Raptors suspected in Audubon nesting failure

David Briggs
Not a single great egret chick survived to fledge from the 32 nests built during the summer at Audubon Canyon Ranch’s Bolinas Lagoon preserve, where they lay their eggs in redwood trees whose tops are rounded from decades of nesting. Scientists believe raptors, including a growing population of bald eagles, may have spooked some birds into other nesting sites and pillaged the young of others.   

The nesting season at the Martin Griffin Preserve just north of Stinson Beach started as most seasons do: the great egrets descended in April with their spindly legs, svelte necks and creamy plumage. In Picher Canyon on Bolinas Lagoon, they courted, bonded and began building nests in the tops of coast redwoods to cradle their turquoise eggs.

But in May, nest building started to falter. By the end of summer, all 32 nests had failed. Not one chick fledged from the canyon, resulting in the first total failure in the 46 years scientists have tracked the birds. 

“We don’t know what happened,” said John Kelly, director of conservation science for Audubon Canyon Ranch, the nonprofit that manages the preserve. “It was the worst year we’ve seen.” 

The nonprofit believes avian predators, including bald eagles, might be to blame, though evidence is elusive.

The great egret came close to extinction in the late 1800’s and early 1900’s, while the lanky bird was sought for its wispy plumes, and Audubon Canyon has been studying them on the 1,000-acre preserve since 1967. Scientists track nest abundance, or the total number of nests built, and reproductive success, or the number of nests that fledge at least one chick. 

The number of nests built each season here has fluctuated since the 1960’s. Although the numbers rose for a period in the early 1980’s, following the ban on D.D.T., nest abundance has been slowly declining since 1990. Over the past several decades, the canyon has averaged 87 nests a season. Nest abundance has lagged behind that number since 2007. And in 2013 there were just 32 nests, the lowest number ever observed. 

“That normally wouldn’t bother us so much, except they also didn’t nest successfully,” Dr. Kelly said. In fact, it was also the third consecutive year of poor reproductive success—resulting in the first recorded instance of poor reproductive success for three years running.

Every year there are some nest failures. A review of Bay Area nesting data from 1991 to 2005 conducted by Audubon Canyon found that 78 percent of almost 4,700 Great Egret nests were successful. But at Picher Canyon in 2011, less than 30 percent survived. In 2012, less than a tenth did. 

This defies past instances of feeble breeding at the canyon that were quickly followed by rebounds. In 1975 a band of violent raccoons tore apart most of the nests, and they raided again in 1981. A golden eagle disturbed the birds in 1989 and 1990, but breeding quickly

The preserve’s lackluster performance this year does not reflect regional trends, however. Dr. Kelly said one colony at the Marin Islands National Wildlife Refuge, off the coast of San Rafael in the San Francisco Bay, had fewer than expected nests, but not a total failure. Other Bay Area sites had normal levels of nesting and reproductive success. (There have been an average 25 great egret nesting sites along California’s central coast region from 1991 to 2005.)

Egrets can move between nesting sites from season to season, or even during a season, depending on food availability, disturbances and other environmental conditions. The fate of nearby nests can also influence whether the birds return or choose a different site to breed. 

A nesting site across Bolinas Lagoon just yards from the downtown pier had a surge in nest abundance this year, with 15 nests. The site is an established great blue heron colony, and saw its first four great egret nests in 2011, though they failed that year. Three more nests were built in 2012, and one pro-

duced two chicks that fledged. In 2013, five of 14 nests had fledged chicks. (The fate of one nest was undetermined.)

Sarah Millus, the biologist who monitored the egrets in the canyon during the summer, said birds that left a nesting site due predation or lack of food would be more likely to nest in an established nesting area than to found a new colony, so it would not be surprising for Audubon Canyon birds to use the heron colony in Bolinas. Still, scientists could not confirm that the egrets at the Bolinas site had previously nested in Picher Canyon. 

Though Dr. Kelly was careful to point out that the reasons for the nesting failure remained a mystery, he said predator birds were the likely culprits. 

Bald eagles have been successfully nesting at Kent Lake since 2008, and this year a nest was also observed on the Inverness Ridge. Although correlation does not prove causation, the four-year decline in Great Egret nest success at Picher Canyon coincides with the colonization of Marin County by bald eagles, scientists concluded in a report on the nesting season, which also noted the presence of a raptor in the canyon.

The report said that avian predators might have directly preyed on nests, but their mere appearance can scare egrets into abandoning eggs and chicks. Imagine a bald eagle or a sinister hawk circling above, Dr. Kelly said. “That’s a pretty scary sight to a nesting bird.” 

But Ms. Millus, who visited the colony every three or four days last summer, said it is difficult to know for sure exactly how avian predation affected nesting, both because there is not round-the-clock monitoring and because they cannot see directly into most of the nests. 

The nonprofit is seeking funding to purchase high-resolution cameras to ramp up observation, especially at night, and help scientists come to firmer conclusions regarding next year’s nesting season.

“We’d love to see the birds come back to Picher Canyon,” Ms. Millus said. “The people who visit the Martin Griffin Preserve would as well.” 

So would Ms. Heistand, who spoke evocatively of the momentous act of fledging. “My favorite time used to be when the chicks first hatched and you see those little fuzzy tiny things, but now I think it’s the fledging,” she said. “Once the chicks are ready to fledge, they’re the same size as the adults, but they’ve never flown before. And so I just love to think about that. We can watch it here; we watch it all the time. Imagine being a bird at the top of a 150-foot redwood tree and taking that first step off.” 

But the future of the egrets is hardly in the control of the scientists who meticulously observe and track them. “Mother Nature’s in charge of this colony. We’re not in charge of this colony,” Dr. Kelly said. “There’s not much we can do about it.”