How to attract birds to prey upon rodents
by Jessica Bern
The initial question addressed was “How do I attract raptors as well as any other birds and wildlife to my farms?” The answer was presented by Matt Johnson from Cal Poly-Humboldt and Breanna Martinico from UC-Davis during a part of “The Role of Birds on the Farm” series.
Johnson began by discussing studies which point to the idea that “attracting owls and raptors to your farm with nest boxes can help contribute to pest management … and it can also be much more cost-effective than using rodenticides.”
Regarding barn owls, the most widely distributed species of owl in the world, he said data show a diet consisting of about 99% rodent pests. “In many cases,” he added, “their eating seems to be affected by the surrounding landscape and crop types.”
According to Johnson, the hunting range of a barn owl is up to about a mile and a half from its nest and in some cases, even farther. However, around 50% of the hunting still occurs within 500 meters. It was also estimated that a barn owl family, both adults and three to four young, will kill somewhere between 3,000 and 4,000 rodents every year. To bring these helpful hunters to you, you can build nest boxes for them to live in.
“The occupancy is highest for nest boxes that are at least three meters or about 10 feet off the ground,” said Johnson, adding that the box itself should not be too short (about 24 inches tall or more) and that the birds seem to prefer wooden boxes over plastic. In addition, he noted, “Nest box occupancy is higher when there are some uncultivated habitats nearby, especially more open habitats.”
Other suggestions were that the boxes should be installed on a pole, ideally one made of metal. Adding grooves on the box would help the owl get a better grip and the size of the opening should be small enough to prevent predators, such as ravens or hawks, from entering. “For added safety,” he said, “putting in a partition wall … will keep a hawk’s talon or a raccoon’s arm from being able to reach around and grab the chicks.”
According to Johnson, larger boxes are also preferred by owls. “It allows them to have more work room, mitigates stress and enables them to exercise their wings,” he said, “and, like other nest boxes, adding in heat deflecting panels in areas that are prone to high temperatures are also helpful.”
To know if your nest boxes are working, he said, “Since colonization can take around one to three years, after that period, you should look for bones or portions of pellets that birds regurgitate underneath the box as evidence that it’s being, or has been, occupied.” If you don’t find anything, he suggested placing the boxes elsewhere.
Inspection is also important during non-breeding season. “If the boxes are falling apart, pull out old pellets and debris and put in wood chips or [any type of] straw.” The latter advice also goes for when you first put up the box.
Johnson also spoke about the American kestrel. Kestrels are far more territorial and secretive than barn owls and therefore a little harder to attract into nest boxes. Also known as sparrow hawks, they are the smallest and most commonly found falcon in North America. In addition, he said “the frequency of nest box use declined with shrub and vegetative density.” As a result, he suggested the boxes be installed at least 300 to 500 meters apart and away from areas with human disturbance.
Notably, “kestrel boxes can occasionally be used by other species including some beneficial species like Western screech owls, Northern flickers and other native songbirds. On the downside, they could also be used by some pest species, including European starlings, so it would be good to monitor your boxes at the end of the breeding season to see who’s using them and consider moving them,” Johnson said.
There is evidence that kestrels prefer wooden boxes. “However, if you already have plastic boxes up, and especially if they’re being used, it’s fine,” Johnson said.
Martinico introduced the topic of attracting raptors to agricultural areas using resting perches. Raptors, she explained, fall into different groups such as hawks, falcons, eagles, harriers and kites. She noted the presence of the type of raptor will depend on the region and time of year.
Interestingly, she noted that “having predators present creates what we like to call ‘a landscape of fear.’” This is because “pest species have to spend more time being vigilant and protecting themselves from predators … In turn, they spend less time foraging or causing damage.”
Since raptors have incredible eyesight, and they like to be at the highest vantage point, Martinico said, “It was found that perches on hilltops were used more frequently than those at the bottom of the hill.” Therefore, she claimed, perches will be very effective if placed around trees or directly in crop fields. Lastly, like most nest boxes, she recommended they not be placed along any type of road that gets regular vehicle traffic.
Martinico also focused on integrated pest management – using multiple approaches to tackle a pest problem – because raptors cannot reduce rodent population to zero. Many control methods can have “non-target effects.” As a result, she found that secondary poisoning can be a deleterious side effect for barn owls and raptors preying upon pests that have ingested anticoagulant rodenticides.
In various studies of mammalian predators and raptor carcasses, Martinico pointed out that more than 75% presented with residues of anticoagulant or rodenticides in their livers. “This indicates that they [had come] into contact with these compounds at some point during their lives or within the past six months to a year,” she said. With second generation anticoagulant rodenticides available, she noted that there has been an increase in toxicity which persists longer in the tissues. The caveat here is to be judicious with your use of rodenticides, as they have been proven to have long-lasting and often negative effects.
Information about kestrels and good designs for their boxes nest boxes can be found at the American Kestrel Partnership (kestrel.peregrinefund.org).
To learn how to build nest boxes for barn owls, visit barnowltrust.org.uk/barn-owl-nestbox/barn-owl-nestboxes.
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