Action: Provide artificial nesting sites for owls
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- Three studies from the UK appeared to show increases in local populations of owls following the installation of artificial nests, although the authors from one note that they could not rule out birds merely switching from natural nest sites. Another UK study found that providing nesting sites when renovating buildings maintained barn owl Tyto alba populations, whilst they declined at sites without nests.
- Four studies from the USA and the UK found high levels of breeding success in artificial nests, three finding equal or higher productivity than natural nests. A replicated, controlled study from the USA found lower productivity from artificial nests, whilst a replicated, controlled study from Finland found that artificial nests were only successful in the absence of larger owls and a replicated, controlled study from Hungary found that fledglings from artificial nests were less likely to be found alive after one year.
- Four studies from the USA and Europe found that artificial nests were used at least as frequently as natural nesting sites. Five studies from across the world found that owls used artificial nests, with one finding that use increased over time, although only for one of two species.
- Three studies found that owls differentiated between nests in different positions, whilst five studies found that different designs of nests differed in occupancy or productivity. Three studies found occupancy did not differ between designs and two found no differences in productivity for different designs.
Supporting evidence from individual studies
A controlled study 1977-9 in riverine forests in Louisiana, USA (McComb & Noble 1981), found that Eastern screech owls Megascops asio (formerly Otus asio) used nest boxes more frequently than natural cavities (0.7% of 5,374 nest boxes inspected contained nests vs. 0.03% of 3,993 natural cavities). Frequently used nests faced north and were situated under tree limbs, in trees with lianas. Barred owls Strix varia also used nest boxes, but at very low frequencies. This study also examined nest box use by other birds (wildfowl, woodpeckers and songbirds).
A short 1984 review of several nest box programmes in the USA (Johnson & Follen Sr.. 1984) found that barred owls Strix varia appeared to successfully use nest boxes provided in a range of habitats. Owls nesting in nest boxes in Minnesota produced more chicks than those in natural nests (2.75 fledglings/nesting attempt for 12 attempts in nest boxes vs. 2.00 fledglings/nesting attempt for six attempts in natural nests); in Wisconsin, two nest boxes produced ten chicks over four years; in Michigan, a single box produced young in three of the four years it was monitored (1979-82). Boxes varied in design, but most were modified wood duck boxes, approximately 33 x 34 x 36 cm, with 18-22 cm entrance holes.
A replicated, controlled study in 1970-83 in boreal forests in Hedmark, Norway (Sonerud 1985), found that three species of owl appeared to nest preferentially in nest boxes, compared to natural cavities. Pygmy owls Glaucidium passerinum showed the weakest preference (55% of 20 nests were in nest boxes), followed by hawk owls Surnia ulula (75% of 12 nests in boxes) and Tengmalm’s owls Aegolius funereus (97% of 167 nests in boxes). The number of nesting cavities available is not recorded. Tengmalm's owls used boxes on isolated trees in clear-cuts most, and those closed mature forest the least. Pygmy owl boxes were smaller (with a 45 mm entrance hole) than other boxes (with a 58 mm entrance) and only one (5%) was predated, compared to 69 (37%) Tengmalm’s owl clutches and four (33%) hawk owl clutches.
A replicated, controlled study in 1979-85 in west Finland (Korpimaki 1987) found that Tengmalm's owls Aegolius funerus nested in nest boxes at least as frequently as natural cavities (3-12% of nest boxes used, depending on design vs. 8.5% of natural cavities). Small boxes (internal diameter <26 cm, entrance holes <15 cm) were used more frequently than larger nest boxes (10-12% of 894 small boxes used vs. 3% of 165 larger boxes). Boxes made from hollowed logs (17-20 cm internal diameter, 8-10 cm entrance hole) and natural cavities were used less than small nest boxes, but not significantly so (7% of 677 log boxes and 9% of 177 natural cavities used). The study area was increased each year of the study, until it reached 1,300 km2, with 450 nest sites. A total of 1,736 nest boxes were used and 177 natural cavities searched.
