Urban development reduces fledging success of Barn Owls in British Columbia, Canada

Birds associated with agricultural landscapes have experienced strong population declines and range contractions across Western Europe and North America (Fuller et al. 1995, Brennan and Kuvlesky 2005, Donald et al. 2006). Declines have been attributed to habitat degradation due to agricultural intensification (Fuller et al. 1995, Donald et al. 2006) and the loss of agricultural land to urbanization (Filippi-Codaccioni et al. 2008, Ludwig et al. 2009). The value of agricultural land as wildlife habitat is degraded when small fields with rotational crop practices and hedgerows are converted to large, intensively utilized fields with monocultures (Fuller et al. 1995, Wilson et al. 2005). In addition to habitat loss, urbanization creates road networks that fragment habitat and expose wildlife to traffic (Underhill and Angold 2000, Borda-de-Água et al. 2014). These changes in landscape composition have been linked to the reduced reproductive output and survival of many farmland birds (Fuller et al. 1995). Newton (2004) argued that most of the declines in populations of farmland birds in Britain (~70% of species) are associated with reduced reproductive output rather than reduced adult survival. Changes in agricultural practices can reduce the reproductive output of farmland birds in a variety of ways. Breeding habitat can be lost as a result of the drainage and conversion of grasslands for cultivation (Vickery et al. 2001, Wilson et al. 2005). Intense grazing regimes can reduce nest cover and increase nest predation rates (Chamberlain and Crick 2003). Increased herbicide and pesticide use can reduce food availability (Rands 1985, Taylor et al. 2006) and reduce the number of breeding attempts and breeding success (e.g., Browne and Aebischer 2004, 2005). Increased use of fertilizers also allows earlier and more frequent haying, which reduces the nesting success of ground-nesting birds (Green and Stowe 1993, Schekkerman et al. 2008). Range contraction and population declines of Barn Owls (Tyto alba) in Europe and North America have also been attributed to changes in agricultural landscapes that reduce both adult survival and reproductive output (Bunn et al. 1982, Colvin 1985, Taylor 1994, Toms et al. 2001, Ramsden 2003). For example, increases in road networks are associated with reduced survival of Barn Owls because the owls hunt on grassy verges along roads, making them vulnerable to collisions with vehicles (Ramsden 2003, Preston and Powers 2006, Boves and Belthoff 2012, Borda-de-Água et al. 2014). Increased use of second-generation anticoagulant rodenticides, which are lethal to Barn Owls, may also contribute to population declines by poisoning adults and young (Newton et al. 1990, Albert et al. 2010). Agricultural intensification reduces the number of potential breeding sites by removing wooden barns and old trees (Taylor 1994, Ramsden 1998). Finally, conversion of grasslands used by Barn Owls for foraging is linked to lower breeding success (Colvin 1985, Butet and Leroux 2001). In the Fraser Valley of British Columbia, Canada, 30% of the nest sites used by Barn Owls in the 1990s are no longer available because old buildings were removed or replaced with structures unsuitable for owls (Hindmarch et al. 2012). The occupancy of remaining nest sites is negatively affected by the presence of highways (Hindmarch et al. 2012). The composition of the agricultural landscape could also influence the breeding success of Barn Owls in this region. Breeding success may vary with the amount and location of grasslands within a home range, because these are expected to influence prey availability. Breeding success could also vary with the amount of urban cover or roads within a home range. Finally, the amount of grassland, urban cover, or roads could influence breeding success by altering the composition of the prey community. Barn Owls are vole specialists, and breeding success increases with the proportion of voles (Microtus spp.) in the diet (Gubanyi et al. 1992, Taylor 1994). We investigated the influence of landscape composition on fledging success of Barn Owls in the Fraser Valley. We described the landscape composition surrounding nest sites occupied by Barn Owls, quantified their diet composition during the nestling period, and monitored fledging success (number of young fledged per nesting attempt). We examined the relationship between landscape composition and (1) fledging success, (2) brood condition (the mean standardized asymptotic mass of chicks in a brood), and (3) diet composition (the proportion of prey biomass consisting of voles). We also assessed whether landscape effects on fledging success were a consequence of landscape effects on clutch size