Zoo anxious for its birds tries to sterilize foxes of Druid Hill


September 29, 1992|By Douglas Birch | Douglas Birch,Staff Writer

For the past 3 1/2 years, Baltimore Zoo staffers have been matching wits with a wild, and wily, group of animals living in the heart of the urban wilderness. And to some extent, zoo staffers say, they've been outfoxed.

Zoo veterinary workers have been trying to trap and surgically sterilize a thriving population of wild foxes that live outside the fences of the 158-acre zoo, in dens scattered through the sprawling Druid Hill Park. It may be the only birth control program for wild foxes run by any zoo in the country.

These cunning animals sometimes slip into the zoo and attack its exotic birds -- including flamingoes, peacocks, cranes and ducks. And the zoo's veterinarian worries that one day an infected fox may spread disease.

But only one gray fox and about 14 red foxes have been caught, tagged and neutered, out of a total of perhaps 40 animals, zoo officials say. While the effort so far has reduced the zoo's losses, staffers say they are catching fewer and fewer animals that have not already been sterilized.

Neutered animals that blunder into the traps seem to realize it represents a risk-free lunch. Others seem to have learned to steer clear.

"It's the feeling of the staff that it's reduced the number of animals that feed upon the zoo's collection," said Dr. Michael Cranfield, the zoo's veterinarian. But he added, sounding mildly exasperated: "If they were a little bit easier to catch, it would go faster."

"We're trying to establish a peaceful coexistence" with the wild foxes, said Sandra Kempske, the zoo's curator of mammals. "What we would like to do is reach an accord with them."

Normally, wild foxes -- which can grow to between 3 feet and 3 1/2 feet in length -- dine on the urban park's free-range rats, mice, birds, bird eggs, rabbits and squirrels. But in the spring, the females -- called vixens -- give birth to from one to 10 cubs or "kits," and then need more nourishment to produce milk. (The cubs remain in the den for about five weeks and then are cared for by both parents throughout the summer.)

So they venture into the zoo seeking captive-bred meals.

A bigger worry is disease. While none of the 15 foxes caught so far was ill, zoo staffers say an infected fox might someday spread distemper or parvovirus among the captive animals. There is also a concern that a rabid fox might turn up, Dr. Cranfield said. None has so far. And the zoo's mammals are all inoculated against rabies.

About four years ago, the zoo gave up efforts to trap, relocate or destroy the foxes, because every time an animal was removed from the park, a new one would show up to claim the old one's territory. (The foxes are thought to migrate south from suburban and rural areas of Baltimore County along the Jones Falls corridor, which includes long, wooded stretches inside the city limits.)

"We weren't making any headway," said Dr. Cranfield. And zoo officials disliked killing the foxes, he added, since a major part of its mission is preserving wildlife from destruction.

For about six months, zoo scientists put out food for the foxes laced with birth control drugs. Ms. Kempske said that didn't work very well.

The drugs were expensive. The foxes were probably wary of the human scent on the bait. And scavenging crows boldly snatched most of the drugged meat before the foxes had a chance to eat it, even when the food was thrown down into their dens.

"The only thing I think we did was sterilize the crows in the park," Ms. Kempske said.

So Dr. Cranfield began taking the trapped animals, tagging them, vaccinating them and worming them. He performs vasectomies on the males and ties the fallopian tubes of the females.

"So they show all the natural behavior of territory and breeding, but they can't have babies," he said.

The sterilized animals defend their territories against fertile foxes. Because the females do not have litters in the spring, they do not need to increase their hunting in order to produce milk. And the annual crop of kits is smaller, so there is less competition among foxes for prey outside the zoo's cages.

"Our way of approaching it is probably the most practical way," Ms. Kempske said. "Foxes are so urbanized and so adaptable, we decided to try to keep the animals we have rather than remove them."

But in recent years zoo staffers have begun trapping the same animals over and over again, while untagged animals are rarely caught.

Dr. Cranfield thinks the repeaters may have decided there is no danger in snatching bait from the zoo's traps. Most other foxes, he fears, have learned to avoid the traps.

Dr. Cranfield still regards the program as a success, judging from the reduced number of zoo birds killed during recent springs.

Beth Zebrowski, a public affairs assistant with the American Association of Zoological Parks and Aquariums, said a number of zoos were trying to control the population of wild pests using birth control methods.

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