Dearborn portion of the Detroit River CBC, 2012

The Detroit River Michigan-Ontario Christmas Bird Count was held, as it is each year, on January 1. This was the 35th year for the count, which is centered at I-94 and Warren Ave, and the 18th year that RRBO has coordinated the field work in the city of Dearborn.

The day began with mild temperatures which had been the hallmark of the autumn and winter season up to that point. All water was open, and there was (and had been for the most part) no snow cover. Waterfowl and seed-eating birds were dispersed far and wide. We'd seen an excellent fruit crop in late summer and fall, but most had been stripped by the time New Year's Day arrived.

We ended the day with 39 species, well below the average of 46 because the party covering the Ford Rouge Plant was denied access this year (not by Ford, but by the private security firm of another company). Thus, we missed a number of species of waterfowl and the two dozen or so Black-crowned Night-herons that typically roost in a small pond on the property.

Nonetheless, the day was not without highlights. Covering the UM-Dearborn campus, Greg Norwood found one of the Gray Catbirds first found on 20 December. This is a new species for the Dearborn portion of the count and brings the cumulative total to 87. A Sharp-shinned Hawk seen on 29 December couldn't be located, but is tallied as a "count week" species and is also new for the count. Greg also found the Yellow-bellied Sapsucker that has been hanging around for weeks. This is only the second time sapsucker has been found on the Dearborn portion of the count.

Darrin O'Brien and I covered the various plantings on Ford properties. Like last year, most of the seed-eating birds were found at the fields in front of Ford World Headquarters. Most numerous were House Finches (over 900) and House Sparrows (over 1800). The day's total of 1038 House Finches was a new high for the count. Many of the fields had a lot of standing water, making it harder for the little birds that like to forage on the ground. The total of 26 American Tree Sparrows was a new count low. Also present at Ford HQ was a Peregrine Falcon. It chased around a Red-tailed Hawk before landing on the Ford building.

Arrow points to grooming Peregrine.

Cathy Carroll turned up 18 Great Blue Herons along the concrete channel of the Rouge. Often this group of birds roosts along the river on campus, but forages all along the river. She also saw one of the seven American Kestrels in the city, which is a new high for the count.

Finally, only one American Crow was found all day. This is a new low, and represents a decade of counts with fewer than 20 crows (most years fewer than ten). The local population has simply not recovered from West Nile virus, and I am beginning to wonder if I will ever see new birds move in.

Former RRBO bander's research in NYT

Julie Jedlicka was an RRBO bander from 2000 to 2003, when she was a graduate student at the University of Michigan-Ann Arbor. She went on to receive her Ph.D at the University of California, Santa Cruz. She is now at the University of California, Berkeley on an NSF Postdoctoral Fellowship. Her research focuses on ecosystem services and avian conservation potential in northern California vineyards.

Julie and I have kept in touch throughout her academic career, and it has always been exciting to see her accomplishments accumulate. She has published a number of papers, and her most recent one -- Avian conservation practices strengthen ecosystem services in California vineyards -- was featured in the New York Times!

I like to think that RRBO had a small part in her success. But I know it is because Julie is a dedicated, creative scientist. Way to go, Julie!

Myth-busting: Birds, buckthorn, and diarrhea

[This post is in response to local interest in the effects of buckthorn fruit on the birds that eat it.]

For the last 5 or 6 years, my research has focused on the use of fruit by birds in the fall, especially non-native fruit and migratory birds. One of the most abundant fruiting shrubs in my southeast Michigan study area is Common Buckthorn (Rhamnus cathartica), an invasive fruiting tree not native to North America.

Before I continue, let me issue this statement: I strongly believe in the use and preservation of native plants. I am not "pro-buckthorn." I believe it has many ecological liabilities. One prevailing notion is that buckthorn causes diarrhea in birds and is harmful to them. Based on a peer-reviewed literature search, I cannot find evidence that this is true.

Much of the background information here is from an excellent overview paper by Izhaki (2002). Other research is also cited.

Background: Chemical properties of buckthorns

Many plants have what are known as "secondary compounds," chemicals that play important roles in plant fitness and survival. These compounds or metabolites are generally believed to deter the consumption of unripe fruit, since the seeds in unripe fruit are not yet viable. See Cipollini and Levey (1997) for a discussion of other very interesting hypothetical functions of secondary compounds in ripe fruit.

The relevant secondary compound in buckthorns is emodin, a free-form (aglycone) anthraquinone found in 17 families, 28 genera, and 94 species of plants, including 23 species in the genus Rhamnus*. It can be found in many plant parts, including the leaves and fruit. Emodin has many properties. In regards to fruit, emodin can act as an antimicrobial, inhibiting fruit damage. Levels of emodin in fruit pulp typically decrease as the fruit ripens. As explained above, this may help to protect the seeds from being dispersed before they are mature, or eaten and destroyed by seed predators.