A replicated, controlled study in 1967-75 in urban woodland in central Texas, USA (Gehlbach 1994), found that eastern screech owls Megascops asio (formely Otus asio) had equal nesting success in nest boxes as in natural nests (average of 3.9 eggs/clutch and 51% of eggs producing fledglings in nine nest boxes vs. 3.8 eggs/clutch and 57% of eggs producing fledglings in nine natural nests). The wood used in nest boxes (plywood, pine or cedar) and nest box size did not appear to affect use or reproductive success. Nest boxes had basal areas of 225, 400 or 625 cm2, with a 6.8 cm entrance hole 25 cm above the base. Boxes were erected 3-4 m above ground on large trees.
A before-and-after study at a 150 km2 in Norfolk, England (Johnson 1994), found that barn owl Tyto alba population density increased from 15 pairs/100 km2 in 1989 to 27 pairs/100 km2 in 1993, following the provision of 60 nest boxes. Nest boxes were used at the same rate as natural nest sites, and pairs using boxes in trees produced more eggs (but not significantly more fledglings) than other nest types. Nest boxes were located in buildings (43 boxes, a maximum of 11 used in a single year) and on trees (17 boxes, a maximum of five used in a single year).
Two before-and-after studies in pine forests in the UK (Petty et al. 1994) found local population increases in tawny owls Strix aluco and barn owls Tyto alba following the provision of nest boxes, although the authors note that the tawny owl population may have responded to an increased food supply. In 1980-91, 90-160 boxes were erected in an area of northeast England. All local birds used boxes by 1983 and the population increased from 40 to 66 pairs. At a site in southwest Scotland, 33-87 nest boxes were provided for barn owls in 1984-90. Resident birds did not move nest sites, but new breeders moved into the area and used boxes (37 pairs using boxes in 1988), increasing the population from five to approximately 42 pairs by 1993.
A replicated study in California, USA, between 1988 and 1993 (Trulio 1995) found that 18 burrowing owls Athene cunicularia that were ‘evicted’ (using one-way doors) from their original burrows at five grassland sites, apparently occupied artificial burrows created 7-75 m away from original burrows. A pair provided with three burrows 165 m from their original burrow did not use them. The authors note that owls were not ringed, so those in artificial burrows could not be confirmed as the evicted birds.
A replicated and controlled trial in 1989-94 in boreal forests in Central Finland, Finland (Hakkarainen & Korpimäki 1996), found that Tengmalm’s (boreal) owls Aegolius funereus successfully used nest boxes provided. However, of 15 possible breeding attempts in Ural owl Strix uralensis territories, only four were made (27%) and all failed. In contrast, 15 of 20 possible attempts in eagle owl Bubo bubo territories were made (75%) and all but two were successful. The presence of Ural owls was therefore found to significantly reduce both the probability of occupation and the chances of success for the smaller Tengmalm’s owl.
A replicated, controlled trial in 1993-5 in arid shrubland in New Mexico, USA (Botelho & Arrowood 1998), found that burrowing owls Athene cunicularia (formerly Speoty cunicularia) nesting in artificial burrows produced significantly more nestlings, but significantly fewer fledglings than pairs in natural burrows (3.5 nestlings/pair and 1.5 fledglings/pair for eight pairs in artificial burrows vs. 2.2 nestlings/pair and 1.9 fledglings/pair for 59 natural burrows). Only 12 of 28 nestlings (43%) in artificial nests survived to fledging, with most being predated or cannibalised. Artificial burrows were constructed from a 19 l plastic bucket buried and connected to the surface with 5 m of 10 cm diameter PVC pipes. Both bucket and pipes had holes drilled in to ensure drainage.
A replicated study in 1981-96 in a reed-dominated wetland site in Cambridgeshire, England (Garner & Milne 1998), found that long-eared owls Asio otus readily used two designs of wicker baskets, with 77 nesting attempts over the study period. Of the 71 nest monitored, 42 (59%) hatched eggs and 36 (51%) fledged at least one chick. Between one and nine baskets were used each year, with three to 23 baskets available. It was not possible to confirm whether the apparent population increase was genuine or caused by owls switching from natural nest sites. Baskets were either local ‘fruit-picker’ baskets or dog baskets (30 cm diameter and 15 cm deep) and replaced every 4-5 years. Baskets were placed in trees, mostly hawthorn, 3.5-5.0 m above the ground.
