AUTHOR BLOG: Migrating Birds That Eat Northern Spicebush Berries Are Fat and Healthy Birds

Yushi Oguchi

Linked paper: Fruits and migrant health: Consequences of stopping over in exotic- vs. native-dominated shrublands on immune and antioxidant status of Swainson’s Thrushes and Gray Catbirds by Y. Oguchi, R.J. Smith, and J.C. Owen, The Condor: Ornithological Applications 119:4, November 2017.

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Collecting a small blood sample from a Gray Catbird to assess its health during stopover. Photo credit: Zak Pohlen

“We should not only conserve avian populations; we should conserve ‘healthy’ avian populations.”  – Dr. Jen C. Owen (my M.S. adviser, Michigan State University)

We investigated whether the health status of fall frugivorous Swainson’s Thrushes and the Gray Catbirds differed depending on their use of shrublands dominated by exotic versus native plants. In the process, we came to appreciate the value of one native fruit, northern spicebush (Lindera benzoin), a fruit we couldn’t even identify at first. Our story represents a unique insight that ecophysiology can bring to conservation science and habitat management.

Our prediction was that habitat may influence bird health through their frugivorous diet and that landbird migrants may be able to enhance their immune system by resting and refueling during stopover. In fall, many shrubs are loaded with fruit, and this fruit contains essential nutrients, including antioxidants, which help a bird neutralize reactive oxygen species produced during exercise (flight).

A question of conservation interest is whether birds that forage on exotic fruits and in exotic-dominated shrubland experience a deleterious effect on their health. Many exotic shrubs such as autumn olive and honeysuckle (introduced to the Midwestern U.S.A.) produce fruits that are generally lower in energy than native fruits such as northern spicebush, but some exotic fruits may have high antioxidants, including immunostimulatory carotenoids. We wouldn’t know unless we tested it!

A state-managed land in Michigan had the perfect habitat matrix—exotic-dominated shrubland and native-dominated shrubland occurring side by side, bisected by woodland. The exotic habitat was largely autumn olive, honeysuckle, and multiflora rose, and the native habitat was largely dogwood species, common winterberry, and northern spicebush. During fall migration in 2012 and 2013, we captured more than 800 individual birds, from which we collected blood samples for comprehensive health assessments.

Where Swainson’s Thrushes foraged during stopover had no impact on their health. On the other hand, Gray Catbirds using exotic shrubland experienced poor health; they lost mass and had reduced immune function and lower antioxidant capacity compared to catbirds using native shrubland. We also saw annual variation, with catbirds exhibiting more deleterious effects in 2013 compared to 2012. We further found that the pattern of habitat effect on catbird (but not thrush) health could be at least partly predicted based on the fruits they ate (math on fruit nutrition and bird fecal data).

SPICEBUSH

Fecal sample showing the remnants of northern spicebush. Photo credit: Jen Owen

It is through this nutritional analysis that we saw the value of northern spicebush. Spicebush fruit has very high energy and antioxidant capacity relative to other species. It was a preferred fruit by both bird species and likely contributed to the bulk of the nutrients they acquired in native shrubland. In exotic shrubland, thrushes consumed more exotic fruits like common buckthorn than catbirds did—so they got antioxidants from exotic fruits. Catbirds, apparently more “reluctant” than thrushes to eat exotic fruits, instead consumed other native fruits that were low in antioxidants. Not eating enough exotic fruits in exotic shrubland may be why catbirds experienced a dip in their health there.

To summarize, migrant health may be influenced by human alteration of habitat, and the effects may depends on the diet of the birds. And again, northern spicebush is a great fruit for birds if you are in eastern North America! In a future paper (accepted by The Condor), we will cover more on shrubland habitat use by the birds at our site.

Dr. Jen Owen continues to direct the Burke Lake Banding Station (BULA) where this research was conducted. She and her undergraduate researchers are doing more intensive monitoring of the fruit abundance and phenology at the site, particularly on the interannual variation in spicebush fruiting in relation to climate. BULA is now in its 7th year of operation, and it continues to provide research opportunities for many students as well as inspiring bird enthusiasts of all ages. Visitors are always welcome while they are open during fall and spring migration. Check them out on their website (burkelakebanding.com) or like them on Facebook (facebook.com/BULAbandingstation).

I’ve done a return migration to the University of Wisconsin–Madison for my Ph.D. this fall. Following my renewed interest in nutrient acquisition so key to health, I now study molecular mechanisms of digestive enzyme modulation in the avian gut.

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