The biological activity of emodin (as well as many other compounds) is different in mammals than in birds. Emodin is well-known as a laxative in humans as well as other mammals. What about birds?

Buckthorn and birds

Wherever it occurs, buckthorn tends to be distributed by birds. There are two commonly held and frequently repeated notions about buckthorn fruit: that it causes diarrhea in birds, and that birds in North America don't "know" not to eat unripe fruit and can become very sick or even die from eating it. (Actually, there is a third notion, that buckthorn fruit is "junk food." That isn't entirely true either, but it a subject of a future post.)

The diarrhea myth, I think, starts with the fact that when people see birds eating fruit, they also see the birds poop a lot. This is, in fact, true of most birds and most types of fruit, especially wet, pulpy fruit and fruit with bulky seeds. Fruit passes quickly through bird guts -- many fruits are composed of mainly water and seeds, and nutrients (especially sugars) that are quickly assimilated. Birds only have so much room in the digestive system and it makes sense for them to process fruit as quickly as possible, including jettisoning the seeds. Birds are also able to consume more fruit if they choose species that have short retention times (Cipollini and Levey 1997). Hence, the appearance of diarrhea. I suppose if you define diarrhea as loose, wet, and frequent defecation, then many fruits give birds "diarrhea." But the implication in public discussions is that the frequent, conspicuous defectations of buckthorn pulp is abnormal or harmful. I've seen similar messes near large stands of Pokeweed (Phytolacca americana) and Red Mulberry (Morus rubra), both native plants, and do not hear similar concern for birds' health.

Studies of secondary compounds in fruits (e.g., Wahaj et al. 1998) have indicated that plants exert "control" over the gut retention time of their seeds in birds through these chemicals; this includes both speeding up and/or slowing down of the passage through the digestive system.  This is presumably to influence the number of seeds expelled per defecation, the dispersal distance, and/or reduce or enhance the effects of gut passage on the seed coat. This control is advantageous to the plant if it increases its reproductive success.

Regarding emodin, Tsahar et al. (2003), working with Yellow-vented [White-spectacled] Bulbuls (Pycnonotus xanthopygos), found that emodin increased digestive efficiency in birds, aiding in the processing of dry matter, nitrogen, and other nutrients. They determined that this effect was the result of emodin slowing transit time of food through the gut. This study, then, found that emodin has a costive (constipating) effect, not a laxative one, at least in bulbuls.

In their review, Levey et al. (2007) noted, 

Despite the widespread detrimental effect of emodin on fruit consumption by vertebrates, ripe fruits that contain emodin are frequently consumed by a wide variety of seed dispersers... suggesting that the net benefits of fruit consumption somehow outweigh the negative effects of emodin. In particular, the nutritional reward of carbohydrates, lipids, and proteins in fruit pulp can be viewed as more important to consumers than the presence of co-occurring secondary compounds.

Do North American birds unwittingly eat unripe non-native buckthorn fruit, which contain higher concentrations of emodin than ripe fruit? Could they consume enough to harm them?

There are at least 14 native species of Rhamnus and Frangula in North America. Several species are widespread, and have similar fruit characteristics (phenology, size, color) as the non-native species. Given the widespread occurrence of emodin in this family, presumably it also occurs in the fruit of the native species. This indicates that North American birds are not naïve to buckthorn or emodin.

Even if they had no experience with fruits containing emodin, studies have shown that many bird species are sensitive to it and adjust their consumption accordingly. Tsahar et al. (2002) found the highly frugivorous Yellow-vented Bulbul could distinguish among artificial diets containing a range of emodin concentrations similar to what is found in ripe fruits, while House Sparrows could only discern emodin amounts similar to what is found in ripe versus unripe fruits.

Both of those bird species are Old World in origin. One of the only studies to take a long look at R. cathartica and North American birds was a doctoral dissertation by James Sherburne (1972). He found American Robins avoided feeding on fruits of other plants coated with emodin, and that even if starved for 12 hours, would only voluntarily eat a few unripe R. cathartica fruits. The unripe fruit contained between 0.6 to 1 microgram of emodin.

Sherburne went on to force American Robins and Gray Catbirds eat unripe Rhamnus fruit or capsules containing emodin. The birds showed signs of diarrhea after consuming 50 micrograms to 5 mg of emodin. At the lowest concentration showing effect (50 micrograms) and the highest concentration given for the unripe fruit (1 microgram), it would require ingestion of at least 50 unripe fruits to cause diarrhea.