A small controlled study in 1990-3 in Devon and Cornwall, England (Ramsden 1998), found that activity in buildings used by barn owls as nesting and/or roosting sites dropped by 68% in nine areas following the conversion or demolition of the building, but was maintained in three other areas where a cavity and access hole were incorporated into the conversion or another nearby (<50 m away) building. There were no changes in eight control areas.
A randomised, replicated study in prairie, shrubland and farmland in southwest Idaho, USA, in 1997-8 (Smith & Belthoff 2001) found that western burrowing owls Athene cunicularia hypugaea preferentially used artificial burrows with large (1,750 cm3) nest chambers, compared to small or medium (707 cm3 or 900 cm3) chambers (31 large chambers selected vs. six medium and seven small, a total of 81 burrows of each type available). Burrows with small (10 cm diameter) tunnels were also preferred, compared to those with large (15 cm diameter) tunnels (30 burrows with small tunnels occupied vs. 14 with large burrows, 72 of each type available). However, there were no differences in reproductive output between nest types. Burrows were arranged in clusters containing all burrow types and designed to resemble natural nests. Chambers were lined with soil and natural burrows nearby were blocked with rocks to ensure owls used artificial burrows.
A replicated study in boreal forests in Gansu, China (Fang 2005), found that boreal owls Aegolius funereus, but not Sichuan wood owls Strix uralensis (formerly S. davidi also known as Ural owls) used nest boxes provided in 2002-4. A total of 120 nest boxes were provided, 50 for boreal owls and 70 for Sichuan wood owls. No boreal owls used nest boxes in 2002, four bred in 2003 (fledging eight chicks from three nests) and six bred in 2004 (fledging eight chicks from four clutches). Several species of songbird also use boxes. Boxes were erected at least 3 m up on tree trunks, lined with grass and mosses and of the same design as used in Europe for Ural owls. From 2003, some were encased in bark to make them appear more natural.
A replicated study on four agricultural sites in northeast Arkansas, USA (Radley & Bednarz 2005), found that, in 2001, barn owls Tyto alba nested in four of 48 nest boxes erected in 2000, making up 29% of nests in the area. All occupied boxes were on a single site with a high owl density and were on artificial structures, although boxes were equally distributed on trees and artificial structures. No nest boxes were used in 2000, possibly because they were erected after birds had settled for the year (boxes erected between January and March). Boxes were made from 39 cm diameter PVC piping with the ends blocked (one only partially, leaving half the end as an entrance hole) and drainage holes drilled in. Boxes were placed 2.5-6.8 m above the ground.
A replicated, controlled study in Hungary in 1995-2003 (Klein et al. 2007) using ring-recapture data found that juvenile barn owls Tyto alba fledged from nest boxes were significantly less likely to be recovered alive than those reared in church towers (25% of 75 nest box-reared birds recovered alive one year after fledging vs. 40% of 116 church tower-raised birds). This difference in survival was only apparent in the first year after fledging, with similar proportions being recovered six years after fledging (28% of nest box-reared birds vs. 41% of church tower-raised birds).
A replicated study in eucalyptus stands in farmland in Lower Galilee, Israel (Charter et al. 2010), in 2008-9, found that barn owls Tyto alba nested in 67% of 51 nest boxes with large (15 x 30 cm) entrance holes, but none of 49 nest boxes with small (7.5 cm diameter) entrances. In contrast, common scops owls Otus scops only nested in boxes with small entrances, using approximately 10% of 49 small entrance boxes, but no large entrance ones, possibly due to competition with the larger barn owls. Boxes were attached to shaded parts of eucalyptus trees and the positions of large and small-entranced boxes were exchanged between years. This study also investigates nest box use by kestrels and songbirds.
