Nandina toxicity to birds?

Is Nandina domestica (a.k.a. heavenly bamboo) killing birds? This controversy is raging in gardening circles on social media, with some people claiming Nandina is killing thousands of birds every year, and some people claiming it’s entirely harmless and that claims about its toxicity are debunked, bad science, or fake news. Have any iNaturalist users observed any such toxicity, or better yet helped document it? Is this a project that iNaturalist can help address in any meaningful way?

Nandina domestica (Berberidaceae) is a widely cultivated species in North America, originally from Asia. It’s an attractive evergreen shrub that produces bright red berries. The controversy started when a mass mortality event of cedar waxwings was investigated in Georgia in 2009. The research paper suggesting they were killed by ingesting Nandina berries (Woldemeskel & Styer 2010; link below) has been widely derided in the gardening community. In reality, it was pretty good science; the authors provided very strong evidence that Nandina berries had killed the birds. They were also fairly cautious in their conclusions. The “bad science” came in the secondary literature, generalizing and extrapolating from that single research paper, about a single mortality event, involving a single species of bird. These overly broad interpretations led to widely cited but largely unsupported and very unscientific claims about Nandina toxicity. This was followed by a lot of incredulity and pushback from gardeners who frequently observe birds eating Nandina berries, apparently without ill effect. But the broad claims that Nandina berries are harmless to birds, and that there is no evidence of their toxicity, are equally problematic.

An excellent recent research paper (Zona 2022, link below) provides support for the claims of toxicity, but with some major qualifications.

The bottom line is that people want simplistic answers to simplistic problems, but science often doesn’t provide them because reality is often a lot more nuanced. No, Nandina berries probably aren’t killing thousands of birds. But yes, they can be deadly to birds, under certain circumstances. If you don’t read anything else, please read this non-technical summary “Nandina Fruits Can Be Toxic to Birds” by botanist Scott Zona, who has recently investigated the potential toxicity of Nandina domestica: https://ncbg.unc.edu/2022/05/04/nandina-toxic-to-birds

References:

Woldemeskel & Styer 2010, “Feeding Behavior-Related Toxicity due to Nandina domestica in Cedar Waxwings (Bombycilla cedrorum)”: https://www.researchgate.net/publication/49720490_Feeding_Behavior-Related_Toxicity_due_to_Nandina_domestica_in_Cedar_Waxwings_Bombycilla_cedrorum

Zona (2022), “Fruits of Nandina domestica are (sometimes) cyanogenic and (sometimes) hazardous to birds”: https://www.researchgate.net/publication/359867037_Fruits_of_Nandina_domestica_are_sometimes_cyanogenic_and_sometimes_hazardous_to_birds?

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As you said, the truth is very likely somewhere in the middle. Nandina poisonings are undoubtedly not isolated to the one documented mortality event. Most species likely aren’t at great risk; Cedar Waxwings are a exceptional case because of the sheer number of berries they will consume.

That being said, I would love to see Nandina removed from big box stores (outside its native range). Here in the southeastern US, Nandina spreads rapidly into natural areas. While things like privet, English ivy, and nonnative honeysuckles are far worse (from a monoculture standpoint), the impact of Nandina still is not worth it.

Sure, many bird species can likely tolerate the moderate number of Nandina berries they are willing to consume, but why not plant something like native elderberries that are similar in height and structure and coevolved with the birds and insects that feed on the berries and foliage.

Nandina toxicity is likely overblown, but in my opinion, so is its appeal. I find the smell repulsive and the foliage unattractive. The berries are admittedly attractive, but there are many native plants that produce abundant red fruits that are far prettier overall.

I feel like this would be a hard thing to document conclusively on iNat. Especially since people are likely to stick to their own sides of this controversial topic regardless of where the evidence points.

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@DCTropics Thanks for those references, John. I’m going to dig into them. I’ve always held to the toxicity side of the argument, if only for the reason that it gives me extra ammo to argue for its removal from local parks and greenbelts and a good argument to avoid planting it, and trading it out for native berry bushes.

