Action: Provide artificial nesting sites for falcons
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- Four studies from the USA and Europe found that local populations of falcons increased following the installation of artificial nesting sites, with one reporting that there was no decline in natural nest use following the installation and use of nest boxes. A replicated study from Canada found that the local population of American kestrels Falco sparverius did not increase following the erection of nest boxes.
- Eight studies from across the world found that the success and productivity of falcons in nest boxes was high and equal to, or higher than those in natural nests. Four studies from across the world found that productivities in nest boxes were lower than in natural nests or in previously published results, or that some falcons were evicted from their nests by barn owls Tyto alba.
- Four studies from across the world found no differences in productivity between nest box designs or positions, whilst two, from Spain and Israel found that productivity in boxes varied between designs and habitats.
- Twenty-one studies from across the world found nest boxes were used by falcons, with one in the UK finding that nest boxes were not used at all. One study from Canada found that falcons preferentially nested in nest boxes over natural nest sites; a study from Mauritius found that most breeding attempts were in nest boxes
- Four studies found that use increased over time. Seven studies found that position or design affected use, whilst three found no differences between design or positioning.
There is a large literature on artificial nest use by falcons, falcons are known to use relatively small nest boxes, compared to other birds of prey and some species (e.g. the lesser kestrel Falco naumanni) nest communally. Therefore we have separated out studies investigating artificial nest use by falcons from those investigating other birds of prey.
Supporting evidence from individual studies
A small study in a pine forest in California, USA (Boyce et al. 1980), found that a pair of prairie falcons Falco mexicanus successfully used a nesting ledge installed on a nesting cliff in autumn 1978. Four eggs were laid on the artificial ledge in 1979 and two chicks hatched and fledged. The platform was made of steel, with rock added to it to encourage use and was held in place by expansion bolts.
A replicated study in 1976-80 in juniper and pine forests in the Great Basin, California, USA (Bloom & Hawks 1983), found that 31% of 208 nest boxes examined were used by American kestrels Falco sparverius and that 82% of these (53 nest boxes) successfully fledged at least one chick. Clutches contained an average of four eggs, with an estimated fledging rate of 3.1 chicks/active nest box. Nest boxes were 18 x 20 x 33 cm in size, with a 7.6 cm diameter entrance hole and erected at 2-6 m from the ground in trees. The use of boxes increased year on year, from 20% in 1976 to 38% in 1980.
A replicated study at reclaimed surface mine sites in West Virginia and Pennsylvania, USA (Wilmers 1982), found that American kestrels Falco sparverius preferentially used nest boxes sited away from a woodland edge; 10 of 65 (15%) woodland edge boxes were used compared with 47% located 50 m or more from a woodland edge. In 1980, 60 nest boxes were erected at 18 mines, and in 1981 a further 91 at 24 mines. Kestrels used 14 (23%) boxes on 10 of 18 (56%) mines in 1980, and 33 of 91 (36%) on 19 of 24 (79%) mines in 1981. Mine sites where boxes were used had significantly less bare ground cover and a deeper litter layer.
A replicated study in two upland pine forests in Wales and England (Petty 1985) found that Eurasian kestrels Falco tinnunculus did not use any of the 41 nest boxes provided between 1973-8. Twenty seven nest boxes were erected on 2 m posts in Wales between 1973 and 1976 and lined with peaty turf, whereas in northern England, 14 boxes of the same design were attached to mature spruce trees (five to six whorls from the top). Only one pair of kestrels bred in the 20 km2 around the Welsh site, with two pairs breeding in the 10 km2 around the English site.
A controlled before-and-after study in Missouri, USA (Toland & Elder 1987), found a large increase in nesting and overwinter population densities of American kestrels Falco sparverius in a 78 km2 area, where 125 nest boxes were erected in 1982-3 (0.05 birds/km2 in 1977-81 vs. 0.32 birds/km2 in 1984). There was no increase in a 90 km2 control area, without nest boxes (0.02 birds/km2 in 1977-81 vs. 0.03 birds/km2 in 1984), but there was in an urban control area (0.13 birds/km2 in 1977-81 vs. 0.23 birds/km2 in 1984), possibly due to increased food availability. Overall, kestrels used 53% of the 125 nest boxes available. Nesting success was significantly higher in boxes mounted on man-made structures (64-78% on buildings and utility poles) than on trees (33%), but still lower than in natural sites (86–88%). However, they produced as many young through double broods and replacement clutches.
