Developing Evidence-Based Solutions

25 April, 2023: CTOM biologist Mark Hopey monitors tower work from his ground-level observation point as an osprey sits, incubating, in a nest at the top of a 192-foot cell tower in Albemarle, North Carolina (pictured below).

The adult female settles low in the nest, only her head visible over the nest-lip. 12 feet below, a climber repairs equipment to restore cell service on the tower.

Experience has taught us that ospreys are remarkably tolerant of human disturbance. Ospreys readily nest on cell towers, even in areas with high anthropogenic activity.

In fact, studies show that the proportion of osprey pairs nesting on artificial structures has increased; by as much as 22% between 1977 and 20061. The telecommunications industry reports an increase in tower-nesting from 1% in 2010 to over 10% of their cell towers in 2020. The generational habituation hypothesis likely accounts for this phenomenon. The hypothesis posits that birds will imprint on the type of nest structure their parents selected and go on to select a similar site once they enter the breeding population.

This exponential growth pattern is good news: ospreys are no longer struggling! However, their choice of nesting structures presents challenges to the industry tasked with providing uninterrupted essential services to meet our modern lifestyle needs.

Most states and the US Fish and Wildlife Service prohibit disturbance that results in harm to nesting ospreys. When work must be done on a tower hosting an active osprey nest, precautions should be taken against causing nest failure. To this end, CTOM created a process to serve the access needs of the telecom industry while avoiding harmful disturbance to birds. Our approach is evidenced-based. To affect change on outdated state and federal recommendations, we developed a study to assess the impact of tower work on nesting ospreys.

Previous research is sparse on the subject of nesting ospreys’ tolerance to human disturbance. Ospreys are known for their adaptability in a variety of conditions and are in the top five of 3,768 analyzed bird species that are found in the most urbanized environments2. A 1982 study found that ospreys nesting closer to humans (therefore experiencing more frequent disturbances) generally stayed on their nests longer and were more territorial as their nests were approached3. Another study found that nest visits, brief handling of young, and brief capture and banding of adults did not impact nest productivity4.

Because the presence of nesting ospreys is an important indicator of habitat health, especially water quality5, ospreys are considered an excellent sentinel species. Their nearly worldwide range, diet, tolerance for human disturbance, and sensitivity to contaminants are just a few of the reasons we believe continued monitoring of their population levels is merited. Currently, the osprey conservation status is of least concern, but much like other birds of prey in North America, ospreys were once in decline due to high concentrations of environmental contaminants such as DDE, DDT, and PCBs6. The population rebounded following the banning of these pesticides in the 1970’s. CTOM believes the rise of human-engineered structures, such as cell towers along coastal and inland waterways, has likely contributed to the nesting expansion of ospreys, whose breeding range now overlaps with all lower 48 states.

It is our hope that by illuminating the osprey’s level of tolerance to tower work, we will better understand how to provide a safe and productive environment for both cell tower climbers and nesting birds.

1 Henny, C. J., & Kaiser, J. L. (1996) Osprey population increase along the Willamette River,
Oregon, and the role of utility structures, 1976–1993. Raptors in Human Landscapes.
London: Academic, 97-108.

2 Neate-Clegg, M. et al. (2023) Traits shaping urban tolerance in birds differ around the world.
Current Biology, 33, 1-12.

3 Van Daele, L. J. & Van Daele, H. A. (1982) Factors affecting the productivity of ospreys nesting
in west-central Idaho. Condor, 84, 292-299.

4 Poole, A. (1981) The effects of human disturbance on osprey reproductive success. Colonial Waterbirds, 4,

5 Grove, R. A., Henny, C. J., & Kaiser, J. L. (2009) Osprey: worldwide sentinel species for
assessing and monitoring environmental contamination in rivers, lakes, reservoirs, and
estuaries. Journal of Toxicology and Environmental Health, Part B, 12(1), 25-44.

6 Ames, Peter L. (1966) DDT residues in the eggs of the osprey in the north-eastern United States
and their relation to nesting success. Journal of Applied Ecology, 3, 87-97.

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About the Author: Kathy Maechtle

Sr. Avian Outreach Specialist Kathy Maechtle is the principal and owner of Cell Tower Osprey Management (CTOM). Maechtle specializes in avian conservation, providing industry and other stakeholders with technical assistance to understand wildlife regulations, apply scientific principles and sound management practices using evidence-based solutions for their avian challenges.

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