Wednesday, October 1, 2008

On the Wing, Over Prairie and Sea

Peregrine Falcon. Photo by David Powell

They’re coming soon, to a forest or shrub stand near you! In fact, many are already here. September is just about the best month to look for migrants winging their way south toward their wintering grounds. It is an especially good time to look for those uncommon species, like Yellow, Canada, Prairie, or even Chestnut-sided Warblers that nest in the northern forests, but winter in Central or South America.

The good news is that huge numbers of migrants can be found in any area with a good supply of insects, often in mixed flocks that just might include that target bird on your wish list. Just about anywhere with good stands of shrubs beneath the cottonwood canopy should host more than enough birds to satisfy your birding appetite. Just find a likely patch and stop and listen. Shortly, you should begin hearing the little chips that tell you songbirds are around. At this time of year, only one thing is on their minds – food! Their entire focus is to add as much fat reserve as possible to help them survive their long migration.

Just as all species look and sound different, their feeding styles vary too. Yellow-rumped and Wilson’s Warblers are very active feeders, flitting about at all levels from ground level to canopy tops. Watch for motion in the leaves that indicates a songbird is actively feeding. For others, like MacGillivray’s Warbler and Common Yellowthroat, a forceful chip note arising from near ground level in dense vegetation reveals their presence. Still others, like our resident Virginia’s and Grace’s Warblers, tend to skulk in the vegetation, moving slowly and deliberately, often evading detection. And, lastly, there are those that never make a sound, like Warbling Vireo. Luck has to be on your side to detect them.

Although the songbirds moving south through New Mexico have a land-based migration, many species in eastern North America have evolved to migrate over the Gulf of Mexico. Their southbound journey exactly coincides with hurricane season. Have you ever wondered how birds cope with the wrath of Mother Nature at her most ferocious? So far in 2008, Gustav, Hannah, and Ike have affected an already perilous journey, one in which there is no natural respite for an exhausted bird.

It is clear that hurricanes have a devastating effect on migrants caught in the storm, with some estimates of hundreds of thousands dead as a result of a single powerful hurricane. Seabirds, in particular, have nowhere to seek shelter on the open waters. Birds caught up in these storms are blown far off course, often landing in inhospitable places or simply arriving too battered and weak to survive. Although the toll may be extreme on individuals, healthy bird populations are able to withstand such losses, and have evolved to do so. However, severe storms can have devastating consequences for endangered species. Such was the case in 2007, when all 18 endangered Whooping Crane youngsters drowned in their protective cage in the lowlands of Florida following an unexpected storm surge.

There is evidence that birds are sensitive to changes in air pressure and instinctively take shelter. A sharp drop in barometric pressure indicates a big storm is on the way. Some birds fly away from the storm’s path, while strong-flying birds, like the Peregrine Falcon, fly ahead of the storm. In 1998, a second-year, female peregrine was captured in Virginia, and fitted with a satellite transmitter. Scientists monitored her migration southward to Venezuela, including during the extreme weather created by Hurricane Mitch. The peregrine initially followed a track that would have taken her island hopping along the eastern Carribean. As Mitch’s winds altered the normal southerly flow that aids migrating birds, she was pushed toward the west into the open waters of the Gulf. Suddenly, over open water, the falcon appeared to stop for nearly two hours, apparently taking refuge on a ship. For the next three days, she moved against the prevailing winds in a direct line toward Galveston, at a speed of ~10 miles per hour, again presumably on a ship. She rested near there for six days before departing in a southeasterly direction, again possibly hitching a ride on a ship based on her average departure speed. She arrived in Venezuela approximately five weeks later after a sojourn in the Yucatan Peninsula.

Although, much has been published about the effect of Hurricane Katrina, little has been written about its impact on avian migrants. One study, conducted by U.S. Geological Survey scientists, used radar ornithology to measure the effect of the storm on migrant use of lowland forested wetlands of the Pearl River and upland pine-dominated woodlands near Slidell, LA. Their study used archived Doppler weather radar data to measure bird use of this particularly important stopover location. What they found was that for the 3 weeks following Katrina, when virtually all vegetation had been stripped in the wetlands, the birds foraged in the less heavily damaged pine uplands. However, about five weeks after the storm, when much of the surviving forest in the Pearl River bottoms began to sprout new foliage, a corresponding increase of migrant use of the lowlands was documented.

Ultimately, the real threat to birds may not be from Earth’s natural forces, but rather human alterations to the planet that reduce or eliminate surge-protecting coastal wetlands and warming ocean waters that generate increasingly more powerful storms.