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American Kestrel Nest Box Guidelines

For information regarding current research and programs on American kestrel nest boxes that is being conducted by Warne in the Wild go to the Kestrel Nest Box Program or the Nest Box Research page.

Biology:

American kestrels (Falco sparverius) are the smallest falcon in North America and are about the size of a blue, Steller's or gray jay (depending on your area). They were commonly known as "Sparrow Hawks" but that name is no longer used except by a lot of farmers and ranchers who grew up using that name. Kestrels are often seen sitting on power lines hunting with the males being more colourful with various white, black, brown, rufous, orange and blue colours while the females are mainly rufous and white with black and brown bars and streaks with limited orange and blue on their head. 

Kestrels naturally nest in tree cavities, woodpecker nests or other cavities because they do not build their own nests but are secondary cavity nesters. Kestrels do readily nest in nest boxes and can also be found nesting in buildings or structures where there are smaller cavities.

Kestrels are often observed hovering over pasture or hay fields when not on power lines because their 

American Kestrel Photo Gallery

preferred prey species live in open areas. Kestrels need at least a few trees to roost and nest in but otherwise can live in grasslands, parkland, hay land, agricultural land or a mix of habitat types. Kestrels are able to see into the ultraviolet light spectrum allowing them to see the urine along rodent trails, which provides kestrels with an advantage when hunting small rodents in pastures and hay land. 

The best habitat for kestrels is pasture land because of prey species type and abundance, as well as, ease of access to the prey. Hay land is also good for prey type and abundance but the hay can be an obstruction to seeing prey when it grows tall prior to being cut. Agriculture is the least effective habitat for kestrels to raise young because of reduced prey species and how difficult it is for kestrels to see the ground through the crop. Canola crops are one of the worst crops for this. However, if the crops have shelter belts, ditches and/or other grassy areas nearby they can still be decent habitat for kestrels because of the rodent populations present in crop land. Mixed lands also is suitable with a lot of the kestrel boxes I maintain on Judah Hill have 25-50% of the surrounding land use being crop and they are able to raise as many young as pasture/hay kestrels.

Best =

Pasture

Great =

Pasture & Hay

Good =

Hay only

Decent =

Mix with <50% agriculture

Has Potential =

Only Agriculture

Nest Box Size Specifications:

The nest boxes that I build are a "basic" rectangular design. I use 10" pine (prefer at least #2 grade) instead of plywood because rodents and porcupines chew plywood, which damages the corners, entrance and roof (shown below on a barred owl nest box). The sides are 16" long, the back is 22" long and the roof is 12" long.

 

I do not slope the roof for three reasons; 1) it is rare that when installing the nest boxes they will be level 2) it is harder to properly seal the roof to the back piece when it is sloped 3) it takes more effort to build sloped nest boxes.

I do not use hinges to open the nest box because that does not keep the wind and weather out of the box as effectively, plus is more expensive. Instead I access the nest box through the side of the box by pivoting the side piece on the screws near the top of the board (red arrows) and a screw or forming nail to keep the side in place (angled arrow).

 

I also do not install a piece of wood along the inside at the bottom of the nest box to keep the wood chips in place. Instead I leave it without, which means a few wood chips may fall out when I open the side for access. However, cleaning nest boxes that have this extra side piece is significantly harder and does not allow the nest boxes to be cleaned as well. Plus once kestrels use a nest box the majority of the wood chips are one solid block and its far easier to loosen this block and slide it out of the box than to break it up and get it over the extra side piece.

Wood chips are needed to be placed in the bottom of the nest box. This allows the kestrels to form a small nest cup where they lay their eggs since they do not make a nest. Two to three inches of wood chips are needed to be placed in each nest box. Do not use wood shavings or anything like that because those soak up moisture instead of drying out like wood chips.

When I use #2 pine there is limited blue or red coloration (shown below) in the wood compared to #3 pine. I still think the blue and and red colouration looks attractive but it will fade in sunlight relatively quickly. I do use #3 pine but am more selective with it because it is more prone to cracking. 

