The CR16 Nest

A New Pair-2016 Beginnings

The CR16 Bald Eagle pair were dirty headed subadults when we first observed them in February, 2016 (photos 1 and 2). At the time, they were busy adding on to a small nest that they commandeered from red-tailed hawks in a prominent old-growth cottonwood tree along Boulder Creek, in western Weld County (photo 3). These young eagles continued to linger in this old- growth cottonwood through March of that season, although the nest remained relatively small, and no eggs were laid. While we were busy studying other productive nests during that season, we lost track of the young CR16 pair until the following fall.

Photo 1: CR16 female soaring above ponds near original nest tree.

Photo 2: CR16 male searching for fish above a pond near their original nest tree.

Photo 3: CR16 pair and small red-tailed hawk’s nest in March 2016.

Season Two—The CR16 Pair Reaches Adulthood

What we presumed to be the same pair was once again spotted in the same old-growth cottonwood in mid-November, 2016 (photo 4). They had begun adding material to the nascent nest from the previous winter, and by this time the nest size had grown substantially (photo 5).  Now almost 7 months older, both eagle’s heads were nearly all white, although the female retained some dark feathering on her head, and some dark areas were mixed in her mostly yellow beak (photo 6a and 6b).

Photo 4: New and larger nest in November 2016.

Photo 5: CR16 male and new nest in November 2016.

Photo 6a: CR16 female still with dark mottled beak and remnant dark feathering behind eyes.

Photo 6b: More recent photo of CR16 female, showing her distinguishing feathering indentation behind her eye.

The nest continued to grow through the end of January, 2017, but ferocious winds in late January and early February destroyed most of the new nest (photo 7).  Meager attempts at nest reconstruction were attempted thru mid-February—mostly by the adult male (photo 8)—although, very little progress was made. Please see slide show with original music from this time period.

A few more rounds of nest rebuilding were noted through early March. During that time, the pair were commonly observed perched at a tiny nest–reported by residents to have been previously built by osprey– in a nearly dead tree about ½ mile west of their original nest tree (photo 9).  Could the pair have claimed this tiny nest to lay their eggs that were imminently due, rather than do so on the ground? A suspicion that we were unable to confirm.

Photo 7: Remnants of new nest after punishing winds in January 2017.

Photo 8: CR16 male brings stick in a futile attempt to repair wind damaged, and largely destroyed nest in early February 2017.

Photo 9: Small osprey nest in what we call perch D, where CR16 adults began spending time after their nest was destroyed by wind.

The CR16 Nest Territory—Bucking A Trend?

It is notable that the CR16 pair established their first nest site only 1.5 to 2 miles from adjacent Bald Eagle nests. This is nearly one-half the average nearest-nest distance of 15 active nests in our 2,000 km2 (1,243 mile2) study area.  Since nesting Bald Eagles establish nest density or distance to nearest-nest based on available resources, we wondered if this abbreviated nearest-nest distance might have negative consequences due to territorial competition for limited resources. While over time, some aggressive interactions with adjacent territorial Bald Eagles were recorded, the majority of competition and disturbances originated from other sources including: 1) non-breeding, wintering bald eagles utilizing their nest tree and nearby resources (photo 10), 2) spring through summer camping and boating at the nearby ponds (photo 11), 3) oil and gas development (photo 12) , and 4) periodic shooting and other activity by the landowner near the base of the nest tree.

Photo 10: Non-breeding Bald Eagles share the original CR16 nest tree and adjacent perches. While the CR16 pair (white headed adults) occasionally would defend their nest tree, they spent little energy chasing away the plethora of eagles that came to utilize their nest tree during migration up the Boulder Creek corridor.

Photo 11: Jet ski during fall of 2017 traveling along the pond below the CR16 nest tree.

Photo 12: Oil and gas “workover” drill rig next to the CR16 nest tree in January 2017.

The CR16 pair continued to occupy their territorial perches within about 3/4 mile radius of their original nest tree through spring and summer of 2017. During late spring through early November, the pair were observed most nights roosting side by side in a weathered old-growth cottonwood about ¾ miles west of their original nest tree (photo 13). We continue to marvel at the sight of the CR16 pair in this same tree while heading home from field work most nights during this same time of year.

Photo 13: Perch E or night roost tree that the CR16 pair occupies most nights from May through middle November.

Fall of 2017—Time to Find a Quieter Area to Nest

A dramatic shift in perch activity was noted in early October, 2017, as the pair began spending much of their time on perches localized about 1 mile south of the original nest tree (photo 14-16). This shift in perch locations was correlative with construction of a new nest along a relatively remote section of Boulder Creek, just to the west of these new perch sites (photo 17).  The nest was built upon an existing heron nest, located at the exposed top of a tree that would sway even in modest winds.  The young pair began incubation that year around February 18, and first hatch was noted on March 23. The hatchlings were regularly attended to by both adults, but something was clearly wrong on April 4th, when both adult eagles were observed perching side by side about 1 mile away from their nest, in their previous nest tree. Concerns of a disaster were confirmed minutes later, as the entire nest and tree had been blown to the ground during recent windstorms.

Photo 14: CR16 pair at perch K. Boulder Creek is in the background marked by the dead old-growth cottonwood. Starting in the fall of 2017, the CR16 pair regularly started to utilize a number of common perches in this area. Not until December of 2017, did we finally discover that the CR16 eagles had built a new nest nearby along Boulder Creek.

Photo 15: CR16 pair perched in a dead cottonwood tree along Boulder Creek in Fall 2017. Their new nest was located only 1/3 mile from this perch.

Photo 16: Another common perch for the CR16 pair in Fall 2017 (perch Q). The metal building is abandoned and belongs to the U.S. Department of Commerce. Fortunately, all these power lines are de-energized and no longer pose an electrocution risk.

Photo 17: The new CR16 nest built upon a former heron nest, atop a relatively thin and spindly cottonwood tree. Photo taken in February 2018, about 1 week after onset of incubation.

The CR16 pair remained in their established nest territory following the loss of their nest through the following summer, and in the early fall began constructing a new nest, within a few hundred feet of their fallen nest tree (photo 18). Once again, the new nest was constructed upon a heron nest situated at the exposed pinnacle of the tree. Like the previous nest tree, the current nest is in a young cottonwood tree, and unlike its old-growth counterparts, sways wildly side to side in strong winds.

The short video clip was taken on May 21, 2020 during a late afternoon windstorm. The CR16 juvenile, still more than 3 weeks prior to flying, rides out the storm in the nest.

Photo 18: Currently occupied CR16 nest that was built in winter of 2018, only a few meters from the previous nest tree that fell. This nest was also built upon a former heron nest, and while the tree is more stout than its predecessor, it still sways wildly during strong winds.

Polygamous Merging Between the CR16 Pair and the Erie Nest—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.

Spring 2019 at the Erie Nest—Undivided Attention from the CR16 or “Shared” Male

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).

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 (photos 31-35 collage below). 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 nearby perches (photo 36)—this was not the case this year at the Erie nest.

Photos 31-35:  Erie Juvenile 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 accounted for.

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 being unobserved for 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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