Eucalyptus trees do NOT kill birds!

The claim that eucalyptus trees kill birds originates with an article in the publication of the Point Reyes Bird Observatory, written by Rich Stallcup.  (1)  Mr. Stallcup was well known as a knowledgeable birder, but he was not a scientist.  He based his claim that eucalyptus trees kill birds on his observation of two dead birds in two different eucalyptus forests over a period of many years.   

Bird mortality rates

Given that Mr. Stallcup was a serious birder who spent much of his time in the field observing birds, we begin our critique of his hypothesis by pointing out that a sample of two is absurdly small from which to extrapolate to a general rule about bird mortality in eucalyptus forests.

Furthermore, the scientific literature about bird mortality informs us that a sample of two does not justify the assumption that these deaths were caused by eucalyptus trees.  For example, the annual survival rate of adult song sparrows has been reported as only 30%.  That is, in a single year, 70% of adult song sparrows will die of predation, disease, starvation, or other factors including old-age. Mortality rates are greater for small birds.  (2)

Mortality rates for young birds are substantially greater than for adult birds.  For example, a longitudinal study of Eurasian kestrels found that 51% of 245 kestrels were dead within their first year.  All of the 245 kestrels in this study were dead by the end of 10 years.  (2)

We have a beautiful Coast Live Oak tree on our property, under which we have found dead birds. Yet we have not concluded from those observations that the birds were killed by the tree.  Of course, they were not.  Although there were probably many different causes of death of these birds, we don’t feel the need to speculate about those causes because bird death is not a rare or unusual event unless, of course, you are looking for an excuse to blame the tree.

Flowers of Blue Gum eucalyptus
Flowers of Blue Gum eucalyptus. Photo by John Hovland

Stallcup’s Theory

Spotted pardalote is an Australian bird with a short beak that feeds in eucalyptus forests in Australia.  Creative Commons
Spotted pardalote is an Australian bird with a short beak that feeds in eucalyptus forests in Australia. Creative Commons

Stallcup speculated that eucalyptus trees kill birds by “gumming” up their beaks or nostrils with the nectar of the eucalyptus flower which blooms from about December to May in California.  He supports this theory by claiming that the birds that are found in Australia, where eucalyptus is native, have long, curved beaks which enable them to eat nectar from the flower without gumming up their beaks or nostrils. 

This theory is not consistent with the “evidence” that Mr. Stallcup uses to support his theory:

  • He found a dead ruby-crowned kinglet.  The kinglet is an insect-eating bird, not a nectar eating bird.
  • Years before that sighting, he found a dead hummingbird.  Hummingbirds eat nectar, but they have long beaks.
  • Many Australian birds that feed in eucalyptus forests do not have long, curved beaks; e.g., spotted pardalote, striated thornbill, and white-naped honeyeater. (3)
  • One study in Santa Cruz, California found many small birds feeding on insects in red gum eucalyptus, including yellow-rumped warblers, Townsend’s warblers, ruby-crowned kinglets, and bushtits.  The beaks of these birds are not shorter than those found in eucalyptus trees in Australia mentioned by the same study.  The study reported no dead birds or gummed beaks. (3)
  • The nectar of eucalyptus flowers is not “gummy.”  It feels watery to the touch.  Eucalypts are not called “gum” trees because of the nectar in their flowers.  They are named for the sap under their bark.
Ruby-crowned kinglet is a North American bird that feeds in the eucalyptus forest in California.  USFWS
Ruby-crowned kinglet is a North American bird that feeds in the eucalyptus forest in California. USFWS

We can compare Mr. Stallcup’s “data” of two dead birds with a database of a local wildlife hospital.  The report of this database was posted to SFBIRD (San Francisco Bird is an email listserve that anyone can subscribe to and report the birds they see in San Francisco) by Richard Drechsler on March 12, 2012 and is quoted here with his permission:

