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Andy Von Duyke

Andy Von Duyke, using his mechanical engineering background, devised a videocamera surveillance system to help decipher the Lake Peltier heron mystery.

Evidence at last for the mysterious disappearance of the herons

By Jennifer Amie

From eNews, December 1, 2005

Since 1945, great blue herons have nested on the Rice Creek chain of lakes in Anoka County. Their colony at Lake Peltier in the Twin Cities suburb of Lino Lakes numbered 700 nests at its peak and has become a neighborhood icon. The local elementary school is named for the birds, and the town logo bears their likeness. But beginning in 2000, the herons began to abandon the colony in the midst of nesting season, producing no surviving young.

For years, the cause of the herons' disappearance remained a mystery. As the colony's population shrunk to 200 nests, biologists and local residents scrambled to figure out how to save the birds. In response to scientific and public concern, the Minnesota Department of Natural Resources funded a study by University of Minnesota graduate student Andy Von Duyke and professor of fisheries and wildlife Francesca Cuthbert. Their investigation, conducted over the past two nesting seasons, has revealed a shocking clue that may help explain why the beleaguered birds have been forced to flee.

Von Duyke and Cuthbert approached the heron puzzle the way any detective might: they set up a system of surveillance. Von Duyke, who has a background in mechanical engineering, devised a way to install and run eight sensitive, time-lapse videocameras on Lake Peltier island beginning in the spring of 2004. The cameras were lashed to nest trees as high as 80 feet off the ground. Von Duyke canoed to the island every two days lugging two fresh 65-pound marine batteries to power them.

That spring, the nesting season was off to an uncertain start. When Von Duyke arrived in early May to install cameras, a nest tree he had intended to videotape had already been abandoned. Once they were installed in an alternate location, however, cameras documented typical nesting behavior: herons brooded their eggs and fed their newly hatched chicks. Then, in late May, scientists observing the colony from an airplane noticed troubling signs. Along the perimeter of the colony, eggs were missing from nests and some birds had fled. The heronry, it seemed, was unraveling along its inland edge, and the abandonment appeared to be advancing toward the center of the colony.

Soon afterward, Von Duyke found the severed head of a chick at the base of a nest tree, suggesting that a great horned owl might be preying on the birds. He played the surveillance tapes from that tree, expecting to find an owl. What he saw instead came as a complete surprise.

The videocamera was focused on a nest that housed two chicks. The bird that had hatched earliest was already the size of a large chicken. Its sibling, more recently hatched, was still a fuzzy little puffball. Watching over them was an adult heron, which suddenly became agitated, rising up and spreading its wings. "Then," says Von Duyke, "a raccoon came sailing through the frame, tackled the larger chick, and sailed out of the frame." The next morning, says Von Duyke, "The puffball was also gone."

Raccoons are major predators of small birds, but the scientists were shocked to see them (literally) tackle such large prey. The older chicks appeared to be nearly the size of the raccoons themselves. "I was most surprised at the intensity of their predation and their mobility in the trees," says Cuthbert.

With hundreds of hours of video still to review, Von Duyke and Cuthbert have not concluded their research and they stress that other predators--including owls and crows--are also a factor.

Ground surveys indicated that the raccoons might be methodically working their way inward from the outer edges of the colony, easily navigating the spindly branches of the canopy to wipe out all the chicks in one nest over several days. "They seem to work systematically," says Von Duyke. "There would be three chicks, then two, then one."

Discovering the role of raccoons at Lake Peltier in many ways raised as many questions as it answered. Could raccoon predation, at least on this scale, be unique to Lake Peltier? Raccoons have not been observed hunting herons at the nearby Pig's Eye colony, but instances of raccoon predation have been recorded in scientific literature. "It could be that this behavior was learned by a couple of individual raccoons and then passed on to their young," says Cuthbert. "It may also be that this type of predation is more common than we know, but just hasn't been recorded. There are many accounts of heron colonies just disappearing, but no one knows why."

With hundreds of hours of video still to review, Von Duyke and Cuthbert have not concluded their research and they stress that other predators--including owls and crows--are also a factor.

By June 15, 2004, the 200 heron nests at Lake Peltier had been abandoned. None of their young survived. That winter, Von Duyke and his team installed metal flashing on the trees at Lake Peltier Island, hoping to prevent the raccoons from scaling the trunks. "Unfortunately," says Von Duyke, "we underestimated the climbing ability and motivation of the raccoons." The nimble animals were able to skirt flashing on narrow shoots and branches or wedge themselves in the spaces between two trunks and clamber up. Only 25 nests on Lake Peltier were occupied in the spring of 2005 and, despite the efforts of the biologists, these nests were attacked by raccoons. "The chicks were healthy one day, then missing the next," says Von Duyke. Only four chicks survived.

No one knows whether the herons will return next spring to Lake Peltier. "Perhaps the erosion of the Lake Peltier colony is simply the result of a natural process," suggests Von Duyke.

Despite the dramatic video, Von Duyke does not believe that raccoons are the "smoking gun" in this mystery. "The herons are hit from all sides," he says. "Thunderstorms, owls, crows, gulls, and starvation can kill them, too. In general, they can survive despite these threats. But if one of them gains a bit of an edge, the whole thing implodes. The raccoons may be a kind of tipping point--the straw that broke the camel's back."

From Imprint, fall 2005, a publication of the Bell Museum of Natural History.