Described as sounding like the English equivalent of ee-oh-lay, perhaps the first thing that comes to mind about Wood Thrushes is their beautiful song. Usually, Wood Thrushes can be heard off in the distance in mixed forests and deciduous woods, and if you have them in your neck of the woods, you know how lucky you are. There is simply no other sound quite like that of a Wood Thrush. These birds breed in the eastern half of the United States and need large tracts of forest to survive. Sadly, they’ve suffered a two-percent decline every year from 1966-2015.
The Wood Thrush is "listed as a Tri-National Concern species and is on the 2016 State of North America's Birds' Watch List, which includes bird species that are most at risk of extinction without significant conservation actions to reverse declines and reduce threats. The Wood Thrush is one of the most prominent examples of declining forest songbirds in North America.”1
Last June, I encountered this bird, its mate, and their nest in an understory about 13 feet high, not long after dawn. One of the Wood Thrushes had insects in its mouth and the other was flicking its left wing at me from a low branch. This was all by chance, mind you. I hadn’t sought out this bird and wasn’t even birding at the time. As I stood on a seldom-traveled path observing the pair in sparkling rays through the trees, I soon discovered their nest. Realizing how close I was, I immediately backed away from my position and observed the pair from afar. Later, I retrieved my camera and took some photos, some of which are below.
Wood Thrushes are one of many forest bird species susceptible to an ecological model called source-sink dynamics whereas these birds will inhabit both high-quality (source) and low-quality (sink) environments to breed. The source for Wood Thrushes is identified as a large tract of forest which allows for a high probability of reproductive success. The sink is a low quality habitat, such as a small woodlot. The sink population can survive, but their offspring will not, resulting in nest failure. To the casual observer, one might deduce that their small woodlot containing a Wood Thrush nest is indicative of a healthy population, but in a sink dynamic, no young ever fledge the nest. Without the protection of forest cover, predators such as crows, grackles, parasitic cowbirds, and squirrels are more likely to find their nest. In addition, cats can easily take these birds while they forage on the forest floor, their primary source for food.
Acid rain may be another factor contributing to the decline of the Wood Thrush. “A byproduct of burning fossil fuels for our vast energy needs, acid rain occurs when nitric and sulfuric acids combine with water in the atmosphere and return to earth as rain, snow, or mist. Acid reaction with the ground depletes soil calcium levels, leading to a host of forest ills. A calcium-poor diet can easily lead to egg shell defects and a smaller clutch of eggs. Both of these factors may contribute to breeding failure not only for the wood thrush but also for a variety of other songbirds."2 It's important to note acid rain's effect on wildlife warrants further research, but evidence suggests it may play an important role in the decline of some species, notably the Wood Thrush, primarily due to its calcium-rich diet.
Specifically, Wood Thrushes need approximately one or two forested acres to survive in their ideal habitat. The acreage from where I observed the pair of Wood Thrushes was sizable, however partially fragmented. It wasn’t apparent they were in their source habitat; and it wasn’t a slam-dunk they were in the sink either. Not more than a stone's throw away, there were open, non-forested areas on three sides of the nest. On the other hand, the birds had a fairly direct route of forested landscape to the east and southeast of their nest.
My photos revealed at least one baby’s beak rising above the top of the nest, so it was obvious the pair had reproduced provided it wasn't a cowbird's beak, but I did wonder whether there were more babies in the nest. I chose not to get better views of the clutch size simply because I didn’t want to disturb the family any more than my presence already had, but I was definitely curious!
I can only hope the trees remain standing for generations to come in the location where I discovered this Wood Thrush family. The realist in me, however, has a heavy heart. When I see homeowners chipping away at treed acreage all around my community, commercial development expanding further and further from the city, and the shocking images of expansive land-clearing from here to eternity as shown from online mapping resources, I am disheartened. I ask myself, when will it stop? How much do humans need? Why do we need so much? As a positive step forward, we have begun to reduce our dependence on fossil fuels, so perhaps acid rain will one day become a thing of the past. It is my fervent belief, however, that these actions alone will never contribute towards a sustainable planet for all creatures here on Earth unless human population control is also part of the equation.
Wood Thrushes are currently migrating to the tropical forests in Central America, another area of shrinking songbird habitat.
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| My photo of a Wood Thrush with suspected flea beetle, crane fly, and green lacewing. Special thanks to Jim (James) Walker, M.S. Entomologist, Department of Entomology, University of Minnesota, for insect identification. Photo ©Becca Mulenburg |
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| My photo of a Wood Thrush nest. Look closely to see the yellow-colored beak rising above the nest. Plastic was used for some nesting material, which is not uncommon for this bird. Photo ©Becca Mulenburg |
1Information from the All About Birds website, https://www.allaboutbirds.org/guide/Wood_Thrush/lifehistory, © Cornell Lab of Ornithology. Retrieved September 9, 2021.
2McLane, Eben. "The Disappearing Wood Thrush." https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100519715. Retrieved Sept. 9, 2021.
Graham, Sarah. "Acid Rain Linked to Bird Decline." Aug. 13, 2002. Scientific American. https://www.scientificamerican.com/article/acid-rain-linked-to-bird/ Retrieved Sept. 9, 2021.
Kricher, John. Peterson Reference Guide to Bird Behavior. Houghton Mifflin Harcourt Publishing Company, New York, 2020.
Ralph S. Hames, Kenneth V. Rosenberg, James D. Lowe, Sara E. Barker, André A. Dhondt "Adverse effects of acid rain on the distribution of the Wood Thrush Hylocichla mustelina in North America" Proc Natl Acad Sci U S A. 2002 Aug 20; 99(17): 11235–11240. Published online 2002 Aug 12. doi: 10.1073/pnas.172700199 PMCID: PMC123239. Retrieved Sept. 9, 2021.
"Source-sink dynamics." https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100519715. Retrieved Sept. 9, 2021.
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