A small before-and-after study in a pine forest in northern California, USA (Pagel 1989), found that a pair of peregrine falcons Falco peregrinus reproduced successfully following the enlargement of their nesting ledge through the use of ditching dynamite in December 1983. In 1984-8, 13 chicks fledged from the site (2.7 chicks/year), whereas all previous nesting attempts had failed due to eggs and chicks falling from the small ledge. The ledge was 36 m up on a dolomitic limestone cliff and the authors caution that only two of four sticks of dynamite used detonated and, had all four exploded, the ledge may have been destroyed. Previous attempts to enlarge the ledge with hand tools had not worked.
A replicated study in 1988-93 in boreal forests in Saskatchewan, Canada (Bartolotti 1994), found that American kestrels Falco sparverius nested preferentially in large nest boxes over small (81-94% of 66 kestrels in nest boxes nesting in large boxes when given the choice). Nest boxes were also preferred over natural cavities (a maximum of 5-15% of natural cavities used vs. 53-88% of 17-19 nest boxes used each year). There were no differences in reproductive success or predation rates between large and small nest boxes (40-87% success for 54 clutches in large nest boxes vs. 33-86% success for 23 clutches in small nest boxes). Comparisons with natural cavities were not possible due to small sample sizes. The author argues that providing 345 nest boxes over the study period did not increase the local population. Nest boxes had a basal area of 241 cm2 (small boxes) or 469 cm2 (large boxes) and a 7.5 cm diameter entrance hole.
A study of an integrated conservation programme for the endangered Mauritius kestrel Falco punctatus from 1973-1994 in montane forest habitat and a captive breeding centre in Black River, Mauritius (Jones et al. 1996) found that nestboxes in areas where natural nest sites were limited were used by released birds, with 90% of 105 documented nesting attempts, during 1988-1989 and 1993-1994, occurring in nestboxes. In the 1993-1994 breeding season, 49% of all monitored wild pairs used nestboxes and several returned to nest in the same ones.
A replicated trial at nine mixed agricultural sites in Iowa, USA (Craft & Craft 1996), found that American kestrels Falco sparverius occupied 66% of 56 nest boxes for at least one year between 1989 and 1992, with a maximum of 42% occupied in any one year. Clutches contained an average of 4.4 eggs (49 clutches) and 4.2 chicks fledged on average from each successful box (33 boxes, average of 2.7 chicks/box). These values are similar to previously recorded productivities for American kestrels. Four wooden nest boxes were erected in 1988-9 at each site on pylons, windmills, barns or wooden posts. An additional two PVC boxes were erected in 1990 at each site, plus a final two at one of the sites.
A replicated trial in 1987-91 in mixed agricultural habitats and woodland in Pennsylvania, USA (Rohrbaugh & Yahner 1997), found that American kestrels Falco sparverius used 76% of 130 nest boxes at least once over the five-year study period, with 49% of 259 nesting attempts raising at least one offspring. Kestrels most frequently used unconcealed nest boxes in open habitats away from forested areas, and with a lot of light entering. Nest boxes with southeast orientations were used most frequently. Nesting success was also higher in nest boxes with high light intensities. Nest boxes were 26 × 24 × 33 cm with a 7.6 cm diameter entrance hole and were attached 2.0-6.5 m above the ground.
A replicated study in agricultural sites in southern Finland (Valkama & Korpimaki 1999) found that Eurasian kestrels Falco tinnunculus occupied 18-22% of 161 nest boxes between 1985 and 1995, with no differences between small, intermediate and large boxes. Boxes sheltered from prevailing weather were more likely to be occupied than exposed boxes (25% of 80 sheltered boxes used vs. 17% of 81 exposed boxes). There were no significant differences in clutch size or number of fledglings produced between nest boxes types and orientations, with success related to laying date and vole abundance. Occupied boxes were, on average, further from forest edges, roads and inhabited houses, and closer to grassy ditches than unoccupied boxes. Boxes were 25 × 27.5 × 25 cm, with a 12.5 x 25 cm entrance (small); 34 × 35 × 20 cm, with a 12 x 34 cm entrance (intermediate); or 33.5 × 45 × 30 cm, with a 12 x 33.5 cm hole (large).