“I have access to a database containing 56,960 records of birds brought to a local wildlife hospital between 1992 and 2010.  During this period this facility also accepted 2500 birds who were classified as DOA (dead on arrival).  I volunteered in that facility for seven months during 2010 where I worked with ‘small birds’ in order to get a better idea of how birds are being injured.  I was aware then of the belief that resin, gum, nectar, etc. from Eucalyptus trees might harm feeding birds.  During my time there I did not encounter any birds whose passages were blocked with any natural resins that would have prevented them from eating or breathing.  This evening I compared my observations with the 19 years of data by searching on the following words:  ‘euc,’ ‘euk,’ ‘bill,’ ‘beak,’ ‘tree,’ ‘asphy,; ‘breath,’ ‘mouth,; ‘nar,’ ‘gum,’ ‘resin,’ ‘nectar,’ ‘nostril,’,’ starv,’ ‘horn,’ ‘stick,’ ‘stuck,’ ‘glue.’ 

Preliminary Data Findings:

(1)    There is no reference to Eucalyptus or any of its byproducts.

(2)    There is one reference to a bill or mouth being restrained…by a synthetic adhesive.

(3)    There is one vague reference to an Anna’s Hummingbird that was “Stuck in Resin”…treated and released one day later.

(4)    Querying on ‘sticky,’ ‘stuck,’ or ‘glue’ yields many records and a wide variety of species being trapped by synthetic adhesives such as rodent traps and building adhesives.”

 Given the ubiquitous presence of Eucalyptus in this area and the birds craving for its nectar one would expect more incidents of starving birds or ones surrendered DOA.  Also, if birds did not have the capacity to preen or molt away this nectar, wouldn’t every (indulging) bird ultimately display this residue?”

 Bird anatomy trumps the absence of data

There is no empirical evidence to support the claim that eucalyptus trees kill birds, but the most compelling evidence that this claim is not factually correct is that it contradicts the basic facts of bird physiology and anatomy.  That is, birds can and do clear their beaks and nostrils with their feet or by rubbing their beaks on branches when necessary.  If their nostrils are obstructed, they can breathe through their mouth and vice versa.

Ask yourself this question to appreciate the absurdity of the claim that birds would passively suffocate rather than using the tools they have at hand:  If you were suffocating because there was something stuck in your mouth or nose, wouldn’t you raise your hands to your face and clear the offending obstacle?  Is there any reason to assume that birds are not physically or mentally capable of the same defensive behavior?  If you have a cold and your nose is stuffed up, don’t you breathe through your mouth?

In 2010, a student taking the Cornell Lab of Ornithology correspondence course on bird biology asked his instructor this question:  Do eucalyptus trees kill North American birds?”  This is the email reply from his instructor:

“I have no firsthand knowledge of the effect of eucalyptus gum suffocating birds (or not), but I share your skepticism for the reasons you mention. The story has birds feeding in flowers and getting gum on their faces. The first bird mentioned in this saga appears to have been a Ruby-crowned Kinglet. They don’t feed on or in flowers much; they’re leaf gleaners and flycatchers. Is there even “gum” to be picked up on the flowers? Again, that seems unlikely. Birds can breathe through their mouths, so just plugging up the nostrils won’t kill them. (Nestling crows often get the feathers covering their nostrils so encrusted with food matter that there is no way they could breathe through them, but the young are fine. I chip it off when I band them.) It seems like this story could be investigated rather easily, but I see nothing about it in the scientific literature. I would need to see evidence before I believed it.”  

Those who love to hate eucalyptus

The Cornell ornithologist would need to see evidence before he believed the claim that eucalyptus trees kill birds.  Native plant advocates apparently don’t need any evidence.  They have been repeating this absurd, baseless claim since 1996, when it was originally fabricated.  One subscriber to the SFBIRD email listserve mentions this claim often, although he never offers any new actual dead birds to add to the two that were used to fabricate the story. 

 So, why do we try, once again, to set the record straight despite the stone wall built around this fable by native plant advocates?  Because we find this claim in the “assessment form” used by the California Invasive Plant Council to justify its classification of eucalyptus as “invasive:” “purported to cause mortality in native bird species.”  The California Invasive Plant Council classifies eucalyptus as “invasive” based partly on the existence of two dead birds. 

This story has been repeated by native plant advocates for nearly 20 years without any supporting evidence.  It has taken on a life of its own until those who repeat it apparently are unaware that there is no evidence to support the myth.

