Measuring the health of Mt. Tam

Maintaining a healthy, vibrant and diverse Mt. Tam begins with understanding how key ecological resources are faring, and how we can better care for this iconic and beloved place.

One Tam partners and Bay Area scientists have come together to try to answer the question: How healthy are Mt. Tam's natural resources?



Photo by Michael Utin


Why Was This Indicator Chosen?

Because they feed almost exclusively on fish, the breeding success of Ospreys (Pandion haliaetus) in the lakes and reservoirs in the One Tam area of focus is a good indicator of water quality and fish abundance. The Kent Lake Osprey colony was first established in the mid-1960s and has been monitored continuously by the Marin Municipal Water District since 1981.

Persecution and environmental contamination in the 19th and early 20th centuries led Osprey to be listed as a California Species of Special Concern. The Kent Lake Osprey colony peaked in the mid-1990s, then entered a period of gradual decline over the subsequent two decades. Recent monitoring suggests the colony is currently about half of its former size.

What is Healthy?

A healthy nesting population of Osprey on Mt. Tam would remain stable over time. High levels of pair occupancy and annual reproductive success would be maintained within the normal range of variability, or above long-term average values based on recent historical monitoring.

What Are the Biggest Threats?

  • Climate change and drought may negatively impact Osprey fishing and reproductive fledging success
  • Contaminants such as mercury and residual DDT in nearby areas where Osprey are known to forage

What is The Current Condition?

The overall condition of Kent Lake’s Osprey colony is Good. Adequate dead tree nesting habitat is present at Kent Lake, and nest site occupancy has been above 75% for all but four of the past 13 years. Because reproductive success has not been measured since 2000, that remains unknown, but the numbers of active nests are encouraging.

Measures of Kent Lake Osprey reproductive effort, 2003–2015

What is the Current Trend?

The Kent Lake Osprey colony was founded sometime after the original filling of Kent Lake (built in 1954). The first nest survey in 1981 showed 11 active nests (Evens, 2015). Additional flooding in 1983 killed many of the edge trees, contributing to the number of nest sites available to Osprey. The number of active nests continued to rise until 1994 when it seemed to plateau (between 35-46 active nests until 2005), then the measure follows a downward trend to 21 in 2015 (Evens, 2015) giving this population an overall trend of Declining.

Kent Lake Osprey nesting pairs, 1981-2016 (used with permission by Jules Evens)

How Sure Are We?

Overall confidence in these assessments is Moderate thanks to consistent nest survey data since 1981, recent tree counts, and encouraging signs of high levels of nest activity.

What is This Assessment Based On?

Monitoring and assessments of the number of occupied nests, nest site occupancy rate (the number of active nests as a percent of occupied nests), number of nestlings fledged from active nests, and the number of dead trees that provide nesting habitat (Evens, 2015).

What Don’t We Know?

Key information gaps include:

  • Annual observations to determine reproductive productivity
  • Foraging patterns and locations, prey availability, and local ecological dynamics of prey species (e.g., top smelt)
  • The presence of mercury and other contaminants in the birds
Osprey Indicator Overview

Learn More


Evens, J.G. (2015). Osprey (Pandion haliaetus): The 2015 Nesting Season at Kent Lake, Marin County, California (Annual report). Corte Madera, CA: Marin Municipal Water District.