and/or brood reduction. We monitored the breeding and diet of Barn Owls within the municipalities of Delta and Surrey, an area of 681 km2 within the Fraser Valley, British Columbia, Canada (49°8′0″N, 122°18′0″W; Figure 1). Our study area is bounded by the Fraser River to the north, the U.S. border to the south, Georgia Strait to the west, and the municipality of Langley to the east. Agricultural land in the Fraser Valley was traditionally used for pasture and hay production. Over the past 50 yr, first vegetable production and then berry production have increased (Elliott et al. 2011). These changes have resulted in a net loss of grasslands (Hindmarch et al. 2012). Map showing the study area in the Fraser Valley, British Columbia, Canada. Dark shading indicates the study area in the municipalities of Delta and Surrey located south of the city of Vancouver. Breeding pairs were located by surveying all known sites documented as being occupied by Barn Owls in the 1990s (Andrusiak 1994), all old wooden barns or other tall structures with suitable openings near the roof and single, and old standing trees on farm properties. Pairs were monitored in 2007 and 2008 from early March until the end of August, when the majority of breeding occurs in British Columbia (Campbell et al. 1990). We obtained data on the breeding success of pairs occupying 40 nest sites (2007, n = 29; 2008, n = 34); 23 nest sites were monitored in both years. No pairs raised >1 brood at a nest site during the 6-mo observation period. Sites were visited every month at the start of the season, every 10–15 days after clutches were initiated, and every 6–10 days from hatching until chicks were 45–55 days old. To minimize disturbance, we visited at night until the females no longer roosted with the chicks. Visits ceased when the oldest chick in the brood was ~55 days, so that chicks would not fledge prematurely (Smith et al. 1974, Bunn et al. 1982). Where the condition of the building allowed, nests were accessed during each visit and any chicks present were weighed. Prior to banding, chicks within each brood were identified by marking them on the back of their heads with nontoxic colored paint. A blood sample (100 μL) was collected by puncturing the brachial vein when chicks were ~35 days old. Where possible, we recorded clutch size, brood size at hatch, and brood size at fledging. When first measured, nestlings were aged on the basis of their mass in one of two ways. If nestlings weighed ≤20 g, they were considered to have hatched within the past 24 hr (Howell 1964, Rich and Carr 1999). Nestlings that weighed >20 g were aged using the predicted age–mass relationship for chicks. We determined this relationship using a logistic growth curve estimated from 11 nestlings monitored from hatching (for details, see Hindmarch 2010). The growth of Barn Owl chicks can be approximated using a logistic growth curve, which reaches an asymptotic mass at ~40 days (Ricklefs 1968, Taylor 1994, Durant and Handrich 1998). To estimate chick size and quality at fledging, we estimated the asymptotic (maximum) mass for each chick that survived to fledging and was measured ≥4 times over the nestling period (mean = 6), by fitting a logistic growth curve. We determined the sex of 23 nestlings using a molecular technique based on polymerase chain reaction. Molecular sexing was conducted using the primers P2 and P8, which bind to the Z and W chromosomes (Griffiths et al. 1998). Female nestlings had a higher asymptotic mass than males (females: mean = 539.3 g, 95% confidence interval [CI]: 503.7–575.0 g, n = 12; males: mean = 474.8 g, 95% CI: 443.0–506.7 g, n = 11). The remaining nestlings (n = 106) were assigned a sex on the basis of this variation in asymptotic mass. One chick had an asymptotic mass that fell between 503.7 and 506.7 g; it was sexed on the basis of its relative size within the brood. We calculated a standardized mass score for each chick using the deviation from the mean sex-specific asymptotic mass during the nestling period divided by the sex-specific standard deviation. The mean score for all the chicks within each brood was then used to determine the condition of a brood. We collected regurgitated pellets from nest sites visited every 6–10 days during the late incubation and nestling periods (March–August, 2007–2008). Prey items in each pellet were identified using bone remnants, and the number of individuals of each prey type was determined by pairing each skull with the correct numbers of ischia, left and right mandibles, and tibiae–fibulae; or, in the case of birds, of each skull with sternum, gizzard sac, and feet. The remaining bones contained within the pellet were assembled to determine the minimum number of additional individuals whose skulls may have been crushed. We used the