January 2016 Issue of Wines & Vines

Attracting Birds of Prey

Nest boxes make it relatively easy to attract predators that help control pests

by Nathan Gogoll
Alternative text
Kestrels hunt large insects as well as small mammals and other birds.

Grapegrowers can attract certain types of birds to their properties to help deter other pest species. While native birds of prey won’t necessarily be the free and quick solution to prevent bird damage in vineyards, attracting the predators could be an interesting addition to the strategy. Plus, referring to your local family of barn owls as “vineyard beneficials” seems kind of cool.

Advice from the U.S. Natural Resources Conservation Service (NRCS) suggests the introduction of barn owl boxes, kestrel boxes and raptor perches could be part of a “multifaceted solution to pest control” that could include other conventional methods such as netting and audible deterrents.

The NRCS said barn owls and kestrels are relatively easy to attract to farmland by installing nest boxes. Nesting pairs of predatory birds focus their hunting near the nests and will capture increased amounts of prey for their growing chicks. These species are easy to attract with nest boxes because natural nesting cavities can be difficult to find.

While owls primarily prey on nocturnal rodents, they are known to kill and stockpile more prey than needed. Kestrels, formerly known as sparrow hawks, will hunt large insects such as grasshoppers, crickets, beetles and moths, as well as small mammals and birds.

Attracting birds of prey may also help with avian pests, such as starlings, by changing their behavior. Additionally, the presence of predators may make pests more cautious and less likely to enter the area to feed.

Attending to small details will make a nest box more suitable to attract and fledge birds. Important factors include the number and location of boxes, timing of box setup, predation and competition, management of the area around the box, and box design.

Number of boxes and location
Rarely will all the installed nest boxes be occupied in a given year. It is recommended that you put up at least twice as many boxes as the number of nests desired. Territoriality, availability of resources and numerous other factors determine which and how many boxes will be occupied. The idea of putting up more boxes is based on giving the birds options to determine which sites best suit their needs.

If a box is not used the first year, wait a few more years to see if it becomes occupied. If a box is consistently not used for a number of years, the location should be changed. For barn owls, the recommended density is up to one box per 5-10 acres; for kestrels, the recommended density is up to one box per 10 acres. This density is recommended for areas with ideal habitat and a serious pest problem.

Boxes should be put up in areas with clear flight access, preferably away from stands of large trees. Barn owl boxes should face away from the heat of the afternoon sun and be 15 to 30 feet high, and kestrel boxes should face east or south and be 10 to 20 feet high. Boxes should be set up as far away from busy roads as possible, since raptors may get hit by cars while hunting.

Timing and competition
Barn owls begin selecting nesting territories at the end of winter, but they may take until early autumn to fledge their chicks. Kestrels begin selecting nesting territories later in spring and may not fledge until the end of summer. Set nest boxes up in the selected areas by the beginning of the nesting season.

Owls and kestrels are predators, but they can also be preyed upon by other animals. Their eggs and chicks may also be eaten. It is important to minimize predation to the extent possible. Other predators can also have beneficial roles in pest control, so it is important not to eliminate them, only to minimize their effect on the nesting barn owls and kestrels.

Effects from terrestrial predators (including snakes and cats) can be significantly reduced by putting conical predator guards on the wooden post below the nest box, using metal poles and placing the box away from branches where predators can enter the boxes. Competition for nest boxes from other species can also reduce the value of the boxes. Nest boxes mimic natural cavities, which are valuable nesting and cover sites for many species, so they may attract non-target wildlife.

Box design, installation and management
Be aware that the Internet shows many inappropriate designs for nest boxes. Boxes can be purchased online and at some home stores, or you can build your own. Ideally boxes are located on wooden or metal poles, but they can be installed on existing structures if they are safe from predation (see above). Wood shavings are often added to the boxes to increase their attractiveness to potential nesting birds.

Boxes should be cleaned and repaired annually to maintain the attractiveness to birds. If occupied, the boxes should not be disturbed during nesting season, since disturbance could result in nest abandonment. Normal farming operations are usually compatible with nesting barn owls and kestrels.

Snags, which are standing dead trees, are important for many types of wildlife. Snags provide cavities for nesting birds and other wildlife, and perching sites for many species of birds. It is important to maintain natural snags whenever possible.

Plentiful perching sites are important for attracting birds of prey. Artificial snags and perches also can be installed to benefit wildlife. Materials can be dead trees or branches, or wooden or metal posts. Artificial perches should be 10 to 30 feet high, and they benefit from a small crossbar of 1 to 3 feet. Different heights and structures will attract different species, so a variety is ideal. Kestrels show a preference for perching on fence lines and wires. Installing wire perches, where perching wires are not already present, may attract hunting kestrels.

Australia-based Bayer’s Farm Advisor has explored this topic and offered some extra areas to consider in Q&A format:

Q: Where do I put the nest boxes?
Put the boxes up wherever it is convenient for you. They can go in trees, on posts out in the field, on the wall of a building. Each site has positive and negative points. In trees, the birds of prey will receive some protection from the elements, but the young will be exposed to predators. On a post, the young will be protected from most predators, but the box may get hot during a heat wave. On a building, whatever is below the box will probably get splattered with fecal matter.

Q: Where should I not put the nest boxes?
Don’t put nest boxes above locations where vehicles or equipment are parked.

Outside your bedroom window is not a good idea either, because the young birds of prey can compete very noisily each time the adults return to the nest with food. Owls prefer not to hunt in the area of their nest box so as not to attract the attention of potential predators. Multiple nest boxes in this vicinity will solve this problem because the hunting areas of the different nesting pairs will overlap.

Q: How many nest boxes do I need?
Six boxes across 50 acres is a good start. Figure you have enough nest boxes when 20% to 30% are not being used at any time during the year. If you put up several nest boxes in the same field, the owls’ territorial instincts will not necessarily cause conflicts. Barn owls are not considered to be very territorial. The number of owls you attract is a product of the number of rodents in your field and the number of nest sites they can find in your area. This means that you can erect as many nest boxes as you have space for. And the more nesting pairs of owls you attract, the more rodents you will get rid of.

Q: Will routine farm operations bother birds of prey?
No, regular farm activity will not bother them as long as the box or the post is not bumped. In most cases they will remain quietly inside the nest box.

Q: Can I peek in my boxes to check on the owls?
It is always best to leave the owls in your nest boxes alone. This is especially true while the hen is sitting on the eggs during mid- to late summer. Scaring her away at this time may result in her refusal to return. When the chicks have hatched, you can safely peek in to see how many you have without worrying whether the parents will return.

Q: What do I do if I find a young owl on the ground?
Chances are good that the youngster’s parents know where it is, and they are probably taking care of it. So unless the owl is injured or in danger, just leave it alone. If the owl appears to be hurt or is potentially threatened, take it to your nearest raptor rehabilitation center.

Q: How do I capture a grounded barn owl?
To capture the barn owl, you will need a sheet, a pair of heavy gloves and a paper grocery bag. Start by throwing the sheet over the owl. Wearing the gloves, grab the owl by both legs just above the feet while the bird is still covered with the sheet. Remove the sheet and turn the owl upside down. Put it into a paper grocery bag and fold the top of the bag over to keep the bird inside.

This article first appeared in the November 2015 issue of Australian & New Zealand Grapegrower & Winemaker magazine and is republished here with permission.

Print this page   PRINTER-FRIENDLY VERSION   »
E-mail this article   E-MAIL THIS ARTICLE   »
Currently no comments posted for this article.