The US Forest Service social scientist, Dr. Paul Gobster, interviewed native plant advocates while a visiting professor at UC Berkeley about ten years ago.  At the end of his visit, he delivered a lecture at the Randall Museum about his observations of the local native plant movement.  He said they were victims of “incestuous amplification,” the trading of misinformation in a vacuum caused by their isolation.  The ridiculous story about eucalyptus trees killing birds is surely an example of incestuous amplification.   


(1)    Rich Stallcup, “Deadly Eucalyptus,”  Point Reyes Bird Observatory, Fall 1996

(2)    Handbook of Bird Biology, Cornell Lab of Ornithology, Princeton University Press, 2004

(3)    Julie Lockhart and James Gilroy, “The portability of food-web dynamics:  reassembling an Australian psyllid-eucalypt-bird association within California,” Global Ecology and Biogeography, 2004, 13, 445-450

Monarch butterflies in California need eucalyptus trees for their winter roost

Monarchs are probably the best-known butterfly in North America, partially because they are distinctively beautiful, but also because of their epic migration.  East of the Rocky Mountains, monarchs migrate from the Canadian border and the Atlantic Coast to spend the winter months in fir trees in Michoacan State in Mexico.  West of the Rockies, monarchs migrate from the Canadian border and the Pacific Coast to overwinter along the coast of California from Mendocino County to San Diego County, near the Mexican border.

Monarch Butterfly.  Creative Commons
Monarch Butterfly. Creative Commons

No single monarch makes the entire journey.  It takes two to three generations of monarchs to make the entire round trip.  How each successive generation knows the route remains largely a mystery, although theories exist.  There are a couple of fascinating books about the migration that we recommend to our readers.  Four Wings and a Prayer is a book about the 38-year effort of Canadian entomologists, Fred and Norma Urquhart, to understand the migration.  It reads more like a suspenseful mystery than the non-fiction book that it is.  Flight Behavior is by Barbara Kingsolver, one of our favorite novelists because nature is often the subject of her work.  Although it is fiction, it has been carefully researched by Kingsolver who studied biology before becoming a writer.  It is engaging both as a cautionary tale for environmentalists and as a personal redemption story.

The western migration of the monarch

Monarch butterflies roosting in eucalyptus tree.
Monarch butterflies roosting in eucalyptus tree.

We will focus on the western migration of the monarch because that’s our neck of the woods, but also because this migration is one of the reasons why many people who care about nature and wildlife object to the destruction of eucalyptus trees.  Eucalypts are the preferred trees for over-wintering monarchs“Three types of trees were used most frequently by roosting monarchs:  eucalyptus (75% of the habitats primarily Eucalyptus globulus), pine (20% of the habitats primarily Pinus radiata), and cypress (16% of the habitats Cupressus macrocarpa).  Twelve other tree species were identified…with a combined prevalence of only 10%.” (1)

For those who may not know the botanical names, that’s Monterey pine and Monterey cypress that are the runners-up to eucalypts as the most popular trees for over-wintering monarchs.  Although monarchs roost in those trees in their native range on the Monterey peninsula, they also use those species outside their native range.  Unfortunately, just as the eucalyptus is a target of native plant advocates who demand their destruction because they are not native, both Monterey pine and Monterey cypress are targeted for destruction outside their native range.  For example, both Monterey pine and Monterey cypress will be eradicated from hundreds of acres of public land if the FEMA grants are funded in the East Bay.  This is just 150 miles away from where those trees are native and there is fossil evidence that they existed in the East Bay in the distant past.  In other words, most of the trees used by monarchs for their winter homes are in jeopardy of being destroyed by the native plant movement.

Another nativist myth BUSTED!

One of the reasons why we are telling this story is that it is a tidy little example of the justifications fabricated by native plant advocates to support their destructive agenda.  In the case of the monarch, native plant advocates claim that prior to the arrival of Europeans, before eucalypts were planted and Monterey pines and cypresses were planted outside their native range, the monarch used native trees for their over-wintering habitat.  The “assessment form” used by the California Invasive Plant Council to classify Blue Gum eucalyptus as invasive says,  “[The Blue Gum] provides roost sites for migratory monarch butterflies…ecological niches for butterflies and raptors probably formerly filled by native plant species.”  No evidence is provided in support of that statement.  We have also read that claim in comments of native plant advocates on internet articles in response to those who defend eucalypts because they are needed by monarchs.