Nest Box Placement:

When installing the kestrel nest box(es) there are a few things to keep in mind:

1) Install the box on the edge of open field (kestrels want a wide open approach) near preferred hunting areas is a big part of attracting kestrels. Pasture is the best option with hay land being the next best option or a mix of hay and pasture. Cultivation is also possible but it is the last choice for kestrels. 

 

2) The best locations are along the south or east sides of tree lines or shelter belts where there are little to no branches that are near the nest boxes. Without branches near the boxes it provides easier access to the boxes, fewer places for predators to hide and the boxes are easier to find by the kestrels.

3) Ideally you will face the box away from the prevailing wind (or at a 90), have it partially shaded from the hottest sun (hot areas of the province) and have the entrance face into the open field or at a minimum a bit less than parallel. The key is to have an open flyway that is easy to see. If your choice is between the entrance facing into a treed area or slightly into the wind I would recommend the wind because kestrels are unlikely to use the treed entrance and squirrels are more likely to use it.

4) Install the box 8ft or higher. 

5) The box can be installed on a tree, post or building. 

6) When installing it on a tree (I have 99% of the boxes on trees) you don't want any branches or shrubs within reach of the box or in the flight path. Aspen trees work really well since they often do not have any branches. You do not need very many trees around the nest box/territory since most areas have power lines that kestrels use as perches to hunt from.

7) If you don't have a tree or a tall post you could attach two 2x4s on each side of an existing normal fence post, which would allow you to mount the box 8ft or higher and still be near trees.

8) Each kestrel has a territory of approximately one quarter section (160 acres) but the habitat and prey abundance varies on how big their territories are.

Best =

No branches 

Good =

A few branches nearby

Decent =

Open access to box

Poor=

Mostly Branches

Unlikely =

Solid Branches

Other Nest Box Users

Other Kestrel Nest Box Users:

-Red Squirrels: this is the most common species I have found using kestrel nest boxes in the Peace Region. To date no squirrels and kestrels have tried using the same nest box with the squirrels using the nest boxes with a higher number of branches near the boxes. I had originally installed boxes in a variety of different sites to determine if there was any preference and the kestrels stick to the very open aspen trees while the squirrels stick to the semi-open trees.

-Other small mammals: other mammals will use the kestrel nest boxes including flying squirrels and mice. However, mice seldom use the nest boxes unless there is a severe lack of natural cavities or tunnels to hide in. Flying squirrels prefer more wooded areas than the edges of tree lines but could use the boxes.

-European Starling: this is by far the worse species to have using your nest box. They are large enough that they may keep kestrels from using the nest box due to their size, aggression and long spearlike bill. Starlings will readily kill native bird species in bird boxes or natural cavities if the starlings can fit into the entrance. However, I have not knowingly had starlings using nest boxes that kestrels have attempted to use or use annually or semi-annually. The only nest boxes starlings have used are boxes that kestrels are showing no interest in. This is because the starlings have initiated nesting in those boxes in late June to July, well after the kestrels initiated nesting. 

 

If you do have starlings beginning to nest in a kestrel nest box you should remove the nesting material, eggs, chicks and/or the adults. However, you must make sure they are starlings and not a native species that is protected (starlings are not protected).

-Small Owls (northern saw-whet, boreal, northern pygmy): All small owl species use the exact same nest box as the kestrel. However, the owls prefer to nest within wooded areas and not on the edge like kestrels. This isn't a golden rule though but if you're trying to get both species you are better off building enough boxes to install them within the wooded areas (owls) and along the edge (kestrels).

-Small birds (house wren, house sparrow, chickadees, swallows): All of these species are cavity nesters and will readily use nest boxes. However, they prefer smaller nest boxes with significantly smaller entrance holes so the likelihood of small birds using kestrel boxes is low but has been documented before. I have had house sparrows use one kestrel nest box but that box was placed poorly (on the edge of a yard of a ranch with a large population of house sparrows). 

