Hurricanes and Droughts Beat Back Migratory Bird Populations

Ever hear the line that frog health is a great indicator for ecosystem health? That if frogs are dying, something is seriously wrong with the entire network? Well the same can be said for birds, which act as indicators for the health of their environment as a whole -- true "canaries in the coalmine" for swaths of habitats and species.

Because of birds' telling positions in their ecosystems, they make good research subjects for studying the effects of changes in climate. Two new NASA-funded studies do just that, taking a look at the effects of hurricanes and droughts on migratory bird populations.

The first study, published in Global Change Biology, found that hurricanes cause punishing, long-term effects on migratory bird species located even a distant 60 miles away from the path of the storm. The destruction of nearby forest prolongs more immediate problems, causing up to five years' worth of recovery time.

Before the study began, co-author Anna Pidgeon of the University of Wisconsin-Madison and her colleagues grouped 77 species into "migratory guilds." Each guild contained species with similar migratory habits. The researchers then focused on five hurricane-affected areas -- Atlantic coast and Gulf sites struck by storms between 1984 and 2005. They compared information on each guild's breeding seasons with the records of hurricane tracks, digital images from a NASA satellite and data from the North American Breeding Bird Survey.

While researchers expected some guilds, like the tree nesters, to show losses after a hurricane season, they expected others, like the shrub or ground nesters, to show gains since hurricane debris and downed trees could increase their resources. As it turned out, hurricane destruction didn't benefit any of the guilds. All species showed a loss in abundance and diversity that could stretch on for years. Pidgeon believes hurricanes shouldn't seriously threaten bird populations, so long as the animals have unaffected forest to retreat to.

The second study, also published in Global Change Biology, focused on drought, ultimately questioning the wisdom of considering all green space viable bird habitat. The amount of bird refuge is usually measured by the Normalized Difference Vegetation Index (NDVI), which measures the density of green leaves on the ground. However, since green cropland that can’t serve as bird cover during a drought still gets picked up on the index, scientists suspected there could be a better measure of refuge quality. They compared the NDVI data with 15 years of precipitation data from the North American plains. Precipitation proved to be a much more accurate indicator of population health and diversity than the NDVI. But the data also revealed that migratory bird species can experience population losses up to 13 percent during extreme droughts.

So it seems that green space -- and green space that's welcoming to birds -- is the crucial protective barrier between migratory birds and changes in climate like hurricanes and droughts. And since birds are such good indicator species, consider that an early warning.

In the NASA image above, the white diagonal shows the path of Hurricane Hugo across Charleston, S.C., and the Cooper River in 1989. Dark gray represents non-forested area, while light gray represents non-forested area after the disturbance. Dark green represents forest, light green represents post-disturbance forest and burgundy shows forest disturbed in 1989.

Images: Hurricane Hugo making its deadly way across the West Indies toward Charleston in 1989. (Susan Greenwood/Liaison/Getty Images); NASA image by Robert Simmon, based on data from Chad Rittenhouse, Department of Forest and Wildlife Ecology, University of Wisconsin-Madison.