Like many of the “cover stories” of native plant advocates, this is just not true.  A search of the scientific literature about monarchs enables us to bust this particular myth to smithereens.  It would be simple enough for native plant advocates to look at the evidence before spinning their tales, but it is apparently easier to make it up, especially when they are rarely questioned.  Million Trees exists to fill this informational void.

The historical record of the western migration of monarchs

The earliest record of over-wintering monarchs in California is from 1864, when monarchs were observed over-wintering in Monterey pines in their native range.  Richard Vane-Wright, the scientist who reports this record, explains why he believes it is probably the first incidence of over-wintering monarchs in California:

“’Previous to that, no mention has been found of this interesting phenomenon…The early Spanish chronicles and traditions make no mention of it, although Monterey, a scant three miles distant, was gay with life when the last century came in…even David Douglas, the world famed botanist, and the keenest-eyes of all the strangers who came [to California] is silent regarding it.’…Douglas, the indefatigable fir tree collector, appears to have made no mention of the phenomenon in 1830-1832, despite spending two winters at Monterey.” (2)

Vane-Wright believes the eastern monarch migration to Mexico also began around the same time.  His theory is that the agricultural practices of early settlers, which cleared trees, created a population explosion of the milkweed that is the host plant of monarchs.  More milkweeds resulted in more monarchs and monarchs began to migrate in response to population pressure, he believes.  He calls this the “Columbus Hypothesis.”  (2)

Biological facts explain why monarchs choose these species of trees

Aside from the historical record, the biology of monarchs and the physical characteristics of the trees in which they over-winter explain why these species of trees are required by the over-wintering monarch.  During the late fall and winter, monarchs enter a dormant phase called diapause.  They continue to need nectar and moisture during that period, but they are not very active, so these resources must be close by.  Although they migrate to the coast from Mendocino to Mexico, they are most abundant around the mid-point of that range, where temperatures and rainfall are moderate.  Most of the approximately 250 roosting sites are within 2.4 kilometers of the ocean, so wind protection is important for them while they are roosting.  All of these factors predict the ideal conditions provided by eucalyptus trees:

  • Monarchs need tall trees (of at least 60 feet) because they roost in the intermediate level of the canopy where wind protection is greatest (3)
  • The forest must be dense enough to provide wind protection,
  • The tree canopy must be open so that the roosting monarchs receive filtered sunlight to keep their bodies warm enough.
  • The monarchs need enough moisture for hydration, but not so much that they are soaked and lose their body heat.  So, dew and/or fog provide the ideal amount of moisture.   (1 & 4)

All of these requirements for the monarch’s winter roost point to their dependence on eucalyptus, pines and cypress.  The trees that are native to the narrow strip of the coast of California do not meet these criteria.  They are not tall enough and they do not grow that close to the ocean because they do not tolerate wind.  The native vegetation of that narrow strip of California coast is predominately dune scrub and coastal grassland prairie.  And these are the vegetation types that the ecological “restorations” in the Bay Area are trying to re-create.  These vegetation types will not be suitable habitat for over-wintering monarchs.  Furthermore, plans to drastically thin eucalyptus forests on hundreds of acres of the East Bay Regional Park District will render those habitats useless for over-wintering monarchs.

