Bio-Diversity News

News on using wildlife to control pests from Bio-Diversity Products

Fall 1999

Health issues regarding nest box cleaning: Potential threat from hantavirus

For years, Bio-Diversity Products has recommended the practice of cleaning nest boxes annually, usually in the late fall, as preparation for the coming nesting season. We have felt that this practice maintains the integrity of the box. At the same time, inspections may uncover situations that may prevent repeated inhabitancy, such as colonies of wasps, or conditions that might affect the chicks, such as lice infested debris. However, with the risks related to hantavirus coming into view, we feel that our customers might want to re-evaluate this practice, or at least become aware of the problem.

Hantavirus is a danger whenever a person comes into contact with wild rodents. This is most particularly true about deer mice, a species known to be the hantavirus's main vector. The deer mouse is common across much of the United States and is easily identified by their white undersides. While the deer mouse carries the virus without showing signs of being infected, it can contaminate almost everything it comes into contact with. Exposure to this deadly disease can come from the rodent's hair, their fecal matter or their nesting material.

Since 1993, 138 case of hantavirus have been diagnose in Arizona, Colorado, New Mexico, Idaho and Oregon. 23 people have died as a result of contact with the virus. No cases to date have been recorded in California.

Hantavirus is a concern for growers who use nest boxes to attract barn owls. Barn owls cough up castings, or owl pellets, that contain the undigested hair and bones of their prey. Many casting can be found in an occupied nest box. As well, uneaten rodents may also be found in nest boxes. Debris in an owl nest box can potentially be infected.

While cleaning out nest boxes at least once a year may still be beneficial as far as the owls are concerned, this act can expose a person to hantavirus. Certainly, the benefits of having a clean nest box are hardly worth the risks of exposure to this grave illness. However, if a nest box is to be cleaned, the person must use rubber gloves and a dust mask. The debris from the nest box should be contained in a sturdy plastic bag and disposed of. Every effort should be made to stay out of the dust.

for more information about the hantavirus and the disease it causes, the Hantavirus Pulmonary Syndrome, please consult the following webistes.

New World hantavirus and Hantavirus Pulmonary Syndrome

Outbreak - Hantavirus Pulmonary Syndrome and Hemorrhagic Fever with Renal Syndrome

All About Hantavirus

W W W . H A N T A V I R U S . N E T

Our customers comment...

Coping with the GHO

Joe Sebastiani, property manager of of Viansa Winery, reports that great horned owls, GHO's, have been seen perching on either the roofs or landing perches of the 18 barn owl nest boxes his operation has purchased from Bio-Diversity Products. To reduce the threat of GHO's attacking young inside or outside the nest boxes, Sebastiani has removed the exterior portion of the dowel from the box.

While the farmer admits that no one has actually seen these predators invading the interior of the nest boxes, he hopes these measures will reduce the slaughter of his gopher eating allies.

It is estimated that as much as 50% of all barn owl deaths may be attributed to predation by the great horned owl. One way to avoid exposing the barn owl to the wrath of the GHO is to place nest boxes at least a half mile away from areas recognized as great horn habitat, such as riparian corridors or densely wooded areas.

Sebastiani states that this would be very difficult in his situation. Located at the mouth of the Carneros Valley near Suisun Bay, the heavily wooded hillsides are perfect for the great horn.

"Great horns are everywhere," he says. "There is no way we could avoid over lapping the hunting grounds of the two species."

For now, he notes, it doesn't seem to be stopping the barn owls from doing their job. All of Viansa's nest boxes show signs of occupancy. Ironically, the two enemies even seem to get along at times. "We have observed great horns and barn owls roosting in the same tree," adds Joe.

Bio-Diversity's design, with an entrance of less than 6 inches in diameter and an interior partition, is intended to keep predators, like the GHO, out of the interior of the nest boxes. However effective as these components are, they will not help an unwary chick, perched on the dowel outside of the box, from a swift, stealthy attack.

For now, Sebastiani is playing it safe. Removal of the landing perch has not reduced the occupancy of the nest boxes since the adult barn owl does not need the dowel to enter and exit. However, Viansa's action may help reduce predation of the young, who use the exterior portion of the perch to exercise their wings before attempting their first flight.

Bio-Diversity Products commends Joe and the others at Viansa for taking this action and recommends that others do the same where pressure from the GHO is a concern. We also recommend shortening the fledgling flight by locating nest boxes within 100 yards or closer to trees. This will reduce the chicks' need for preparation.

Once in the relative safety of the trees, the chicks continue to be nurtured by their parents for several weeks more, while gaining strength and survival skills.

Finding the Balance................ by Tom Hoffman

Since I began promoting the use of barn owls for rodent control, I've spoken with many growers about their successes and failures with this program. I've noticed that those who have a positive story to tell, have one thing in common. They have achieved a natural balance between predators and prey. So what is it that these farmers are doing that others aren't?

The answer lies in the numbers. When attempting to control rodents with barn owls, the biggest mistake you can make is to not provide enough nesting sites. You have to take advantage of one of the unique characteristics of the barn owl: its lack of a sense of territoriality. Where the food source will permit, barn owls will live in what appear to be colonies. That is what makes the barn owl an effective ally. And the right approach is not to attract one or two pairs, but a whole colony of them!

So at this point, the obvious question is: how many nest boxes is enough? And the answer is: that depends. For example, in placing my boxes in my own vineyard, I tend to situate the boxes around the rodent infested areas, figuring about four to six will handle 50 acres, and in most years, most of them are used. In someone else's vineyard, where rodents are not such a serious problem, the same number of boxes might work for 100 acres. What ever the number is in your field, your goal should be to allow as many nesting pairs of owls to move in as your rodent supply will sustain, then permit them to find a balance with the food supply.

To determine when that balance has been reached, look at your occupied nesting sites, counting only the boxes with nests in them. (Solitary roosting adults don't count.) If the occupancy rate is 50 to 80%, then figure the balance has been reached and you probably don't have food for too many more nesting pairs. And when the gopher population increases, you'll still have room to accommodate more owls.

On the other hand, if you continually have close to 90 or 100% occupancy, you could probably use more nest sites. I say probably, because you will never know unless you actually put the boxes up and see if the owls move in or not.

Remember, year by year, the owl population will reflect the rodent population, each fluctuating in response to the density of the other. With a high rodent population, you will be able to sustain more owls and they will remain in until the food supply begins to dwindle. Then the owls will disperse to greener pastures. With fewer owls on duty, the gophers are once again on the rise.

This newsletter was published for the customers of Bio-Diversity Products. For more information about our products or services, contact us at (209)369-8578, or email us at:

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