Evaluating movement patterns of adult Bald Eagles in the vicinity of a U.S. Navy communication facility in Cutler, Maine

Eastern Maine holds one of the highest densities of breeding Bald Eagles between Nova Scotia and Chesapeake Bay. This region, commonly referred to as “Downeast Maine,” was particularly important in sustaining the Bald Eagle population in Maine and New England throughout its recovery throughout the latter half of the 20th century. Despite its value to the broader regional population, many aspects of the ecology of Bald Eagles residing in eastern Maine remain poorly studied. In this study, we investigated how Bald Eagles in Cutler, Maine, used a globally important U.S. Navy communication facility that poses collision risks to birds. To our knowledge, this study provides the only information on seasonal home range area of coastal-dwelling Bald Eagles anywhere in New England.

Overview

Cutler, Maine is home to a Naval Support Activity (hereafter, Installation; managed by Naval Facilities Engineering Systems Command, NAVFAC), which is a globally significant communication facility owned and operated by the U.S. Navy. The Installation is situated in Washington county, which is a stronghold for Bald Eagle (Haliaeetus leucocephalus) populations in Maine.

In 2017 and 2019, BRI scientists instrumented a total of six adult Bald Eagles to characterize their home range and altitude patterns relative to the Installation. Findings support the assumption that territory proximity to the Installation strongly influences the probability that individuals will use the Installation hazard areas. No Bald Eagle core use areas overlapped with the Installation hazard areas. Bald Eagles generally had a higher probability of using the hazard areas during the non-breeding/winter season than the breeding/post-breeding period.

Study Methods

For this and previous studies, the zone of the Installation posing potential flight hazards to aerial wildlife are divided into two areas. The primary hazard area is a 3.1 km2 zone containing both towers and guy wires, which pose collisions risks to Bald Eagles. The secondary hazard area is a 200 m open space buffer zone beyond the primary hazard area that contains some cabling of reducing altitude, but towers are not present within it beyond the primary hazard zone.

Bald Eagles were fitted with GPS/GSM transmitters, attached using a backpack-style harness, which recorded GPS locations every 15-20 minutes.

Utilization Distributions (UD) is a space use probability with respect to time, showing the probability of where an animal might be found at any randomly chosen time. UDs are reported within individual years (starting on June 1st) and additionally during two different time periods roughly delineated by both breeding chronology and season.

  • Breeding/post-breeding: March 15 – Oct 31
  • Non-breeding: Nov 1 – March 14

To evaluate the potential exposure of Bald Eagles to hazards associated with the Installation, scientists evaluated spatial overlap between individual and composite Bald Eagle UDs with hazard areas of the installation provided by GIS boundaries from the U.S. Navy.

Study Findings – Space Use Patterns

All Bald Eagle UDs (utilization distributions) estimated from GPS telemetry were highly centered upon nest sites. Several individuals showed varying degrees of habitat use elsewhere – predominantly in eastern Maine and New Brunswick, Canada. Analyses of space use patterns within designated breeding cycle/seasonal time periods suggests individuals have different space use characteristics during the two periods.

  • UD areas were larger in the non-breeding period than in the breeding/post-breeding period in 6 out of 7 resident eagle years.
  • The pattern of larger space use in the non-breeding period was generally also mirrored at the 50% and 75% contour levels.

Bald Eagles have a high nest site fidelity, often returning to the same site year after year. Distance measurements between the GPS location estimates for Bald Eagles and their respective nest sites demonstrated a relatively high degree of fidelity to territories and nest sites. Within the Installation area, the majority of both individual and daily-summarized altitude estimates were well below the maximum height of the towers within the primary hazard area.

Figure 1: Full extent utilization distributions (UDs) of six transmitter-instrumented Bald Eagles in the vicinity of the Installation.

Figure 2: All-years composite utilization distributions (UDs) of six transmitter-instrumented adult Bald Eagles within the vicinity of the Installation during two different breeding cycle/seasonal periods.

Collision Risk

Bald Eagles face collision risk from the man-made structures and towers within the Installation. Collision risk was estimated two ways:

1) UD areas of the individuals tracked using telemetry to evaluate encounter risk of nest sites

2) calculating mean inter-nest distance of all nesting territories and applying a buffer to evaluate territory overlap with project area.

The overall median daily altitude estimate in the primary hazard area (38.9 m) was 16% of the height of the 244 m towers and 13% of the height of the 305 m towers.

  • Comparisons of daily altitude estimates between primary and secondary hazard areas indicate Bald Eagles perch or fly at higher altitudes in the primary hazard area than the secondary hazard area.

In general, individuals associated with nest sites closer to the Installation had higher probabilities of using the hazard areas than those from more distant nest sites.

  • Based on risk criteria using the first approach, one nest was low risk (17%), one was moderate risk (17%), and four were high risk (66%).
  • Using the second approach, 12 additional nests were included in the encounter risk assessment (Figure 3):
    • Under half (44%) of these nesting territories had a high encounter risk, while the remainder of nesting territories were distributed evenly between low and moderate encounter risk categories (28% each).

Figure 3: Encounter risk assessment for resident Bald Eagles (associated with nesting territories within 15 km of the Installation) derived from transmitter-instrumented Bald Eagle use patterns and distance-based risk criteria relative to the Installation.

Summary and Conclusions

This study provides first-time quantitative characterizations of space use and home range by resident adult Bald Eagles in eastern Maine. Our findings support the general assumption that the proximity of nesting territories is a key factor influencing the probability that resident eagles will enter the hazard areas, with the likelihood of use being greater in the non-breeding period than in the breeding/post-breeding area. The two methods for encounter risk assessment were in relative agreement, suggesting that individuals from 7-8 different nesting territories had a high risk of entering the hazard areas. GPS estimates of Bald Eagle flight altitude indicated that eagles are predominantly using lower altitudes within the primary and secondary hazard areas.

This work, in combination with previous work done on the Installation highlight several management actions aimed at Bald Eagle risk reduction. The home range and characterization of horizontal and vertical habitat use in this study can inform Bald Eagle conservation decisions across North America.

Collaborations and Funding Support

This project was executed through the Cooperative Ecosystem Studies Units (CESU) National Network: Avian and Bat Surveys at Northern Maine Naval Installations & Follow-On Bald Eagle Telemetry and Use Surveys at Naval Support Activity Cutler, Cutler, Maine. View the full report here.

DeSorbo, C. R., A. T. Gilbert, C. P. Persico, D. Hengstenberg, W. Hanson and D. E. Meattey. 2021. Bald Eagle Movements Relative to the Naval Support Activity Cutler in Cutler, Maine. BRI Report # 2021-16 Submitted to NAVFAC PWD-ME, Portsmouth, NH. Biodiversity Research Institute, Portland, Maine. 34 pp

Photo Credits: Bald Eagle Flying © BRI-Kim LeBlanc, Bald Eagle Perched © BRI, Figure 1, 2, and 3 © BRI Staff, Bald Eagle Flying Wings Spread © BRI-Kim LeBlanc, Cutler, Maine © BRI-Lauren Gilpatrick