Reports
Variation in Floater and Migrant Activity at Six Bald Eagle Nest Territories in Northern Colorado’s Expanding Breeding Population
DANA J. BOVE AND HOLLY A. ANDERSON
Front Range Eagle Studies, Boulder Colorado, 80305 USA
ABSTRACT.—Seasonal patterns in outsider bald eagle (Haliaeetus leucocephalus) presence and territorial aggression were analyzed across six focal territories in the northern Colorado Front Range between 2016 and 2025. The study combined standardized camera and field observations to examine non-territorial bald eagle (OBE) counts, aggression events, and seasonal patterns in aggression intensity, with some metrics normalized to 100 observation hours. Results reveal substantial inter-territory variation in both OBE pressure and aggression, indicating that impacts are not uniform across the study area. Stream-connected territories (with direct access to major riparian corridors) generally sustained higher and more consistent OBE pressure than upland sites. Upland territories had consistently lower OBE counts across all years. The data show that territorial pairs are not equally affected by the regional population rise. Some territories—such as a long-established stream-connected site—showed sustained increases in non-resident presence without a proportional rise in aggression, while others—most notably an upland site on the edge of a declining roost system—experienced abrupt spikes linked to regional roost disruption and local instability. Territories adjacent to stable, long-standing communal roosts generally exhibited more consistent pressure and aggression, whereas those peripheral to a declining roost system were more vulnerable to episodic surges. The collapse of the Middle Roost and decline of the Confluence Roost have altered historic winter corridors, shifting pressure onto remaining roosts and previously marginal territories. These findings emphasize the importance of territory-specific context, roost proximity, and seasonal timing in assessing OBE impacts, and highlight management implications for proposed developments near declining roost systems.
READ REPORT – (PDF August 14, 2025)
Master Documentation for the Above Report – Bald Eagle Aggression and OBE Datasets
By Dana J. Bove and Holly A. Anderson (Front Range Nesting Bald Eagle Studies)
Date: August 11, 2025
This report provides descriptions for all three dataset collections available for download.
Each dataset is packaged as a separate ZIP file and contains multiple Excel files, one per nest site.
Users can download any individual ZIP or all three depending on their research needs.
- Aggression and OBE Datasets (Single Nest Files)
Contains combined Outsider Bald Eagle (OBE) and aggression event statistics for each nest, one file per nest site.Files included:
– BOCR Aggression and OBE.xlsx
– CR16.5 Aggression and OBE.xlsx
– Erie Aggression and OBE.xlsx
– ERLA Aggression and OBE.xlsx
– Hygiene Aggression and OBE.xlsx
– Stearns Aggression and OBE.xlsx
Purpose: These datasets summarize OBEcounts, OBE rates (per hour and per 100 hours), aggression event counts, and severity breakdowns for each nest site. - Aggression per 100 Hours (Standardized) – Single Nest Files
Contains standardized aggression event rates normalized to 100 observation hours for each nest site.Files included:
– BOCR aggression 100 hours standardized.xlsx
– CR16.5 aggression 100 hours standardized.xlsx
– Erie aggression 100 hours standardized.xlsx
– ERLA aggression 100 hours standardized.xlsx
– Hygiene aggression 100 hours standardized.xlsx
– Stearns aggression 100 hours standardized.xlsx
Purpose: These datasets allow for direct comparison of aggression rates between sites by standardizing to a consistent observation effort. - Multi-species Aggression Datasets – Single Nest Files
Contains aggression event counts and species details for all species observed interacting with eagles, one file per nest site.Files included:
– BOCR ALL multi species aggression sp edit.xlsx
– CR16.5 multi species aggression sp edit.xlsx
– Erie All multi species agression sp edit.xlsx
– ERLA multi species aggression sp edit.xlsx
– Hygiene multi species aggression sp edit.xlsx
– Stearns multi species aggression spell edit.xlsxPurpose: These datasets expand beyond bald eagle vs. bald eagle interactions to include multi-species aggression events, with species identification and event details.
Usage Notes:
– All datasets are provided in Microsoft Excel format (.xlsx).
– Column names, units, and scoring definitions are consistent across files within each ZIP set.
– Read the README.txt file inside each ZIP for specific details relevant to that dataset set.
