ATHENS, Ga. -- There's a new predator in the forest, and it has neither feet nor claws. The new beast is the chain saw, which is ripping down tree stands so shopping centers and subdivisions can go up. Now there is evidence that forest fragmentation is doing something else: causing a decline in migratory songbirds in the American South.
In a new study at the Ft. McClellan Army post in northeastern Alabama, researchers have discovered that cutting forests can allow large predators like raccoons and opossums access to ground-nesting songbirds. The loss of eggs and nesting areas appear to result in a decline in some species.
"Some 20 percent of songbirds species are ground-nesters, from ovenbirds to Black-and-White Warblers, and many are in decline," said Amber Keyser, a doctoral student in genetics at the University of Georgia. "We wanted to know if the threat was coming from large or small predators."
Keyser's research on songbirds, just published in the journal Conservation Biology, was done while she was a master's student at Auburn University, in conjunction with Dr. Geoffrey Hill, an avian consultant and a faculty member at Auburn, and Eric Soehren of the Natural Heritage Section of the State of Alabama's Natural Lands Division. The work was funded as part of the Legacy Resource Management Program of the U.S. Department of Defense.
There has been both scientific and anecdotal evidence that some songbird species in the United States are in decline. As large sections of contiguous forest are broken into tracts, birds experience a loss of habitat for nesting and foraging, but the problem is even more insidious. Fragmentation can also create avenues in which predators are virtually funneled to nesting sites. While there have been studies in the Midwest and Northeast to discover the kinds of predators that are wiping out nests, little work has been done in the Southeast.
The current study was designed to discover which predators are disturbing nesting sites of neotropical songbirds in the South. (A neotropical bird is one that winters in Latin America but breeds and spends the warm months in the U.S.) Some studies using artificial nests have focused on predation risk, but they have largely used quail eggs to mimic songbird eggs, and this may not give a complete picture of what is happening to diminish reproductive success.
"As an alternative to nest experiments that use quail eggs, some researchers have used artificial nests with eggs constructed from clay," said Keyser. "Clay eggs have advantages because they closely match the size of passerine eggs and retain tooth, bill and claw marks from which predators can be identified." By using both clay and quail eggs, Keyser and her colleagues were able to get a much better idea of which predators were at work.
The researchers studied 12 forest fragments of mixed pines and hardwoods at Ft. McClellan that had been the subject of an earlier point-count census of neotropical migrant songbirds during the breeding season.
In a small area cleaned of leaf litter, the scientists constructed artificial nests that contained two fresh Northern Bobwhite eggs and two clay eggs. Nests were constructed and all eggs handled with gloves to minimize human scent. Most nests were checked seven days after egg placement.
"We were concerned only with categorizing predators as small-mouthed like chipmunks or mice or larger, and we used clues in the depredated nests to implicate small-mouthed predators, large-mouthed or both," said Keyser. In order to recognize the marks of small predators, the researchers gave clay eggs to captive mice to study the tooth and claw marks they made on them. Bill marks were compared to the bill dimensions of Blue Jay and American Crow specimens. The team compared larger canine and incisor marks to skulls of gray fox, raccoon and Virginia opossum. White-tailed deer left extremely large molar impressions on the clay eggs.
While the scientists did not attempt to identify predator species, they did examine the clay eggs for large or small marks. What they discovered was a significant increase in predation as forest fragment size decreased. They also found that small fragments had a much higher incidence of predation from large species than small species. Predation from small species remained constant no matter what the fragment size.
Thus it appears that songbirds nesting in small forest fragments bear an increased burden of predation, and reproductive success is thus affected, sometimes dramatically. Worse, the clustering of nest sites in small fragments increases the chance that large predators will destroy eggs. These patterns of predation could not have been revealed in a more traditional experimental design because the use of only quail eggs samples larger predator activity.
Problems using the method of Keyser and her colleagues remain. Clay eggs often showed marks of more than one species of predator, and tooth impressions left by large predators can mask marks left in the clay by small predators. Also, large predators sometimes completely removed clay eggs from the nesting sites. Nevertheless, the study is a step toward understanding with greater clarity why ground-nesting songbirds are declining in the South.
"Obviously, this has important conservation implications," said Keyser. "The problem is that there is habitat loss on the wintering grounds and on the breeding grounds. It's an issue that needs more study. We also need good land management if we want to maintain sustainable populations of songbirds."
Editors/Writers: The study was published in Volume 12, No. 5 of Conservation Biology, October 1998.