Passenger Pigeons and Lyme

Today my roof is buried under more than a foot of snow, and the drift is sliding off of the high-angle metal in a slow-motion avalanche. Not long ago, my roof hosted an avalanche of a very different kind – an avalanche of acorns.

Back in August, I started hearing weird noises on the roof. A loud plunk would be followed by a series of rattles, then silence, and sometimes a final plink. While working in the garden one afternoon, I figured it out. The noises started, the short silence ensued, and an acorn launched off the roof, sailed over my head and plinked on the roof of my car. Every slight breeze brought the same series of events hailing down on me from the large oak in the yard. The driveway soon became a roly-poly mess of round, brown nuts.

Oaks are mast trees, which means that they save up their energy for three to five years before producing thousands of acorns all at once. And single trees don’t usually mast on their own. Somehow, all the trees in an area communicate to determine which year will be the right one. “In the old time, our elders say, the trees talked to each other,” writes Robin Wall Kimmerer in her book Braiding Sweetgrass.

Scientists don’t go for that explanation. Foresters hypothesize that trees simply wait until they’ve built up enough energy. But that doesn’t explain how trees in rich and poor habitats are ready at the same time. Mycologists now theorize that the networks of mycorrhizal fungi that connect a forest by the roots may be the agent of coordination. “A kind of Robin Hood,” writes Kimmerer, “they take from the rich and give to the poor, so that all the trees arrive at the same carbon surplus at the same time…Through unity, survival. All flourishing is mutual.”

Why does masting together equal flourishing? It is simple math. With thousands of acorns available at once, the hordes of acorn-eating seed predators cannot possibly eat them all. Some, at least, will survive to grow a new tree.

At least, that is how it works today. In the past, there was a seed predator that could clean out an oak forest, even in a mast year. Passenger pigeons, once the most numerous land bird on the continent, traveled in flocks of millions. When they descended on a grove of oaks, the twigs, branches, and even whole trees broke under the weight of their bodies. By one report, not an acorn could be found after they moved on.

Their nesting colonies, by necessity, also followed the mast. The largest nesting area ever recorded was in central Wisconsin in 1871. The size was estimated at 850 square miles. The population was estimated at 136,000,000 birds. It takes a lot of food to support that many birds, even for the two months or less it took to mate, brood, and fledge the squabs.

Now that passenger pigeons are extinct (the last one died 100 years ago this fall), it’s difficult to tease out the significance of their lives, and the consequences of their demise.

What we do know is the impact that extra acorns (presumably the ones no longer eaten by flocks of pigeons) have on several other species. In New York, scientists have shown that Lyme disease increases two years after an acorn mast year. The mechanism seems to be the increase in white-tailed deer numbers and local density, along with an increase in the population of mice (one of the best carriers of the Lyme-causing bacterium).

Deer help the ticks reproduce. Mice infect the ticks with Lyme disease. The ticks infect us. If the pigeons were still here to eat all the nuts, and prevent the local concentrations of deer, mice, and infected ticks, hypothesizes ornithologist David Blockstein, would that also keep the spread of Lyme disease in check?

The world is a complicated place, and we can never fully predict the consequences of our actions. None of the nineteenth century market hunters--who slaughtered thousands of passenger pigeons per day with the conviction that they would always be plentiful—would have ever suspected that their actions might result in their descendants suffering from a tick-borne disease.

Jerry Niemi, from the Natural Resources Research Institute, at the University of Minnesota, Duluth, recently spoke on the rise and fall of bird populations in Minnesota and Wisconsin. Since the year 1600, he recounted, 131 species of birds have become extinct. Today, another 1213 species are facing the threat of extinction. That is 12% of all known species of birds. What cascade of events will their losses trigger? What kind of avalanche will climate change bring? Certainly not one as quaint and harmless as acorns or snow tumbling off my roof.

Kimmerer’s wisdom sums it up for me: “What happens to one, happens to us all. We can starve together or feast together. All flourishing is mutual.”

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! The current exhibit, “Nature’s Superheroes—Adventures with Adaptations,” opens in May 2014 and will remain open until March 2015.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com.