Guest Post: Death of a Million Trees
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. Photo by John Hovland
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
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 Hummingburd 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