Schafer et al. (1983) tested the toxicity of a wide range of compounds on birds, including emodin. The dose of emodin needed to kill half (LD50) of Red-winged Blackbirds and European Starlings was greater than 100 mg per kg of body mass. Under this scenario, an average-sized 65-gram male blackbird would need to consume at least 6500 unripe fruits of R. cathartica (based on the high estimate of 1 microgram of emodin in an unripe fruit from Sherburne) to reach the "LD50" dose. Little wonder the authors calculated a "hazard index" indicating little or no potential for emodin to cause acute poisoning in these species.

Given the ability of birds to detect emodin, their reluctance to eat unripe fruit (three other studies cited in Izhaki [2002] noted that in North America, "most bird species do not consume the unripe fruits"), and the high levels needed to create a negative response, we can probably infer that wild birds do not consume large enough quantities of unripe fruit to cause diarrhea or serious toxicity.

As for ripe fruit, Sherburne found no emodin in ripe fruits of R. cathartica, although other studies have found it in the ripe fruit of other Rhamnus species not yet established in North America (Tsahar et al. 2002). As it is found in far smaller amounts in ripe fruit, the number of fruits that would need to be consumed to cause harm would be many times greater than the examples listed above. Sherburne reported on 11 species of birds that readily ate ripe R. cathartica fruit, but did not mention any negative effects (in Knight et al. 2007).

In the wild, birds tend to mix their diets, choosing fruits and other foods within short feeding bouts, and this may be an attempt to balance nutrient intake or to avoid ingesting too much of a particular secondary compound (Cipollini and Levey 1997) . Further, the varied dietary inputs may contain their own qualities that could mediate any adverse physiological effects of emodin.

There are many ecological crimes we can pin on buckthorn. It doesn't appear that causing harmful diarrhea is one of them. If anybody has seen any other studies or peer-reviewed publications that do show that eating buckthorn fruit is harmful to birds, please post them in the comments. I am fascinated by this topic and interested in learning of new research.

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*Glossy Buckthorn, Frangula alnus, has often been included in the genus Rhamnus as R. frangula. It also contains emodin. When I refer to "buckthorn" here, I am referring to both Common and Glossy Buckthorn in general, unless otherwise noted.

Cipollini, M. L., and D. J. Levey. 1997. Secondary metabolites of fleshy vertebrate-dispersed fruits: adaptive hypotheses and implications for seed dispersal. American Naturalist 150:346-372.

Knight, K. S., J. S. Kurylo, A. G. Endress, and J. R. Stewart. 2007. Ecology and ecosystem impacts of common buckthorn (Rhamnus cathartica): a review. Biol. Invasions 9: 925-937.

Izhaki, I. 2002. Emodin -- a secondary metabolite with multiple ecological functions in higher plants. New Phytologist 155:205-217.

Levey, D. J., J. J. Tewksbury, I. Izhaki, E. Tsahar, and D. C. Haak. 2007. Evolutionary ecology of secondary compounds in ripe fruit: case studies with capsaicin and emodin. Pages 37-58 in A. J. Dennis, E. W. Schupp, R. J. Green, and D. A. Westcott, eds. Seed Dispersal: Theory and its Application in a Changing World. CAB International, Cambridge, MA.

Schafer, E.W. Jr, Bowles, W.A. Jr, and J. Hurlbut. 1983. The acute oral toxicity, repellency, and hazard potential of 998 chemicals to one or more species of wild and domestic birds. Arch. Environ. Contam. Toxicol. 12:355-382.

Sherburne, J. A. 1972. Effects of seasonal changes in the abundance and chemistry of the fleshy fruits of northeastern woody shrubs on patterns of exploitation by frugivorous birds. Ph.D. dissertation, Cornell University, Ithaca, New York.

Tsahar, E., J. Friedman, and I. Izhaki. 2002. Impact of fruit removal and seed predation of a secondary metabolite, emodin, in Rhamnus alaternus fruit pulp. Oikos 99:290–299.

Tsahar, E. 2001. The impact of the secondary metabolite emodin in Rhamnus alaternus fruits on fruit removal and seed predation. M.S. thesis, Tel-Aviv University, Israel.

Tsahar, E., J. Friedman, and I. Izhaki. 2003. Secondary metabolite emodin increases food assimilation efficiency of Yellow-vented bulbuls (Pycnonotus xanthopygos). Auk 120: 411-417.

Wahaj, S. A., D. J. Levey, A. K. Sanders, and M. L. Cipollini. 1998. Control of gut retention time by secondary metabolites in ripe Solanum fruits. Ecology 79:2309-2319.

Fall banding: Weeks 8-10

Good riddance to a wet September. October has been occasionally wet or windy, but overall the weather has been much better. Unlike some Octobers, we have had only two mornings with frost, which can delay us from opening in the morning if the nets get stuck shut. On the downside, deer are mating and they've managed to get in the banding area regularly. One night a buck was presumably chasing a doe and they ran into the gate of the chain link fence so hard they bent open the latch! We are now operating with a reduced number of nets, and all but two of the ones remaining are damaged from their activities.