Protecting the Confluence Roost: Assessing the Impact of Proposed Development on Bald Eagle Habitat
FRNBES (Front Range Nesting Bald Eagle Studies)
ABSTRACT.— The Confluence Bald Eagle Communal Roost, near the junction of Boulder and St. Vrain Creeks, has long been a critical winter refuge, once drawing 40–50 bald eagles nightly. Today, counts often fall below 10, following habitat loss from the 2013 flood, abandonment of the Middle Roost upstream, and increased disturbance along the Boulder Creek corridor. The City of Longmont’s proposed development of the Distel property—just 0.45 miles from the roost—would place a composting facility and fire training center within sight of the site, in violation of CPW roost-protection guidelines. While modern covered compost systems are promoted as low-impact, little research exists on whether pre-processing areas or other facility operations could attract bald eagles or other wildlife in this setting. Given the roost’s role in a diminished network already stressed by upstream losses, and new evidence from FRNBES’s Variation in Floater and Migrant Activity at Six Bald Eagle Nest Territories in Northern Colorado’s Expanding Breeding Population showing heightened territorial instability in the corridor, Front Range Nesting Bald Eagle Studies (FRNBES) recommends delaying approval until targeted research, design safeguards, and long-term monitoring can ensure the facility will not further destabilize this critical habitat.
READ REPORT (PDF — Feb. 24, 2025)
Limited Availability of Old Growth Cottonwood Nest Trees in the Northern Colorado Front Range
By Dana J. Bove (Front Range Nesting Bald Eagle Studies)
The limited availability suitable old-growth or Plains Cottonwoods for bald eagle nests in areas of the northern Colorado Front Range (NCFR)—and specifically those at a sufficient distance from anthropogenic disturbance—has received little attention when it comes to protection of territorial bald eagles and their habitat in Colorado. Although a common assumption is that alternate nest trees should be easy for bald eagles to acquire, this is arguably not the case across the NCFR.
READ REPORT (PDF — October 5, 2022)
READ REPORT (PDF — 13 July 2020)
Expert Opinions and Conclusions Concerning the U.S. Fish and Wildlife Service’s “Take” Permit MB663S7C-O (project AR1237-39), Garrett Construction Co., Impacting the Stearns Bald Eagle Nest. Broomfield. Colorado
State wildlife managers at Colorado Parks and Wildlife (CPW) have publicly expressed concerns about “densely [human] developed areas [in the NCFR corridor] that also contain a high concentration of bald eagles”. This appears to have led to a new management definition applicable to bald eagle nests located in “highly developed areas” (Colorado Parks and Wildlife 2020). Nests in “highly developed” areas are defined by CPW as those occurring in “an area where the existing [building] density exceeds 10 or more daily occupied facilities within a ¼ mile (1320 feet, 400 m) radius of the nest.” Instead of the previous no surface occupancy recommendation (NSO) aimed at protecting all bald eagle nests year-round at ½ mile seasonal or ¼ mile annual radius (Colorado Parks and Wildlife 2008), a new NSO buffer of 660 ft (400 m) now applies to nest territories deemed as “highly developed” (Colorado Parks and Wildlife 2020). Colorado Parks and Wildlife’s new 660 ft buffer for these purportedly urbanized nest territories is derived from U.S. Fish and Wildlife’s bald eagle management guidelines, which have long-recommended 660-foot radius buffers around bald eagle nests (U.S. Fish and Wildlife Services 2007).
The 660-ft nest buffer adopted from USFW guidelines can be traced to studies conducted nearly 25 years ago on impacts of timber clearcutting in dense northern boreal forest habitats around bald eagle nests (Manville 2018).
READ REPORT (PDF — April 10, 2018)
Home Range and Critical Areas for Bald Eagles at Standley Lake
Submitted to the City of Westminster
Colorado Bird Observatory / 13401 Piccadilly Road / Brighton, CO 80601
Due to the steep decline of bald eagle populations from DDT through much of North America after the early 1940s, bald eagles were rarely noted in the Front Range until the early 1980s. The first documented bald eagle nest in the NCFR was established at Barr Lake in 1986 (Colorado Parks and Wildlife 2020), followed by Standley Lake in 1993. Please see the attached 1993 report by the “Colorado Bird Observatory” or now called Bird Conservancy of the Rockies.
READ REPORT (PDF — April 10, 1993)