THIS month is the peak of spring bird migration, when New Yorkers flock to Central Park, craning their necks to catch a glimpse of refueling warblers.        But the treetops hold fewer feathered gems each spring, to the point that a typical middle-aged bird-watcher now feels triumphant upon seeing a single bay-breasted or Canada warbler, two of the dozens of disappearing species common in our youth.
       
Humanity’s assault on migratory birds includes a familiar litany of human-made perils — clearing of forests, predation by cats and poisoning by the toxic byproducts of agriculture and industry. But one of the biggest contributors to the decline in migratory bird populations has gone largely unnoticed: white-tailed deer.
By 1900, deforestation and unregulated hunting had reduced deer populations in the Eastern United States to tiny remnant clusters surviving in remote sanctuaries. But subsequent protective laws and aggressive habitat management allowed deer to bounce back.
To this day, wildlife managers slice intact forests into sunny woodlots that maximize the number of deer and the frequency of encounters between deer and hunters. Private landowners are encouraged by wildlife agencies to crisscross their forest acreage with tasty plantings of clover and wheat in support of what is now a burgeoning population of perhaps 50 million white-tailed deer — in some places as many as 75 deer per square mile.
For some, such an abundance of wildlife might add excitement to a Sunday drive or backyard nature adventure. But most of us have become all too aware that there is a downside to having so many hoofed neighbors in the form of disease-bearing ticks and front-end collisions.
Less appreciated, though, is how these millions of deer are quietly eating every palatable leaf within their reach across the eastern forests of North America. That’s very bad news for migratory birds.
Migratory warblers generally feed in the treetop canopy, but many treasured species — worm-eating, Kentucky and hooded among them — hide their nests in dense vegetation on or near the ground. Deep in the woods, buffered from suburban predators and rural pesticides, warblers should be able to nest in peace. But they can’t do so when hungry deer have demolished the forest understory.
       
Take a quick drive through forested terrain and see for yourself the stark browse lines, missing orchids and denuded shrubbery. The conclusion is inescapable: There are too many deer, and they are endangering the rest of our flora and fauna, including valuable timber and invaluable songbirds.
The typical solutions, like bringing back mountain lions and wolves to control deer, are no longer an option in most places, in part because of the forests’ proximity to humans. Deer hunting has lost its appeal for many younger Americans, and the population of new hunters in the East is most likely declining faster than the threatened cerulean warbler.
       
Nevertheless, the good news is that this is a problem we can fix — and fix quickly.
One easy step is to fence off select sections of the woods, creating deer-less oases. Researchers in Virginia and Pennsylvania have successfully fenced deer out of small forest plots, demonstrating that although deer severely alter the structure and composition of deciduous forests, vegetation and birds come roaring back when deer are excluded.
Fencing, however, is expensive, especially on such a large scale. An even easier solution is to go back to the source of the problem: stop managing our forests for deer. Those early 20th-century strategies are a great conservation success story, but perhaps too much so: the deer are now being managed to the detriment of the rest of the ecosystem.  We need to seek balance and manage public land for fewer deer. Reducing deer numbers will mean healthier forests, fewer ticks and more warblers each May.

Daniel Cristol is a professor of biology at the College of William and Mary.