Bay Area Bald Eagles: Slow but Steady Growth for the National Bird

In the 21st century, Bald Eagles have become a more common than rare sighting in the Bay Area, mostly seen in the wintertime near a supply of ducks or fish. In the spring or summer, nesting Balds may even stake a claim near a large Bay Area lake or reservoir. Over the past two decades, many local sites, such as Laguna de Santa Rosa, Lake Sonoma, Kent Lake, San Pablo Reservoir, and Lake Del Valle, have had single pairs; others, like Calaveras Reservoir and Lake Berryessa, have had multiple nests per year.

So, back on March 10, 2012, it wasn’t a great surprise when a local and diligent birder raised his binoculars and picked up an adult Bald Eagle flying over Crystal Springs Reservoir in San Mateo County. The surprise came when he realized that the bird was carrying a stick to the far, western side of the lake, and fitting it into a 6-foot diameter nest in a tall conifer.

This was the first known Bald Eagle nest on the San Francisco peninsula since a 1915 La Honda nest, and its proximity to San Francisco—10 miles as the eagle flies—made it a particularly exciting event.

For a month, leaders of Sequoia Audubon Society stationed themselves along a roadside, a mile from the eagle nest, and shared their telescopes, expertise, and time with any eagle-watcher who happened along. On April 8, when I stopped in, there must have been 10 other cars and small lines for the two scopes.  Even through the scopes, the view of the eagle on the nest was spotty at best—they were that far away—and the Audubon members carried laminated photos from a high-powered lens camera to help viewers know what they were viewing.

The last sighting of the pair at the Crystal Springs nest was April 18. They had appeared to be “close-sitting” for a month, meaning they were likely incubating eggs, but no one had been able to see into the nest to know for sure.  The cause of the eagles’ abandonment is speculative at best, although several hypotheses have been forwarded. 

One hypothesis is that the Audubon viewing-station was, at just one mile away, too close for the adult eagles to be comfortable on the nest.  While it is true that you never know how individual eagles will tolerate human activity, this pair did, by choice, build a nest one mile from a small roadway with regular car traffic and cyclists. Furthermore, the nest was something like a mile-and-a-half from an eight-lane highway. This leaves me to believe that they were not bothered by the relatively quiet Audubon docents. Also, the day I stopped to see the nest, I saw no behavioral signs of eagle agitation or anxiety.

A second hypothesis has to do with a news helicopter that made a pass near the eagle nest on April 16 or 17. I can’t find out how close or how long the chopper was near the nest tree, but the timing doesn’t look good; the eagles abandoned just a day or two after this incident.

A third hypothesis has to do with the age of the female eagle. She had a dark streak of feathers behind each eye, indicating that she was a young adult eagle (4 to 5 years), and so would have been inexperienced in the nesting process. Young adult raptors often have failed nests. As they become older adults, they have more successful nests and fledge more young.

Many of the Bay Area Bald Eagle nests I mentioned above had false starts; they also had intervening years in which no eaglets where hatched, and for no obvious reason (to a human). But young adult raptors are also thought to be good at colonizing new habitats, so although 2012 looks like a wash for the Bald Eagles at Crystal Springs, this attempted nesting bodes well for future years.

By Allen Fish
Director, Golden Gate Raptor Observatory