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yeah, we removed it from our yard because if it’s not doing any good, and has the potential to do harm, it gets yanked. We planted native blueberry bushes instead.

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Given a choice, birds generally prefer the fruits of native species.* And yes, they can probably consume a certain number of Nandina fruits without harm. I suspect part of the problem is simple availability: in many urban and suburban areas, there are so few native food sources that fruit-eating birds are forced to subsist on non-native species. This may not be a problem in late winter when Nandina fruits are much lower in toxicity; but in some areas, this species may be just about their only food source through the entire winter.

*“We found that birds preferentially consumed native fleshy fruits rather than invasive fruits throughout the autumn season…. Our fruit preference results showed that fruit availability was not reflected in fruit consumption. In particular, birds primarily consumed the fruits of native species throughout the autumn season, despite the increasing relative abundance of invasive fruits in late-autumn, echoing previous studies that have shown birds prefer native fruits.” https://www.sciencedirect.com/science/article/abs/pii/S0006320719314946

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Are these the same people who keep spraying Raid on the cockroach until it finally dies – since it didn’t instantly drop dead, it must not have been poisoned yet?

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We have a few Nandina domestica bushes in our garden, although not as many as when we moved in and we intend to replace them with appropriate native species when we can.

However, I’ve never noticed the flocks of Cedar Waxwings paying the Nandina any attention as they’re too busy gorging on the pink peppercorns from the large Schinus molle tree, also non-native and apparently somewhat toxic to chickens.

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A lot of these people do indeed say “I don’t see any dead birds near my Nandina, so it can’t possibly be toxic to them.”

Some gardeners say they never see birds eating Nandina berries, and others say they see birds eating the berries without any ill effects. I think both can be true; in some areas there may be other more preferred sources of food available, and as I stated above (and as the research suggests), small numbers of berries may be perfectly safe for most birds to eat; the problem is specifically cedar waxwings or any other species that gorges on the berries.

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The “fallacy of few field observations” applies here: Even though few/none of us–even active birders and field observers–have seen Cedar Waxwings or American Robins actively consuming Nandina berries, the evidence is clear that avian dispersal is the normal vector for this species’ spread into native habitats away from suburban yards. The scattered distribution of new individuals of Nandina as an understory colonizer under a canopy of, in our case in Central Texas, juniper-oak woodlands is prima fascie evidence that birds are the vectors. There is a moderately long list of candidate frugivorous birds, both resident and migratory, that might be the culprits, but numerically and seasonally (i.e. when Nandina berries are ripe and available), the above two species are prime suspects. Other candidates here locally might include the resident Northern Mockingbird, wintering Hermit Thrushes, and any number of less common/smaller frugivores.

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I thought it was also interesting that the research showed that unripe Nandina berries were more toxic than ripe ones (not that we can expect birds to be capable of delayed gratification).

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This brings up a whole other discussion–mostly missing from that published literature–on the seasonality of the potential dispersal vectors (e.g. migratory birds) in the region where Nandina is native and strategies by the plants to optimize the utilization of such vectors. It may behoove a plant species to deter fruit consumption earlier in the Fall/Winter because seeds dispersed early in those seasons may be more likely to be consumed or destroyed by secondary consumers (ground foraging animals/insects) or pathogens, or the seasonality of optimal seed germination may not coincide with an early seed dispersal. By contrast, when conditions are better for seed dispersal, predation avoidance, and/or germination (e.g. late winter or spring), the chemistry of the fruits has evolved to take advantage of the changed conditions. Probably all aspects of such hypotheses are testable with field data and/or lab experiments.

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I have a whole list of species I’ve seen eating Nandina berries: Northern Mockingbird, Eastern Bluebird, Cedar Waxwing, American Robin, and Eastern Phoebe.

My apartment complex has a grove right outside my window (I’ve volunteered to remove and replace them with something native to no avail). We also have several American Hollies that fruit at the same time as the Nandinas. I suspect the hollies draw the birds in and they eat the Nandina berries because they are similar in color to the hollies’.

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