proportion of prey biomass consisting of voles as an index of diet composition (Otteni et al. 1972, Meek et al. 2009) for each breeding attempt where we had collected ≥ 16 pellets during ≥4 visits to the nest site (2007, n = 18, 2008, n = 21). Information on body mass for the different species of prey was obtained from British Columbia field guides on rodents, lagomorphs, and birds (Nagorsen 1996, Sibley 2003, Nagorsen 2005). We created digitized data of land use in Delta and Surrey using from on and and land use were obtained from a for the study all to a minimum land area of 2008). on and crop cover in Delta were obtained from an field from 2007 that contained on crop A data was not available for Surrey, so we created a data by fields for land use or crop type and these data (n = Finally, data on and roads within the study area were obtained from a 2007 British Columbia road We these data using to of the agricultural landscape within a of each nest We used a each site because this in an area that the home range of a Barn Owl Taylor 1994, and 1995). The were (1) area of (2) area of other (3) number of a is any area of to these area of urban cover, of and of roads (for and additional of see and Hindmarch 2010). The of roads within a of a nest site was with the of highways = and the area of urban cover = so this was from We to cover and that were or from in to minimize the number of in the of landscape assessed within the estimated home range from the of Barn Owls in the Fraser Valley, British Columbia, Canada. of landscape assessed within the estimated home range from the of Barn Owls in the Fraser Valley, British Columbia, Canada. We calculated standardized of fledging success number of young fledged per nesting brood clutch size, brood of hatched young that to and diet composition of prey biomass consisting of for each by the mean and by the standard deviation for each to For nests monitored in both we then used of these standardized in We the data in because we had a sample of nests with associated landscape data (n = 40 nest than of nests with diet composition data (n = nest we used the to the relationship between each of landscape cover, urban cover, and and success, brood clutch size, and brood We not brood size as a in variation in brood condition because was no relationship between brood size and brood condition (Hindmarch 2010). we used the to the relationship between each of landscape and diet composition at Finally, we examined whether variation in fledging success, clutch size, brood and brood were described by that a landscape a diet composition or both a landscape and a diet composition was landscape The was between urban cover and highways = landscape effects on fledging success, brood clutch size, brood and diet composition a each a and a n = the relative of landscape effects and diet composition in variation in fledging success, brood clutch size, and brood a the landscape it had more than the in first a with a diet composition where was of landscape a with both a landscape and a diet composition n = We used an to and the within each and We calculated Information for small sample and for each with within of the and with were considered to have strong and We calculated the of their standard and 95% using all in the 95% confidence and standard for when across We used the and the standard and confidence to the of the 2010). were conducted in using the We all as habitat on of the area within a of Barn Owl nest sites other cover an additional The remaining habitat and and and and was variation in the amount of urban cover, and of highways and roads within of the nest sites used by Barn Owls of composition of foraging habitat within a of Barn Owl nest sites in the Fraser Valley, British Columbia, Canada (n = 40 For a of land use at occupied and see Hindmarch et al. of composition of foraging habitat within a of Barn Owl nest sites in the Fraser Valley, British Columbia, Canada (n = 40 For a of land use at occupied and see Hindmarch et al. The fledging success of Barn Owls at the 40 monitored nest sites was fledged young per nesting attempt success was higher in 2007 n = than in 2008 n = = = fledging success as the amount of urban cover increased The the urban cover the of the and the estimate for the urban cover had 95% that not and other landscape than the and the for the remaining landscape had confidence that bounded and between urban cover within a of a nest site and fledging success of Barn Owls in the Fraser Valley, British Columbia, Canada km2 of urban cover of the area within a of the success number of young fledged per nest was standardized by the mean and by the standard deviation for any and the predicted relationship 95% from the in the success = urban of for the relationship between landscape composition and