Posted on June 25, 2021 by SF Forest Alliance (sfforest.org)
Of course it’s no surprise to any of our readers that eucalyptus stands are an excellent resource for nesting birds. A colony of double-crested cormorants nests (or did nest) near Lake Merced; great horned owls nest in eucalyptus trees in Glen Canyon and Golden Gate Park; and flickers nest in an old eucalyptus tree in the Presidio.
But we loved this article by Dr. Allison J. Gong, a life-long Californian, and a marine biologist, backyard beekeeper, columnist for Bay Nature Magazine, and college-level biology professor.
This article was originally published at Notes From a California Naturalist and is republished here with permission.
(As a comment, we don’t think eucalyptus is dying of “sheer old age” because eucs in California have not reached the typical 200-400 lifespan of these trees, though we don’t know about guano! And we appreciate her question: “At which point, however, does a species cease to be considered non-native?”)
A BUSY PLACE
In Morro Bay, CA, there is a stand of eucalyptus trees that has been designated a natural preserve. In 1973 the Heron Rookery Natural Preserve was established to protect great blue herons (Ardea herodias) as they nested. Since then other bird species have taken to nesting in these same trees. When we were there at the end of May we saw these species with nests in the eucalyptus trees:
- Great blue heron (Ardea herodias)
- Double-crested cormorant (Phalacrocorax auritus)
- Great egret (Ardea alba)
- Snowy egret (Egretta thula)
This particular rookery is not at all removed from human activity. It is right across the street from the municipal golf course and next to a hotel, and there is a walking/biking trail that runs directly under the trees. Signs advise people to keep their voices down, but pedestrians are walking under the trees all day, dodging the rainfall of guano from above. The birds don’t seem to be bothered.
Unlike the Brandt’s cormorants (Phalacrocorax penicillatus), which nest on cliffs and rocks, the double-crested cormorants nest in trees. Birds build nests with local materials, and there is a difference in what I could see making up the nests of these two species. The Brandt’s cormorants at Natural Bridges in Santa Cruz were using seaweeds as the main building material; I could see birds flying back with algae in their beaks, and then either handing it off to a mate on the nest or tucking it into the existing structure itself. In some cases I could see the pieces of algae well enough to make a tentative ID.
Brandt’s cormorants (Phalacrocorax penicillatus) nesting on rock arch at Natural Bridges 2021-05-14 © Allison J. Gong
Those are the Brandt’s cormorants. The double-crested cormorants nest in the trees, as we saw at the heron rookery. Here’s a pair that have a brood of three chicks:
Double-crested cormorant (Phalacrocorax auritus) parents and trio of chicks 2021-05-23 © Allison J. Gong
At Morro Bay, which is an estuary rather than a rocky area, the double-crested cormorants use a lot of eelgrass (Zostera marina) in their nests. Eelgrass is very abundant in the Morro Bay harbor and Estero, whereas the birds would have to fly a bit farther to gather algae. Eelgrass, being a true plant, is less slimy than the algae are, and these cormorants’ nests look much drier than the mounds of algae used by the Brandt’s cormorants up in Santa Cruz.
A short distance up the coast at San Simeon the double-crested cormorants were nesting in a smaller rookery, also in eucalyptus trees. I liked the pattern of how these four nests were situated in three-dimensional space:
Double-crested cormorants (Phalacrocorax auritus) nesting at San Simeon 2021-05-23 @ Allison J. Gong
Returning to goings-on at the heron rookery in Morro Bay, the herons and egrets were also raising youngsters in that stand of eucalyptus trees. Remember, this rookery is very easily visited by humans. Here’s a view of the trees, taken from the small parking area:
Heron Rookery Natural Preserve in Morro Bay, California 2021-05-23 © Allison J. Gong
It’s difficult to photograph the nests because of all the branches obscuring the view. We were also there near mid-day, with the overhead sun making lighting conditions less than favorable for good photography. I did find one comparatively visible heron nest, containing one parent and one sullen punk-ass teenager of a chick. The nestling had started growing feathers but was still almost half fluff, clearly not ready to fly yet.
Great blue heron (Ardea herodias) nest at the Heron Rookery Natural Preserve 2021-05-23 © Allison J. Gong
Both great egrets (Ardea alba) and snowy egrets (Egretta thula) nest at the heron rookery. Here’s a great egret nest with two chicks:
Great egret (Ardea alba) nest at the Heron Rookery Natural Preserve 2021-05-23 © Allison J. Gong
From what I could see, the herons and egrets don’t use any marine material at all to build their nests. One factor that determines the suitability of a potential building material is proximity—even if a certain material is fantastic in other ways, birds may not use it (or may use less of it, compared to other materials) if it costs too much energy to fetch and bring back to the nesting site. For the herons at this site, sticks are easy to come by. Another thing to consider is that herons and egrets are not marine birds. Although some populations live and nest in coastal areas, most do not. Thus it is not surprising that their nests are built from materials that are terrestrial rather than marine.
I did not see any snowy egret nests in areas where they could be photographed well. However, there were some adult snowies in their spectacular breeding plumage. There was enough of a breeze to ruffle up those long plumes that used to be harvested to decorate ladies’ hats.
Look at these beautiful birds!
Adult snowy egret (Egretta thula) in breeding plumage 2021-05-23 © Allison J. Gong
Adult snowy egret (Egretta thula) in breeding plumage 2021-05-23 © Allison J. Gong
For several decades now, the cormorants, herons, and egrets have been nesting in these eucalyptus trees, which brings to mind the consideration of native versus non-native species. The trees themselves, blue gum eucalyptus (Eucalyptus globulus) are non-native, having been imported to California from Australia starting in the 1870s. This introduction was encouraged by calls to replace native trees that had been cleared for fuel and building material, both of which were desperately needed during and after the Gold Rush. Since ecologists began considering the effects of non-native species in the 1980s there has been a backlash against the blue gums. Given their large size, their having been planted in groups to serve as windbreaks, and their propensity for dropping a lot of debris, they are very conspicuous, and it is easy to get all hot and bothered at how in certain places they dominate the landscape.
Great egret (Ardea alba) in flight 2021-05-23 © Allison J. Gong
At which point, however, does a species cease to be considered non-native? Having been established in California for 150 years, what is the role of E. globulus in the ecology of the Golden State? There are many people and organizations that would like to see the blue gums eradicated, or at least their populations greatly reduced. On the other side of the argument, groups such the San Francisco Forest Alliance posit that blue gums should be treasured as heritage trees.
At the Heron Rookery, some of the eucalyptus trees are dying. One reason is sheer old age. Another is the several decades’ accumulation of bird wastes onto the soil, which is slowly killing the trees. As the blue gums die, the birds will have to find other places to nest. One of the pro-eucalyptus arguments is that many species of native birds—not just these here but other species such as red-tailed hawks, red-shouldered hawks, and a whole host of songbirds—nest in eucalyptus trees throughout the state. If the blue gums are removed, then where will these undoubtedly native birds nest? Especially if the native trees have long been gone?
Taking the long view, my guess is that the birds will figure it out. Ecological communities evolve over thousands of years. The 150 years of the eucalyptus trees’ presence in California seems like a long time, but in terms of ecological time they are merely a blink of the eye. The herons, egrets, and cormorants have been nesting at the Heron Rookery for an even shorter period of time. When this stand of blue gums is gone, due to either natural attrition or removal by humans, the birds will find another place to nest. They might not choose a place that is so easily visited and observed by people, though.