A replicated study in Badajoz, Spain, in 1989 (Aviles et al. 2001), found that European kestrels Falco tinnunculus used 16% of 567 nest boxes placed in seven agricultural and woodland habitats. There were no significant differences in laying date or productivity between habitats. However, when only habitats with more than 15 occupied boxes were analysed, nests in pastures were found to have significantly larger clutch and higher breeding success than those in cereal fields (4.4 eggs/clutch and 4.2 fledglings/nest for 39 nests in pastures vs. 3.7 eggs/clutch and 3.5 fledglings/nest for 19 nests in cereal fields). Nest boxes were erected on power pylons across the habitat types in spring 1989.
A before-and-after study in mixed farmland and oak woodlands in Avila and Segovia, central Spain (Fargallo et al. 2001), found that the local population of Eurasian kestrels Falco tinnunculus more than doubled between 1993 (23 pairs) and 1998 (55 pairs) following the installation of 47 nest boxes over the same period. The number of kestrels in natural nests remained approximately constant (15-25 pairs), whilst the number in nest boxes increased from three (1993) to 35 (1998). Birds in nest boxes fledged more chicks and experienced less nest predation than those in natural sites (3.6-3.8 fledglings/clutch and 12% predation for 79 nest box clutches vs. 2.4-2.8 fledglings/clutch and 37% predation for 37 clutches in natural nests). Nest box chicks had more ectoparasites, but this difference was not significant. Nest boxes were installed in winter: 14 in 1993-4, 11 in 1994-5, 16 in 1996-7 and six in 1998.
A small study in mixed farmland and woodlands in Alentejo, Portugal (Franco et al. 2005), found that lesser kestrels Falco naumanni used 25% of 36 nesting cavities in two ‘breeding towers’ in 2003. The towers were constructed in 1997 and 1999 but not occupied until 2002, after modifications were made to the nest chambers to create an enlarged nest cavity. Three pairs bred successfully in 2002 in addition to common kestrels F. tinnunculus, rollers Coracias garrulus, barn owls Tyto alba and jackdaws Corvus monedula.
A replicated study in a 1500 km2 area of mixed deciduous forests in Pennsylvania, USA (Katzner et al. 2005), found that American kestrels Falco sparverius used an average of 86 nest boxes each year between 1993 and 2002 (32% of the approximately 270 boxes in the area). Pairs laid an average of 4.6 eggs/clutch and fledged 2.7 nestlings/box (171 boxes monitored). First breeding attempts were successful 69% of the time. These productivity levels are similar to those recorded elsewhere. Boxes were 26 × 24 × 33 cm, with a 7.6 cm diameter entrance hole. They were erected 3–6 m off the ground (usually on trees or utility poles, but sometimes on sheds and barns).
A before-and-after study in mixed farmland and woodlands in Alentejo, Portugal (Catry et al. 2007), found that the local population of lesser kestrels Falco naumanni increased by 36% between 2003 and 2006, following the provision of over 450 artificial nest sites over the same period. The number of pairs nesting in artificial sites increased from 29 in 2003 (150 nests available and a total local population of 268 birds) to 121 in 2006 (450 nests and 364 birds in total). Nests included nest boxes, clay pots and multi-cavity “breeding walls” and “breeding towers” (discussed in Franco et al. 2005).