A replicated study in eucalyptus stands in farmland in Lower Galilee, Israel (Charter et al. 2010), in 2008-9, found that long-eared owls Asio otus fledged more chicks from small nest baskets, compared to large ones (1.25 fledglings/breeding attempt for eight attempts in large baskets vs. 3.7 fledglings/attempt for seven in small baskets). Differences in clutch size and hatching success were non-significant (3.3 eggs/clutch and 60% hatching success for large nests vs. 5.0 eggs/clutch and 64% hatching success for small nests) and owls occupied nest types with equal frequency. Nest baskets were metal bowls filled with coconut fibre and were either 30 cm in diameter and 16 cm deep (small) or 40 cm in diameter and 20 cm deep (large). Thirty eight of each type of nest were erected in 2007, with the positions of large and small nests exchanged in 2008. The study also discusses nest box use by Eurasian kestrels Falco tinnunculus.
- McComb W.C. & Noble R.E. (1981) Nest-box and natural-cavity use in three mid-south forest habitats. The Journal of Wildlife Management, 45, 93-101
- Johnson D.H. & Follen Sr. D.G. (1984) Barred owls and nest boxes. Raptor Research, 18, 34-35
- Sonerud G.A. (1985) Risk of nest predation in three species of hole nesting owls: influence on choice of nesting habitat and incubation behaviour. Ornis Scandinavica, 16, 261-269
- Korpimaki E. (1987) Selection for nest-hole shift and tactics of breeding dispersal in Tengmalm's owl Aegolius funerus. Journal of Animal Ecology, 56, 185-196
- Gehlbach F.R. (1994) Nest-box versus natural-cavity nests of the eastern screech-owl: an exploratory study. Journal of Raptor Research, 28, 154-157
- Johnson P.N. (1994) Selection and use of nest sites by barn owls in Norfolk, England. Journal of Raptor Research, 28, 149-153
- Petty S.J., Shaw G. & Anderson D.I.K. (1994) Value of nest boxes for population studies and conservation of owls in coniferous forests in Britain. Journal of Raptor Research, 28, 134-142
- Trulio L.A. (1995) Passive relocation: a method to preserve burrowing owls on disturbed sites. Journal of Field Ornithology, 66, 99-106
- Hakkarainen H. & Korpimäki E. (1996) Competitive and predatory interactions among raptors: An observational and experimental study. Ecology, 77, 1134-1142
- Botelho E.S. & Arrowood P.C. (1998) The effect of burrow site use on the reproductive success of a partially migratory population of western burrowing owls. Journal of Raptor Research, 32, 233-240
- Garner D.J. & Milne B.S. (1998) A study of the long-eared owl Asio otus using wicker nesting baskets. Bird Study, 45, 62-67
- Ramsden D.J. (1998) Effect of barn conversions on local populations of barn owl Tyto alba. Bird Study, 45, 68-76
- Smith B.W. & Belthoff J.R. (2001) Burrowing owls and development: short-distance nest burrow relocation to minimize construction impacts. Journal of Raptor Research, 35, 385-391
- Fang Y. (2005) Conservation action on the endemic owls at Lianhuashan Mountains. Final report of project 'OWL 2002'. Unpublished report to the BP Conservation Programme [now Conservation Leadership Programme] (added by: Ward-Francis A. 2010).
- Radley P.M. & Bednarz J.C. (2005) Artificial nest structure use and reproductive success of barn owls in northeastern Arkansas. Journal of Raptor Research, 39, 74-79
- Klein A., Nagy T., Csörgő T. & Mátics R. (2007) Exterior nest-boxes may negatively affect barn owl Tyto alba survival: an ecological trap. Bird Conservation International, 17, 273-281
- Charter M., Izhaki I. & Leshem Y. (2010) Effects of the risk of competition and predation on large secondary cavity breeders. Journal of Ornithology, 151, 791-795
- Charter M., Izhaki I. & Leshem Y. (2010) Does nest basket size affect breeding performance of long-eared owls and Eurasian kestrels? Journal of Raptor Research, 44, 314-317