standardized of fledging success number of fledged young per nest brood clutch size, brood of hatched young that to and diet composition of prey biomass consisting of of Barn Owls breeding in the Fraser Valley, British Columbia, Canada. For and of see and The for all and the are = number of n = sample size, = between the of each and the with the and = for that of for the relationship between landscape composition and standardized of fledging success number of fledged young per nest brood clutch size, brood of hatched young that to and diet composition of prey biomass consisting of of Barn Owls breeding in the Fraser Valley, British Columbia, Canada. For and of see and The for all and the are = number of n = sample size, = between the of each and the with the and = for that standard and 95% confidence for all in the relationship between landscape composition and standardized of fledging success, brood clutch size, brood and diet composition of Barn Owls breeding in the Fraser Valley, British Columbia, Canada. For and of see and standard and 95% confidence for all in the relationship between landscape composition and standardized of fledging success, brood clutch size, brood and diet composition of Barn Owls breeding in the Fraser Valley, British Columbia, Canada. For and of see and condition (mean standardized asymptotic was to the landscape composition surrounding a Barn Owl The in this the of the landscape that the highways of the landscape all bounded Barn Owl clutches contained n = at nests monitored in both were in 2007 and 2008 = = clutch size not vary with landscape The was the in the no with landscape strong of the landscape all bounded was on of hatched young to fledge (n = of brood not vary between at nest sites monitored in both 2007 and 2008 = = of brood increased as the amount of urban cover increased The the urban cover more than the of the The estimate for the urban cover had 95% that not and The remaining all than the and for the remaining landscape all bounded and between urban cover within a of a nest site and standardized of brood in Barn Owls in the Fraser Valley, British Columbia, Canada. (the proportion of hatched young that to was standardized by the mean and by the standard deviation for the and the predicted relationship 95% from the in the = urban Barn Owls a variety of a of different species were in the pellets collected during the field voles (Microtus also voles were the the proportion of prey biomass consisting of voles was n = The proportion of voles in the diet was between = n = was higher in 2007 than in 2008 = prey species in pellets and and and and and species and was that the landscape composition surrounding a nest site diet composition during the breeding The the of the which the urban cover of the other landscape strong of all the landscape also had confidence that bounded was to that landscape effects on fledging success and brood were due to in diet In the variation in fledging success and brood that the diet composition had than the the landscape (the urban cover had and times more than the the of the and and times the of that also the diet composition was also that diet composition clutch size and brood In variation in clutch size and brood the times and times the of with the diet composition of for and effects on standardized of fledging success, brood clutch size, and brood of Barn Owls breeding in the Fraser Valley, British Columbia, Canada. landscape in which is of landscape effects We present the for all in each and are in and of for and effects on standardized of fledging success, brood clutch size, and brood of Barn Owls breeding in the Fraser Valley, British Columbia, Canada. landscape in which is of landscape effects We present the for all in each and are in and Barn Owls, as a in are predicted to be of changes in quality 2001, et al. Agricultural intensification and urbanization in the Fraser Valley of British Columbia, as in many of the to the loss and of pasture and other the of the landscape to species Barn we not a between the fledging success of Barn Owls and the amount of within their home range. The amount of urbanization was the landscape of Barn Owl fledging success and the of brood reduction. In the landscape of the Fraser Valley, the of urbanization to be a of the of Barn Owl populations than the area of that foraging Barn Owls could be negatively affected by the amount of and land within their home range because urbanization reduces the quality or of prey in surrounding foraging the proportion of prey biomass consisting of been argued to be an of diet quality for Barn Owls and and Taylor 1994). In the Fraser Valley, we that Barn Owl fledging success and of brood were negatively affected by the amount of urban cover within their home range, that individuals breeding in with