The number of birds we have banded so far in October represents over 40% of the total for the season. Most have come after mid-month, when American Robin numbers increased, and the first big push of Yellow-rumped Warblers, Hermit Thrushes, and sparrows began. It is heartening to see respectable Hermit Thrush numbers. The number of Swainson's Thrushes banded this fall was our second lowest. We have already hit our average for Hermit Thrushes, and are getting plenty of seed samples from them, which is excellent. We're still below average for White-throats, and they don't seem as numerous this year as usual. We have only banded four White-crowned Sparrows, which is ridiculously low. Our average is 22.

White-throated Sparrow.

Warblers continue to be banded. In addition to Yellow-rumps, late species such as Common Yellowthroat, Orange-crowned, and Palm still are around, as is Tennessee. Every day we still get a Nashville Warbler or two, and we are far over our fall high of 59 Nashvilles banded. Notable was a Blackpoll Warbler banded on 22 October that tied the late fall date for Dearborn -- but one seen the next day furnished the new late date. An American Redstart was also banded  on 22 October, and that was a new late date for that species.

Orange-crowned Warbler.

Two species were banded this month that we don't get too often: Eastern Towhee and Yellow-bellied Sapsucker.

Young male Eastern Towhee.

Young female Yellow-bellied Sapsucker.

We are now around 1000 new birds banded of 72 species, approaching average but with a capture rate (based on number of nets and hours opened) still well below average. Typically we band into the first week of November, but we may go longer than that this year if the weather holds. The next banding update will be a summary of the season.

Poison ivy: Breakfast of champions

Our study of the fruit composition of the diets of fall migratory birds focuses on Catharus thrushes and the super-abundant non-native fruits found in urban areas. However, we do look for seeds in the "samples" (droppings) provided by all the birds we band. This helps us examine which other bird species are eating fruit, what types of fruit they consume, and whether the diets of different bird species favor different fruit species.

One of the native fruiting plants that is common on our site is poison-ivy (Toxicodendron radicans). The flowers are rather insignificant and can be overlooked.

They do result in fruit, and large mature vines produce good crops of small, off-white drupes. This often occurs overhead, where the vine has climbed up a tree and gets good sun exposure.

This portion of poison-ivy is growing on a fence where it gets direct sun, and each fall it has a lot of fruit.

Poison-ivy fruits themselves are about half the size of a pea, quite dry, and papery. They contain very little pulp. Each fruit has one to three, odd-looking, globular seeds that are very distinctive.

We find the seeds in samples from only a few species of birds. Most often it's woodpeckers, and we've gotten poison-ivy seeds from Downy and Hairy Woodpeckers, and Yellow-shafted Flickers. The sample below is from one dropping from a flicker.

The three seeds in the upper right are from wild grapes (probably Vitis riparia). The other 13 are poison-ivy. Over the past several years, the only other bird species contributing poison-ivy seeds has been Yellow-rumped Warbler.

When we find seeds in a Yellow-rump sample, it's always poison-ivy. The other day I was holding a Yellow-rump while writing down some data. Suddenly, a single poison-ivy seed shot out of the warbler! Usually, we just collect the seeds that are deposited in the holding bags we use to transport the birds from the nets to the banding lab.

This week, we got two poison-ivy seeds from a Ruby-crowned Kinglet, the first seed samples of any kind we've had from a kinglet. This particular bird was first banded on 7 October, when it weighed 6.3 grams. We recaptured it several more times. On 12 October, it weighed 6.7 grams, on 16 October it was 7.2 grams, and on 18 October it was 7.4 grams. It doesn't sound like much, but that's a 17.5% increase in the bird's original weight.

Poison-ivy fruit are very high in fats (over 40%) and low in sugars. Not all birds are able to easily or efficiently digest and absorb fats. Differences in digestive physiology, as well as the varied nutritional needs among residents,  long-distance, and short-distance migrants has a great influence on fruit choice.

In the special case of poison-ivy, the fats in the fruits are also of a waxy (high melting point) nature, and only a few bird species are able to process plant waxes. The eastern form of Yellow-rumped Warbler, found here, is called "Myrtle Warbler" because of its ability to eat the fruit of wax myrtle (Myrica sp.). Poison-ivy fruits are very similar in composition to myrtle fruit.

You may wonder if the compounds (urushiol) in poison-ivy that cause a skin rash in most people are present in the fruit. I'm not entirely sure; some sources say yes, others indicate they are only present in the sap, and therefore only the stems and leaves. In other plants with urushiol, such as mangoes, the fruit does not contain the poison. I'm not very sensitive to poison-ivy, and I have not had a problem handling the seeds once pooped from a bird.