Getting hit high and low: Options for managing bird and vole damage

Blueberry growers must manage several vertebrate pests that pose risks for fruit yield, fruit quality, and plant survival. Many concerns derive from damage by deer, birds, and voles. Basic tactics to manage wildlife conflicts fall into three main categories: block access, deter feeding, or remove animals. Some conflict situations require management via combining tactics or varying approaches over time to yield the best results.

Deer

Adequate fencing is the only way to achieve high rates of protection from deer damage. Although implementation of that option can require significant initial investment, depending on the size of the area to be protected and the value of the potential crop losses, the long-lasting control provided by fencing could be the best long-term solution. Each grower will need to consider the cost-benefit ratio and feasibility of such an investment.

To achieve long-term effectiveness, a fence must be regularly patrolled and checked for damage or compromise. Despite their large body size, determined deer can take advantage of relatively small openings created by other animals’ under digging at the base of fences or nimbly jump through gaps between panels of welded wire. Fences to prevent deer entry should be built of strong and long- lasting materials.

The absolute minimum height recommended for the Willamette Valley is 6 1⁄2 feet tall, but 8-foot fences are preferred. When building the fence, one must keep an eye on topography. A fence sited adjacent to a significant rise outside of the fence can be compromised by deer using topography as a ramp to reduce the necessary in-bound jump height.

Where habitual travel corridors or areas of frequent breach attempts are identified, addition of an electric top strand and/or outrigger can be helpful. Regular upkeep and powering the fence at night will be necessary to maintain the effectiveness of those assets as well, because deer are notorious for “testing” fences of all types for weaknesses. Battery-powered, AC, and solar- powered options are available and offer some flexibility to suit different situations, but growers should first assess the need to augment their fence with these tools before investing in these additions.

Deterrence of deer via area deterrents (e.g., “predator scents”) or taste deterrents (several commercially-available options) can be used to achieve short- to medium-term reductions in damage. However, any surface-applied deterrent will need to be renewed frequently, and deer tend to habituate to smells and flavors after a period of time, making it necessary to use multiple products in rotation. Hazing, such as startling deer with noise-making devices is a temporary means of scaring deer, at best. Because deer habituate to stimuli that do not actually result in harm, using hazing as a damage prevention tool would require constant vigilance and varied application to achieve results. Use of incendiary hazing equipment requires local and state permitting and is only viable in rural areas.

In some cases, reduction of local population might be a management option if a landowner or group of landowners can work with the Oregon Department of Fish and Wildlife (ODFW) to secure depredation hunt permits or to provide hunter access to private property during regular hunting seasons. At the very least, landowners should contact their ODFW District Biologist to gain technical assistance and to explore options for potential cost-share financing of fencing or other options. Call the ODFW headquarters (800-720-6339) with your property’s address or specific location information to identify your District Biologist and their contact information.

Birds

Management of bird damage in fruit crops is a common concern. Scouting in the early morning and early evening can help determine which species of birds are affecting the crop across the growing season. Monitoring throughout the season(s) of damage is also important in determining the effectiveness of management approaches (See tips in Burrows et. al. below).

Although highly effective, netting and frames or guide wire systems for anything but the smallest, most compact plantings will initially be expensive. Deployment and subsequent storage of the nets each season also require a seasonal investment of labor, and bird damage can vary from year to year dependent on many external factors, leading some growers to question the long-term value. Ground-feeding, seed-eating birds, such as house finches, may forage between netted rows. Habitat management via tillage and vegetation management can be an additional tool to use in combination with the netted rows.

Beyond netting to exclude birds, growers can use one or more alternative tools to manage bird damage. Trapping and lethal removal of birds is feasible for only a few non-native, invasive species: pigeons, house sparrows, and starlings. In these cases, contacting the regional U.S.D.A. Animal Plant Health Inspection Service (APHIS) representative will be an important first step to gaining technical assistance and accessing registered avicide product. Depredation permits for some native bird species can sometimes be obtained, but will generally require working in concert with both the U.S. Fish and Wildlife Service and APHIS to obtain necessary permits. Permitting for removals of native bird species is complex (example), so consultation with agency staff is vital.

Similar to deer, hazing birds with propane cannons or incendiary tools is only feasible in rural areas and is subject to the same permitting rules. Better approaches to hazing yield the advantages of predator-avoidance behaviors while sidestepping the always-present tendency of animals to habituate to disturbances that are predictable, yet non-lethal. Especially on larger plantings or across several adjoining plantings, seasonal employment of falconers is being used in multiple states for protection of multiple bird-vulnerable, high-value crops. This tool is one for which cost-benefit ratios need to be calculated early, as falconry companies are still relatively rare, so seasonal contracts should be secured as early as possible.

