A banded adult purple martin wearing a light logger geolocator. Source: Patrick Kramer and Tim Morton, ScienceMag.

In the time it takes each of us to drink a cup of coffee, acres of tropical forest are cut down. Over the past thirty years, more than half the traditional coffee farms in Latin America have been converted to a newer growing method for higher production. Since the 1960s dozens of migratory bird species from northeastern United States have experienced long-term chronic declines in population size, with few signs of leveling off.

All these events are connected. Deforestation is occurring around the world faster than ever for plenty of reasons, but one of the most widespread and impactful ones is agriculture, and sun coffee (that is, coffee not grown under shade but in huge fields with pesticide, herbicide, and fertilizer assistance) is becoming more common to increase yields in the very same countries where North American migratory birds stay for their winters. But more about sun coffee and its negative ecological impacts later.

Bridget Stutchbury, professor of biology at York University and author of “Silence of the Songbirds” and “The Private Lives of Birds,” thinks that Latin American deforestation is the main contributor to the aforementioned population decline, and has some new data to prove it. For the first time, scientists (i.e. Stutchbury and her field crew) have been able to track the exact routes of several migratory bird species on their way back and forth from wintering grounds, using a light logger. This little device, which looks like a little LED bulb on a hip harness the birds wear (weighing a total of 1.5g), records the degrees of sunlight every ten minutes for up to two years while it travels with the bird. Based on this data, Stutchbury and her team can calculate what time the sun rose and set every day the bird wore the logger, and thereby estimate the bird’s latitude—and its longitude, albeit with a little more error—on that day.

Purple martin feeders

Using populations from Lake Eerie, Pennsylvania, Stutchbury chose to track the Purple Martin, Wood Thrush, Bobolink, and Red-eyed Vireo using the light loggers. These birds live in fairly different types of habitat when in North America (e.g. Bobolink in grasslands vs. Wood Thrush in forests), with varied eating habits (Purple Martins swoop through the air for insects like swallows, and the Red-eyed Vireo picks insects off branches), but mostly seem to prefer thick forest—especially rainforest—when migrating south to Central and South America. Populations of Purple Martins from around the whole east coast seem to congregate in a relatively small portion of the Amazon in Brazil, resting in a pristine rainforest canopy that is quite different from the white feeders around New England that we envision when we think of the species. Red-eyed Vireos stay in another area in the Amazon, and Wood Thrushes in northern Honduras; as long as the forested areas these birds migrate to stay that way, you might think, the species should be fine.

But the population declines have to be occurring somewhere. If the untouched Amazon where the birds stay for five months isn’t being deforested yet, then where is the threat coming from? Stutchbury and her colleagues don’t believe North American habitat loss is the main problem, so they have turned to the resting points that birds use along their migration routes. That’s why the light loggers have been such useful tools: we can now estimate how many days an individual Purple Martin takens to get from Lake Eerie to the Yucatan Peninsula (~5 days), how many weeks they rest there (~3), how long they take to reach the Amazon (~2 weeks), and so on. Knowing that Red-eyed Vireos spend three weeks on their spring migration in Colombia, or that the core wintering grounds for all Wood Thrushes in northeastern US is a fairly compact portion of western Nicaragua, allows scientists to identify deforested areas that face habitat destruction. The stops between stretches of migration flying are incredibly important for the birds as resting points to refuel–loss of habitat can have especially dire consequences in territorial species that return to the same places every year, such as the Wood Thrush.

In Part 2 (Coffee), I will explain how shade-grown coffee is being used by the Smithsonian Conservation Biology Institute and its partners to conserve and create habitat for migrating songbirds.