more urban cover not have a lower proportion of voles in their Barn Owls a diet voles to their of the amount of urbanization within their home range. fledging success was reduced at sites with more urban cover because these sites had higher of brood reduction. in birds is by food availability et al. et al. that urban cover reduced fledging success by the of brood at a nest site not influence diet data that landscapes the availability of also increases the of wildlife to a range of For example, are used to and the second-generation rodenticides, in have been to result in poisoning of owls in the Fraser Valley and et al. et al. 2012, et al. 2012). the higher of to could increase adult or chick in Barn we have not measured in these Barn Owls of that this is to have reduced chick survival fledging success in are in that spp.) and (Elliott et al. of which a of the Barn diet et al. 2000, Hindmarch 2010). within was and was with size and hatching which that food rather than reduced fledging success (Hindmarch 2010). to and other are to to increase in the Fraser Valley, and the on Barn Owls and other (Elliott et al. 2011). is associated with increases in and are to the composition of the in to urban et al. for example, that the of was reduced in grasslands where habitat of the we that urbanization not influence the proportion of voles in the which that owls are to and their for voles within a landscape and any effects of urbanization on the prey may be by the of Barn Owls to as adult that are in urban (Taylor 1994). in because Barn Owls a of A consequence of urbanization is increased to Barn Owls are vulnerable to traffic because they hunt in near roads or highways (Ramsden 2003, Preston and Powers 2006, Boves and Belthoff 2012). We predicted that increased to roads would be associated with reduced fledging success of Barn Owls this to higher rates of to the of et al. we no relationship between fledging success and the of highways within the home range of breeding of breeding adults may be the of may have the remaining to their brood on in Barn Owls breeding at sites with increased to highways also clutches than birds at sites (Hindmarch 2010). sites with increased traffic are to be occupied and higher et al. Hindmarch et al. sites may be occupied by breeding owls that clutches than more experienced birds et al. 2011). The and location of habitat increase the and availability of small and be with the breeding success of Barn For example, and calculated that Barn Owls of to in In the Fraser Valley, Barn Owls at nest sites by foraging of nest sites had of within In the number or within an home range not influence diet composition or the fledging success of Barn Our are with other that that cover not influence Barn Owl fledging success (e.g., Meek et al. et al. 2011). may not with the breeding success of species because they not of the foraging If diet composition or prey is linked to an quantified habitat type (e.g., the fledging success of Barn Owls be with the availability of prey or availability is linked to habitat may not with breeding using or of habitat not the predicted with For example, et al. and et al. no between the fledging success of Barn Owls and habitat composition within home the fledging success of Barn Owls in urban and in not occupied sites contained urban and more than sites et al. all these the of for foraging Barn Owls, which that (1) to effects on breeding success to be and (2) effects be at and If landscape composition the availability of owls breeding in with or increased urban cover may in we no that any of the landscape we measured the condition of Barn Owl availability may have on brood condition because within Barn Owl brood when food is (Andrusiak 1994, Taylor 1994, et al. 1999). with this Barn Owls in study had hatching success Hindmarch brood of nestlings in of The of brood increased as the area of urban cover increased within a home range, which that in more are more food than in with urban The of brood size to food availability experienced by may the condition of Barn Owl chicks to be when vary in quality et al. 1999). landscape may influence the and of Barn Our that changes associated with rather than the amount of foraging habitat within the agricultural reduces Barn Owl breeding success, because urbanization reduces prey and is to landscape influence the foraging of Barn Owls, in to urban nest are used as a to the loss of suitable nesting habitat for Barn Owls in agricultural nest be located in with urban and from We and and the many other for their in the We for to use data on Barn Owls collected in the 1990s as of We are most to the for and to Barn Owls on their their and this would not have been for the was to by the of Canada. was by the for Fraser

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