In addition to the physical properties of eucalyptus, the monarch benefits from the fact that it is flowering from about December to May, while the monarch is roosting in the tree.  The flowers of eucalyptus contain a copious amount of nectar which is also important to the honeybee because it is flowering at a time when there are few other sources of nectar.  One study reported observing monarchs feeding on the flowers of Eucalyptus globulus. (5)

Risky Business

We have mixed feelings about reporting this research about monarchs to our readers because there is some risk to the monarchs in doing so.  The evidence suggests that monarchs did not over-winter in California prior to 1864, after the magical date that nativists have selected to freeze-frame California’s landscape to their nativist ideal.  Here in the San Francisco Bay Area, this magical date is 1769, when the expedition of Portola laid eyes on the San Francisco Bay.  Will nativists declare the monarch an alien invader to be eradicated along with the hundreds of plants and animals they claim “don’t belong here?”  This may seem a far-fetched conjecture, but keep in mind that the European honeybee is being eradicated in some “restorations” because it is not native.  The honeybee is essential to the survival of American agriculture, yet its existence is threatened by the radical agenda of the native plant movement.

That’s the risk we take in reporting this evidence because we hope that it helps our readers to understand the absurdity of the nativist agenda.

Update:  Monarchs have returned to Natural Arches State Beach in Santa Cruz in big numbers.  Here is a link to a report that includes a lovely video of the roosting Monarchs.  


(1)    Dennis Frey and Andrew Schaffner, “Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(2)    Richard Vane-Wright, “The Columbus Hypothesis:  An Explanation for the Dramatic 19th Century Range Expansion of the Monarch Butterfly,” in Biology and Conservation of the Monarch Butterfly, Natural History Museum of Los Angeles County, 1993.

(3)    Andres Kleiman and Miguel Franco, “Don’t See the Forest for the Butterflies:  The Need for Understanding Forest Dynamics at Monarch Overwintering Sites,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(4)    Kingston Leong, et. al., “Analysis of the Pattern of Distribution and Abundance of Monarch Overwintering Sites along the California Coastline.” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(5)    Susan Chaplin and Patrick Wells, “Energy reserves and metabolic expenditures of monarch butterflies overwintering in southern California,” Ecological Entomology, 7:249-256, 1982

Weeds: In Defense of Nature’s Most Unloved Plants

We would like to tell our readers about a charming little book about weeds, by the same name.  Weeds:  In Defense of Nature’s Most Unloved Plants, by Richard Mabey, contains an eclectic collection of information about the weeds of Britain, their origins, the history of their use–including as medicine, the role they have played in literature, and much more. 

First, we venture a definition of weeds, though any definition is likely to be controversial.  The concept of “weed” originated with agriculture, some 5,000 years ago in the Old World*, when man began to distinguish between those plants that are edible or otherwise useful and those that are not.  And so, plants that are not perceived as useful or turn up in the wrong place, were defined by man as “weeds.” 

It’s a shifting concept, because a plant that was useful historically, either because it was believed to be a cure for some malady, or was otherwise useful, might be replaced by some superior remedy.   Such a changing concept of the value of particular plants is a central theme to the book as well as to the Million Trees blog.  We often invite our readers to consider that much of the current interest in native plants is a horticultural fad that is likely to change in the future as it has in the past. 

We also often question the designation of hundreds of non-native plant species as “invasive” in California, a designation that makes them a target for eradication.   Mabey’s book about weeds helps us to put this designation into perspective.  Britain obviously has a much longer history of trade with its neighbors on continental Europe, which increased the potential for the introduction of non-native species.  Yet, despite Britain’s longer history of ecological globalization, Mabey tells us that only about one dozen species of plants are presently considered invasive in Britain compared to hundreds in California. 

Rhododendron ponticum, one of only a dozen plants considered invasive in Britain. Wikimedia Commons

Mabey defends several of the non-native plants considered as invasive in Britain.  He believes that some are merely responding to the disturbance of native vegetation by the activities of man.  Nature hates a vacuum.  When native vegetation is no longer adapted to the changed soil, water, and air quality conditions created by man, any plant that will grow in these new conditions is preferable to bare ground.  Plants—including weeds—help the soil absorb rainwater into the ground which would otherwise run off the land, silting streams and causing erosion.

There is much food for thought in this little book.  It invites us to compare our list of nearly 200 plants in California that have been officially designated as “invasive” to a short list of only a dozen plants in Britain.  What accounts for this big difference?  Different conditions or different attitudes?  We don’t know the answer to this question, but we think it is a question worthy of consideration.  

* Crosby, Alfred, Ecological Imperialism, Cambridge University Press, 2009