Predators of Kestrels:

Due to the small size of kestrels a large number of species can prey on them. Predators could include: other birds of prey (hawks, falcons and owls), medium to large mammals (fisher, marten, skunks, coyotes, foxes, dogs, cats, and raccoons) and some small animals which would prey mainly on nestlings or eggs (weasels and larger snakes, such as, bull snakes).

Benefits:

Kestrels eat a lot of prey species that people, especially ranchers, farmers and acreage owners, consider pests. Prey species include: voles, mice, possibly smaller ground squirrels, birds, reptiles, amphibians and various invertebrates including; grasshoppers, beetles, dragonflies along with various other invertebrates. So far in the Peace Region I have had indications the kestrels are eating a large numbers of voles and grasshoppers with occasional birds, reptiles and dragonflies.

Kestrels are too small to prey on pets or livestock, with the exception of free range chicks, poults etc. Overall kestrels would be beneficial to ranchers etc because of the prey species that kestrels consume.

Possible Reasons for Decline:

-Winter Habitat: This is one of two main areas where I think most of the kestrel problems are stemming from. With the net loss in suitable land for kestrels to successfully survive the winter the population will decline. The land conversion through 

Nestlings with a Vole from 2016

the decades has left fewer landscapes available for kestrels and limited amounts of land has been converted back to naturalized habitats. I would like to see a study that compares the land availability trend compared to the kestrel population trend to indicate if this is an important factor.

 

-Breeding Habitat: This is the second of the two main areas of concern but it encompasses the time frame not covered by the winter habitat. This is basically the same idea as the winter habitat loss but encompasses a larger portion of North America and limits the potential for population growth vs maintaining the population (winter habitat). 

-Not Enough Woodpecker Habitat:

With the loss of shelter belts and treed areas the habitat available for woodpeckers has decreased. Although most woodpecker species are still present in a lot of areas their densities will have decreased due to lack of habitat and therefor fewer nest cavities will be created. Because of the reduced cavities available there will be, at least in theory, fewer available for kestrels and fewer kestrels will be raised to increase or maintain the population.

-Lack of Suitable Nest Cavities: This goes hand in hand with the lack of woodpecker habitat, in the Peace River area this appears to be an important factor, at least locally, based upon the use of nest boxes on Judah Hill. Prior to installing nest boxes here there was one maybe two pairs of naturally breeding kestrels by 2010/2011. Farmers and ranchers from the area said in the 1970/80s there were significantly more kestrels present in the same area. After installation of the nest boxes in the spring of 2012 the kestrels have repopulated the area to at least the same level as the 70/80s by 2015/2016 with 12 pairs breeding in nest boxes and four to five naturally nesting pairs. There have been limited changes to the landscape use, with the exception of more treed areas and shelter belts being removed during that time, so the limiting factor appears to be a lack of nesting sites. 

-Cooper's Hawk: Predation by Cooper's hawks has been blamed as a component of the kestrel decline. Although that may be true, in 2016 Cooper's hawks successfully raised their young approximately 750m away from a pair of kestrels that successfully raised their chicks on Judah Hill, along with all of the other kestrels nest box pairs in the area. There was also suspected Cooper's hawk nesting at another location on Judah Hill (but nests never found) during the summers from 2015-2017 and kestrels were successfully nesting in that area too.

-Cats/Dogs: Predation by cats and/or dogs (pets) is more likely to occur on kestrel fledglings because of their weak flying capability. However, when adult kestrels are catching or consuming prey kestrels would be preoccupied and more susceptible for predation by pets.

-Pesticides: the use of pesticides on invertebrates that kestrels feed on is a potential source of mortality for both adults and chicks. However, on pasture and hay land there shouldn't be much use of pesticides so hopefully there are currently limited impacts of pesticides on kestrels.

-Randoms (power lines, wind turbines, vehicle collisions, windows, persecution): these sources of mortality are mainly accident type sources of mortality and will hopefully not be adding much impact into the population trend of kestrels, especially with new technology and policies to limi the risks to kestrels.