A replicated, controlled study in mixed agricultural habitats in the North District of Israel, in 1999-2006 (Charter et al. 2007), found that Eurasian kestrel Falco tinnunculus nesting in small nest boxes produced more chicks than those in large boxes (3.0 chicks/clutch for 37 in small boxes vs. 1.9 chicks/clutch for 44 clutches in large), with no differences between boxes and natural nests (2.1 chicks/clutch for 56 attempts). Large boxes had higher failure rates (48% of 44 attempts) compare to small (20% of 37) and natural nests (20% of 56 in natural nests). When only successful nests were analysed, all boxes fledged more chicks than natural nests (3.6-3.9 chicks/clutch for 52 clutches in nest boxes vs. 2.7 chicks/clutch for 44 in natural nests). Boxes were either: 50 x 75 x 50 cm with a 25 x 15 cm entrance hole and mounted 2.5-3.0 m above ground or 50 x 30 x 30 cm with a 22 x 15 cm hole and 5-6 m above ground. Sixty large and eleven small boxes were erected.
A replicated, controlled trial in five towns in Apulia, southern Italy (Bux et al. 2008), found that lesser kestrels Falco naumanni nested in artificial nest boxes in the first two years after installation. Two hundred nest boxes were placed on flat roofs in 2007, 8% were occupied in 2007 and 17.5% used in 2008. Nest box breeders fledged an average of 1.8 chicks/clutch in 2007 (17 clutches), similar to pairs nesting in attics (1.7 chicks/clutch for 18 clutches) but significantly lower than pairs nesting in wall cavities (2.7 chicks/clutch for ten clutches). Productivity was lower in 2008 (1.5 chicks fledged/clutch, 35 clutches) but no comparison was possible with other nest types.
A replicated study in 2005 in sagebrush steppe and agricultural fields in Idaho, USA (Butler et al. 2009 ), found that 71% of 59 nest boxes were occupied by American kestrels Falco sparverius. Box orientation did not significantly affect occupancy rates (although no east-facing boxes were occupied) but did affect hatching success (43% of 21 southwest-facing nest boxes unsuccessful vs. 25% of 12 southeast-facing boxes and 0% for nine facing north-west). West-facing boxes were approximately 0.6°C cooler on average than east- and south-facing boxes, and also less humid. Nest boxes were 21 x 21 x 46 cm and erected at 2.5-3.0 m off the ground on utility poles.
A controlled before-and-after study in 1989-93 in Florida, USA (Smallwood & Collopy 2009), found that the population of southeastern American kestrels Falco sparverius paulus in an 3,600 km2 experimental area of dry mixed forests and agricultural land increased following the installation of 388 best boxes in 1990-3 (5.0 birds/100 km2 in 1989 vs. 32.3 birds/100 km2 in 1992). There was no corresponding increase in a similar area without boxes (34.4 birds/100km2 in 1989 vs. 34.9 birds/km2 in 1992). The number of boxes used increased each year, reaching 158 in 1993 and a total of 365 nesting attempts (39 of which were re-nesting). Nesting success averaged 67%, with 2.4 fledglings/nest. This is relatively low compared with previously recorded productivities. Boxes had a base of 19.7 x 23.5 cm, with a 8.9 cm diameter entrance hole.
A replicated study in eucalyptus stands in farmland in Lower Galilee, Israel (Charter et al. 2010), in 2008-9, found that Eurasian kestrels Falco tinnunculus nested with equal frequency and equal success in nest baskets of two different sizes (13 of 76 nests used, average of 1.8 chicks fledged/clutch for six clutches in small baskets vs. 2.5 chicks/clutch for six in large baskets). Overall productivity in this study was lower than previously recorded in nest boxes in the same region (2.2 fledglings/breeding attempt vs. 3.2 fledglings/attempt in previous studies). Nest baskets were metal bowls filled with coconut fibre and were wither 30 cm in diameter and 16 cm deep (small) or 40 cm in diameter and 20 cm deep (large). The positions of large and small nests exchanged in 2008. The study also discusses nest box use by long-eared owls Asio otus.
A replicated study in eucalyptus stands in farmland in Lower Galilee, Israel (Charter et al. 2010), in 2008-9, found that Eurasian kestrels Falco tinnunculus nested more frequently in nest boxes with large entrance holes than in boxes with small holes (17% of 51 large-entrance nest boxes occupied vs. approximately 8% of 49 small-entrance boxes). Breeding success of kestrels did not vary between nest box types although 22% of kestrels in boxes with large holes abandoned them because of barn owl Tyto alba interference. There was no such interference in small nest boxes. Nest boxes were 50 x 75 x 50 cm with either 15 x 30 cm (large) or 7.5 cm diameter (small) entrances. In 2008, 27 large and 25 small boxes were erected, with 24 of each in 2009. The positions of large and small boxes were exchanged between years. This study also discusses nest box use by owls and songbirds.
