No evidence that birds are harmed by non-native plants

The garden columnist of the San Francisco Chronicle wrote an article about the harmful effects of non-native plants on birds in 2012.  She quoted an ornithologist as making these gloomy predictions about the harm that non-native plants may be doing to birds:

• “Nonnative berries may not provide the nutrition that particular native birds require.”
• “Nonnative fruiting plants could bring birds into a wildland habitat new to them, not necessarily with a good outcome.  Birds might flock to a new area, feast on the new food source till it’s gone, but then not find enough food for so many birds in the rest of the habitat.”
• “Nonnative plants alter wildland communities ‘in more complex ways than simply providing food for birds.’  A bird-dispersed nonnative fruiting shrub, for example, ‘can form underbrush or thickets in areas that previously lacked an understory…Birds may find their nesting sites disrupted, more cover for predators, etc.’”

Our initial reaction to these dire predictions was that they were entirely speculative.  The consistent use of the word “may” to describe the consequences of non-native plants suggested that supporting evidence was absent.  It seemed that, as usual, the nativist ideology was casting a dark pall on nature.

We were reminded of one of our first encounters with a nativist over 10 years ago.  On a tour of a park in San Francisco, he claimed that non-native plants in San Francisco were creating a “sink” which he defined as attracting migratory birds into a climate to which they were not adapted, where they would eventually freeze to death.  I pointed out to him that it does not freeze in San Francisco.  He was unaware of this fact.

Secondly, we reacted to the implication of the Chronicle article that birds are passive in nature and incapable of making good choices for themselves.  The suggestion is that birds are unable to discern nutritious food from “junk” food and incapable of choosing a safe nesting site to raise their young.

Thirdly, since every plant is native somewhere, we found the suggestion that non-native plants are nutritionally inferior to native plants illogical.  Are we to believe that where these plants are native, birds are malnourished?

Where is the evidence?

And so, we decided to see if we could find any actual evidence that supports these statements.  We started by looking at the research work of the ornithologist quoted by the Chronicle gardening columnist, Clare Aslan.  She earned her Ph.D. in Ecology at UC Davis in 2010.  Her Ph.D. project is described in a publication the year her degree was awarded:  “Avian use of introduced plants:  Ornithologist records illuminate interspecific associations and research needs.”  (1)

In this publication, Ms. Aslan tells us that her project was essentially a questionnaire that was sent to over 1,000 non-professional bird watchers in four American states (California, Florida, New York, and Washington).  These bird watchers were presumed to be skilled because they were members of Ornithological Societies of North America.  Responses were received from 173 of these bird watchers, of which 51% were from California.  Respondents reported 1,143 interactions between birds and plants.   “Interact” is defined as the full range of bird behavior:  eat, nest, perch, glean, etc.

The objectives of the questionnaire were:  (1) to evaluate patterns of bird use of non-native plants to determine the role the birds play in dispersal of “invasive” plants; (2) to examine the food web and guilds formed by bird interactions; and (3) to determine gaps in empirical research to inform future research efforts.

Respondents to the survey reported that 47% of observations of feeding by 139 bird species were of seeds or fruits of non-native plants.  Thirty-five percent of all “habitat interactions” were with non-native plants and 26% of all nesting activity was in non-native plants.  If non-native plants are harming the birds, nearly half of them must be in danger of starving to death!

Ms. Aslan tells us nothing about the relative nutritional value of non-native plants compared to native plants in this publication.  In her concluding paragraph, she suggests that her primary finding is that more research is needed to understand the role that birds play in the dispersal of “invasive” plants which has “direct application for invasion prevention.” 

Perhaps Ms. Aslan was misquoted by the Chronicle garden columnist.  If not, Ms. Aslan does not seem to have any empirical evidence to support her statements about the negative impact of non-native plants on birds.

Research about birds’ food preferences

Million Trees is always looking for the happy ending.  Consequently, our next step was to search the scientific literature for evidence that birds are being harmed by non-native plants.  It was not difficult to find several reassuring articles about the food preferences of birds:

• One study found that birds do, indeed, have food preferences and their preferences are based on many factors, including color, size, and availability.  However, “In addition to these factors the nutritional composition of fruit pulp also influences selection of fruits by birds.”  (2)
• Another study found that what birds choose to eat depends somewhat on their migratory patterns.  They choose foods with more fat content immediately prior to a long migratory journey over “major ecological barriers” such as seas and deserts. (3)
• In a native eastern forest in the United States, most of the nutritionally best fruit was eaten early in the season, prior to the beginning of the migration.  Over-wintering birds were then left to eat the fruit that remained.  (4)
• Finally, a study of fruit in the tropics showed that fruit that is more conspicuously colored and/or displayed seemed to be compensating for lower nutritional value than less conspicuous fruit.  The authors speculate that these are the evolutionary trade-offs that enable plant species to survive. (5)

All of these studies suggest that the birds know what they are eating and why.  We find no reason to fret on behalf of the birds that are eating non-native seeds and berries.  In fact, the eradication of some of the best food sources—such as Himalayan blackberries—may be a greater cause for concern.  And, once again, we find the extreme negativity of nativism to be a bigger problem, with respect to the damage being done to the environment in the guise of saving the planet from the harm they have imagined.

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(1)    Clare Aslan and Marcel Rejmanek, “Avian use of introduced plants:  Ornithologist records illuminate interspecific associations and research needs,” Ecological Applications, 20(4), 2010.

(2)    E.W. Stiles, “The influence of pulp-lipids on fruit preference for birds,” Vegetatio, Volume 107-108, Issue 1, 1993

(3)    F. Bairlein, “Nutrition and Fruit Selection in Migratory Birds,” Bird Migration, Springer Berlin Heidelberg, 1990

(4)    John W. Baird, “The Selection and Use of Fruit by Birds in an Eastern Forest,” The Wilson Bulletin, Vol 92, No 1, March 1980

(5)    Nataniel Wheelwright and Charles Janson, “Colors of Fruit Displays of Bird-Dispersed Plants in Two Tropical Forests,” The American Naturalist, Vol. 126, No 6, December 1985