 28: Shared CR16 male taking care of his juvenile eagles at the Erie nest, just two weeks after his CR16 eaglet was lost.

Photo 29: The shared male would regularly return from duties at the Erie nest to perch at night with the CR16 female. Photo taken during middle July 2019—about 1 month after the two Erie juveniles fledged—at the CR16 night perch upon sunset.

The two Erie juveniles began branching near the beginning of June 2019 at about 5 ½ weeks of age. The larger of the two juveniles (JV1), who was almost certainly a female—eagles are sexually dimorphic such that females are typically larger than males—dominated her smaller sibling (JV2) throughout the nestling stage, especially during feeding. Yet JV2 was by far the most accelerated in pre-flight skills and activity, and was the first of the two to fledge on June 19th, at eight weeks of age (photo 30). Please take a fun look at a video of JV2 spring-boarding during this time period, while his big sister looks on.

Photo 30: The smaller of the sibling juveniles (JV2) spring boarding about two weeks prior to fledge on June 1, 2019.

During the first two weeks post-fledge, both of the Erie juvenile eagles spent a majority of their time perched in the nest tree, interspersed with short duration flights (typically less than three minutes), mostly to and from the nest tree. As is typical during this time period, landing and perch balance was challenging (photo 31 to photo 35). Both juveniles would occasionally disappear from view for short time periods during this time period. Whereas the first two weeks of post-fledge activity is generally characterized by what we call the “herding cats” phase—with parents in close proximity to juveniles on the ground or in perches near to the nest tree (photo 36)—this was not the case this year at the Erie nest.

Photos 31-35: Erie Juveniles practicing landing skills in 2019.

Photo 36: Original Erie male in 2018 during early “herding cats phase” with two of the three juvenile eagles from that year.

At just beyond about two weeks post-fledge, both juveniles began disappearing from view more frequently, and adult presence also began to diminish. Another noteworthy change was that in the week or so post-fledge, the juvenile eagles were left unattended in the nest tree by the adult female at night.

A major milestone for fledglings is the transition from short duration, mostly flapping flight to longer soaring flights. The two Erie juveniles marked this important transition on July 4th, about 15 days after their first flight. The presence of siblings during the early days of soaring flight makes this this one of the most exhilarating time periods, both for the fortunate observer, and obviously for the juveniles as they soar together for the first time. (photos 37 to 39).

Photos 37-39: Erie juvenile siblings in 2019, about 3 weeks post-fledge, enjoying paired soaring flight.

Erie Female’s Ritual Disappearance

As she did in 2018, the Erie female (photo 40) once again mysteriously disappeared about 2.5 weeks after her young eagles had fledged. For the next two weeks, the juveniles appeared to be supported only by the shared male. During this time period, we conducted exhaustive searches for the Erie female, and like the year before, her whereabouts remained undiscovered. Although her absence was unsettling, we have also documented a similar-timed disappearance of the Stearns adult female (see Stearns nest), and with our combined experiences, we remained confident that Erie female would return.

Photo 40: Erie female watching over her nest and eaglets. 

An Awkward and Intense Introduction

Several days after the Erie juveniles began extended soaring, they embarked upon an adventure that would bring them face to face with their dad and their unreceptive stepmother, the CR16 female. Up until that day, this strangely-related family had to our knowledge never met, although both females can clearly see the activities at each other’s nests, even at two miles away. The Erie juveniles added fuel to what would already have been a tense encounter when they landed in the CR16 pair’s former nest tree, which has remained very much under their ownership.

The CR16 adults were perched in a nearby tree when the Erie siblings arrived in the old CR16 nest tree. After a short pause, the CR16 female flew to the top of the nest tree, displacing one of the juvenile eagles to the middle of the tree. For what seemed like an eternity, the CR16 female stared menacingly down at both siblings (photo 41). This détente was broken when the male attempted to feed a fish to one of the siblings which, evidently was the last straw for the CR16 female, who immediately chased both young eagles from the tree (photo 42). One can imagine this drama playing out in a human scenario, with mom coming home to an unexpected meeting with her husband’s children from another mother. This was certainly a strange and awkward experience, both for the eagles and the wide-eyed observer.

Photo 41: CR16 female (top middle blue circle) glaring down at one of the Erie juveniles during an awkward visit to her original nest tree. Male top right.

Photo 42: Moment of allegiance, as the shared male (top left) watches his primary mate (CR16 female) chase after one of the Erie juveniles (blue circle). Moments prior to this photo, the male left a fish for the juvenile—apparently the last straw for the CR16 female.

Return of the Erie Female

We were ecstatic to find the Erie female perched side by side with her two juveniles in the nest tree as dusk approached on July 21st (photo 43). Her disappearance this season lasted nearly two weeks, a time roughly equivalent to her sabbatical during the previous season. Unlike the Stearns female—whose post-fledge sabbatical continues until after her offspring have dispersed–the Erie female has returned the past two seasons to spend the last week or more with her young eagles. While we suspect this same behavior is likely employed by other nesting Bald Eagle females during the post-fledge to dispersal period, females at two other nests in our studies do not leave during this period.

Photo 43: Return of the Erie female who is joined by both of her fledglings after an absence of over two weeks.

During the Erie female’s disappearance, we noted that both juveniles increasingly spent time away from the near-nest area, but after her return they began nucleating back to the nest tree (photo 44). The Erie female and her two juveniles began roosting at night together in the nest tree, something we had not observed since before the juveniles fledged more than a month earlier (photo 45). During this time of increased family bonding, the Erie female seemed especially attentive to her young, which was particularly notable one evening (July 26) when she performed back-to-back prey deliveries to satisfy the needs of both siblings.

Photo 44: Erie female and her juvenile eagles soon after her nearly two-week hiatus from the nest area.

Photo 45: For nearly two weeks after the female returned to the nest area, all three eagles—mom and juveniles—once again began perching at night together in the nest tree.

Disturbance of the Erie Reunion

The week-long reunion period neared its conclusion on July 28, when a crop duster circled repeatedly in very close proximity to the eagles in the nest tree (photo 46). All three eagles flushed and scattered on several of the airplane’s approaches, and collision between the plane and eagles was a major concern. FRNBES had spoken twice with the contracting farmer the previous year about this same activity, and kindly requested that the pilot utilize another approach that would not disturb and endanger the nesting eagles. This incident was reported to federal and state authorities. FRNBES has once again written in 2020 to the farmer, warning to keep a safe distance from the Erie and nearby CR16 nest.

Photo 46: Crop duster circling over the Erie nest on July 28, 2019. The Erie female and both juveniles scattered repeatedly from the nest tree, frightened by the airplane’s close passes.

2019 Dispersal of the Erie Sibling Juveniles

For most of the day following the disturbance by the crop duster, the eagles remained away from the nest tree. However, for the next two evenings at least one juvenile and the Erie female returned to the nest tree to roost at night. Neither of the Erie juveniles were observed after July 30th, marking their approximate dispersal date. The Erie female and shared male were observed in the vicinity of the nest tree until July 31, but neither were seen near the nest again until the end of August (photo 47).

Photo 47 The Erie nest tree after dispersal of both juvenile eagles.

2019 and 2020 Season

At the time of this writing, the 2019-2020 season for both the Erie and CR16 nests is nearing its close. We will provide updates on the progress of these nests very soon.

Page updated 8-23-20
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