- Boyce Jr D.A., Fisher L., Lehman W.E., Hipp B. & Peterson J. (1980) Prairie falcons nest on an artificial ledge. Raptor Research, 14, 46-50
- Bloom P.H. & Hawks S.J. (1983) Nest box use and reproductive biology of the American kestrel in Lassen County, California. Raptor Research, 17, 9-14
- Wilmers T.J. (1982) Kestrel use of nest boxes on reclaimed surface mines in West Virginia and Pennsylvania. M.S. Thesis, West Virginia University, Morgantown. 182 pp. (added by: Showler D.A. 2010).
- Petty S.J. (1985) A negative response of kestrels Falco tinnunculus to nestboxes in upland forests. Bird Study, 32, 194-195
- Toland B. & Elder W. (1987) Influence of nest-box placement and density on abundance and productivity of American kestrels in central Missouri. Wilson Bulletin, 99, 712-717
- Pagel J.E. (1989) Use of explosives to enhance a peregrine falcon eyrie. Journal of Raptor Research, 23, 176-178
- Bortolotti G.R. (1994) Effect of nest-box size on nest-site preference and reproduction in American kestrels. Journal of Raptor Research, 28, 127-133
- Jones C.G., Heck W. & Lewis R.E. (1996) The restoration of the Mauritius kestrel Falco punctatus population. Ibis, 137, S173-S180
- Craft R.A. & Craft K.P. (1996) Use of free ranging American kestrels and nest boxes for contaminant risk assessment sampling: a field application. Journal of Raptor Research, 30, 207-212
- Rohrbaugh R. & Yahner R. (1997) Effects of macrohabitat and microhabitat on nest-box use and nesting success of American kestrels. Wilson Bulletin, 109, 410-423
- Valkama J. & Korpimaki E. (1999) Nestbox characteristics, habitat quality and reproductive success of Eurasian kestrels. Bird Study, 81-88
- Aviles J.M., Sanchez J.M. & Parejo D. (2001) Breeding rates of Eurasian kestrels (Falco tinnunculus) in relation to surrounding habitat in southwest Spain. Journal of Raptor Research, 35, 31-34
- Fargallo J.A., Blanco G., Potti J. & Vinuela J. (2001) Nestbox provisioning in a rural population of Eurasian kestrels: breeding performance, nest predation and parasitism. Bird Study, 48, 236-244
- Franco A.M.A., Marques J.T. & Sutherland W.J. (2005) Is nest site availability limiting lesser kestrel populations? a multiple scale approach. Ibis, 147, 657-666
- Katzner T., Robertson S., Robertson R., Klucsarits J., McCarty K. & Bildstein K.L. (2005) Results from a long-term nest-box program for American kestrels: implications for improved population monitoring and conservation. Journal of Field Ornithology, 76, 217-226
- Catry I., Alcazar R. & Henriques I. (2007) The role of nest-site provisioning in increasing lesser kestrel Falco naumanni numbers in Castro Verde Special Protection Area, southern Portugal. Conservation Evidence, 4, 54-57
- Charter M., Izhaki I., Bouskila A. & Leshem Y. (2007) The effect of different nest types on the breeding success of Eurasian kestrels (Falco tinnunculus) in a rural ecosystem. Journal of Raptor Research, 41, 143-149
- Bux M., Giglio G. & Gustin M. (2008) Nest box provision for lesser kestrel Falco naumanni populations in the Apulia region of southern Italy. Conservation Evidence, 5, 58-61
- Butler M.W., Whitman B.A. & Dufty A.M. Jr. (2009) Nest box temperature and hatching success of American kestrels varies with nest box orientation. Wilson Journal of Ornithology, 121, 778-782
- Smallwood J.A. & Collopy M.W. (2009) Southeastern American kestrels respond to an increase in the availability of nest cavities in north-central Florida. Journal of Raptor Research, 43, 291-300
- 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
- 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