Others have developed predator-prey audio systems that can be easily moved and which have good flexibility in the calls they transmit. These systems can be programmed to deliver the calls of multiple predator (avian) species, multiple distressed prey species, can add variable pitch to each of those calls (i.e., sounds like multiple different individual “voices”) and can be programmed to deliver sounds at random or highly mixed (i.e., unpredictable to the birds) frequencies.

Another advantage of these tools is that they tend to attract little if any attention from human neighbors, allowing their use in a broader range of sites. Still under development in the U.S. and abroad are drones, robo-falcons, and other small, maneuverable devices that could be preprogrammed to deliver audio, visual, and “behavioral” threats to feeding or hiding birds.

Voles

Voles can damage blueberry plantings through creation of their extensive aboveground runs, or paths, through creation of their extensive underground tunnel systems, and through gnawing and girdling plants. At least 9 species of vole (Microtus species) inhabit western Oregon. Because of species-specific habitat requirements however, only two (gray-tailed and Townsend’s) provide the majority of conflicts with production agriculture in the Willamette Valley of Oregon.

One of those species, the gray-tailed vole, is endemic to the valley, meaning that it evolved here. This is significant because across the country, many vole species can be vulnerable to tunnel flooding as a means of population control. Our endemic gray-tailed vole however, evolved in an ecosystem that flooded very regularly, thus it evolved behaviors and tolerances that make it invulnerable to that approach.

The life history and ecology of the majority of vole species makes them a textbook example of species capable of nearly achieving exponential growth, at least over short “boom” seasons. In fact, Townsend’s voles hold some of the North American records for high population densities during those population highs.

Individual voles have short life spans ranging from 2-16 months, but females have very high reproductive potential. Females are capable of breeding at 3-4 weeks of age as they are still attaining their adult size, but some females delay for several months. Individual females are capable of producing multiple litters between March and into November. Gestation for litters of 4-8 pups lasts approximately 23 days. In part because of their “fast” life cycle, voles are especially well suited to respond quickly to good environmental conditions. As-yet undiscovered combinations of priming factors allow vole populations to skyrocket in some years, while in other years the population density will be so low as to escape significant notice or need for investment of management effort. These characteristics make establishment of regular monitoring plans (see Burrows et. al. for suggestions) vital, along with a stepped plan for implementing varying levels of intervention.

Because at all population densities voles use their above- and below-ground pathways to provide access to food and safety from predators, light tillage between rows to interrupt burrows and to manage between-row food subsidies (i.e., grass and forbs) is a good baseline management strategy.

Encouragement or non-harassment of natural predators (e.g., foxes, weasels, hawks, owls) is also a good and low-input strategy. However when vole populations are in a “boom” phase, they overwhelm natural predators’ ability to eat the excess. Again, monitoring early in the season to gauge how much intervention is needed will be greatly helpful.

There is currently an in-burrow Oregon SLN for registered zinc phosphide products, but this is not an option for organic growers. A current cultural practice, laying fabric weed barriers beneath plants or high-mounding mulch, actually creates safe vole refuges from predators, so both traditional and organic producers should consider pulling the matting back during late winter/early breeding season to allow those voles to be vulnerable to both the weather elements and predators.

Finally, some producers use taste-repellant products developed to reduce browsing by deer and rabbits. As with any repellant, these would require frequent reapplication and rotation of different products (flavors). Frequent maintenance of the repellency is especially important given the constant entry of “new animals” into the vole population via reproduction.

See also:

  • Craven, S.R. & Hygnstrom, S.E. Deer. 1994. Prevention and Control of Wildlife Damage. Extension, University of Nebraska.
  • Burrows, C.L., C.B. MacConnell, T.A. Murray, and K.K. Schlamp. 2015. Integrated pest management for blueberries: A guide to sampling and decision making for key blueberry pests in northwest Washington. Washington State University, Whatcom County Extension

Use pesticides safely!

  • Wear protective clothing and safety devices as recommended on the label. Bathe or shower after each use.
  • Read the pesticide label—even if you’ve used the pesticide before. Follow closely the instructions on the label (and any other directions you have).
  • Be cautious when you apply pesticides. Know your legal responsibility as a pesticide applicator. You may be liable for injury or damage resulting from pesticide use.

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