JUST, Vol. IV, No. 1, 2016
Trent University
Urban Raptors: Owl and Hawk Adaptation to Urban
Centers
Matthew Poppleton
Abstract
This report focuses on the adaptation of raptors, specifically hawks and owls, to urban environments. Sighting
data was gathered from 3 online sources using Peterborough based data. In the city of Peterborough, the
broad-winged, red-tailed, sharp-shinned hawks (Buteo platypterus, Buteo jamaicensis, and Accipiter striatus
respectively) and the great gray and great horned owls (Strix nebulosi and Bubo virginianus respectively) are
recorded in high numbers relative to other species. The ability for raptors to adapt to urban settings is dependent
on ecological limiting factors, including prey items and habitat availability. Many owls and hawks seem to adapt
well to urban settings, however there are variances within species and local-populations. Maintaining sufficient
natural resources within urban centers is necessary to conserve healthy raptor populations.
Keywords
Ecology — Ornithology — Environmental Science
Introduction
The ecology of urban wildlife is a relatively recent research
field, which aims to provide wildlife conservationists and
managers with knowledge of how animals respond to urban
environments. This information can also be used by city
planners to build and maintain effective resources and developments to facilitate the continued use of urban centers by
animals. In this report, raptors, specifically owls and hawks,
are of interest because they are a charismatic species that
people have admired for millennia (Kaufman 2000). Urban
wildlife management is more effective when focusing on a
species that citizens want to protect. Protecting and managing
these raptor species also necessitates the protection of wildlife
that raptors depend upon, species which otherwise may be
thought of as pests, including small mammals, rodents and
birds.
This article contains a primary report and a literature review. The primary report provides sighting data on owls and
hawks within Peterborough, Ontario, and discusses the findings. The literature review discusses the current and past
literature on raptor adaptation to urban centers with respect
to habitat and diet; urban hazards and recommended management directions are also discussed. The report aims to
be relevant to Peterborough, however these implications are
relevant for other urban centers as well.
1. Primary Report: Peterborough Hawk
and Owl Sightings
1.1 Methods: Raptor Sighting Sources
I collected hawk and hawk sighting information from three
separate sources: the Cornell Lab of Ornithology and National
Audubon Society’s eBird, the Peterborough Field Naturalist’s
Christmas Bird Count (CBC), and the Atlas of the Breeding
Birds of Ontario (ABBO). This section discusses the different
sources and their methodologies.
The website eBird.com is a citizen science tool, in which
individuals, organizations, and publications with registered
online accounts can input bird species sightings from certain
locations including Peterborough (eBird.org 2016a). The bird
count and date is included with each recorded sighting, along
with options of photos and notes. Sighting totals for each
species are not available so the individual highest counts of
each species from specific dates are used instead. This source
of sightings is particularly useful for urban birds because
individuals can enter sightings from any location within the
city (eBird.org 2016a); the other sources can only record birds
from set locations within the city. An issue with eBird is
that although historical dated sightings are available, active
citizen science records only begin around 2012-2013, creating
a disproportionate amount of recent sightings to past sightings
(eBird.org 2016b; 2016c; 2016d).
Another source of data are The Peterborough Field Naturalist’s (PFN) Christmas Bird Count (CBC), a local chapter
of The National Audubon Society’s data collection (Peterboroughnature.org 2015a). The sighting data is recorded by
registered groups of volunteers located at separate locations
along a 24 km diameter within Peterborough County during
one day in December (Peterboroughnature.org 2015b). It
should thus be noted that each year records one day’s worth
of sightings. Online data is available from the Audubon Society from 1900-2014 (Netapp.audborn.org 2016). I totalled
sightings over the past 20 years.
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 2/12
The Atlas of the Breeding Birds of Ontario (ABBO) record
uses a point count system in which birds are counted within
specific regions. The dataset includes information on breeding
evidence and various point count indices, and so the methodology may be more systematic than the CBC and eBird; however, I did not find the actual methodology used. ABBO’s data
is more limited by time than the other two sources because
it only has data available from 1981-1985 and 2001-2005
(Birdsontario.org 2016). Also, ABBO and the CBC may
be more representative of species within specific areas than
eBird, because they show trends for birds appearing within
set locations, however, they have limited timescales. eBird
is not restricted by time, since the birds can be located anywhere within Peterborough at any time, and the boundaries are
much larger than the plots of the CBC and ABBO. Since each
of the sources used have different timescales and recording
methodologies, the counts cannot be totaled to show anything
in particular other than the relative number of owl and hawk
sightings in Peterborough.
1.2 Sighting Data and Discussion
Before synthesizing the results, it needs to be understood that
all of the recording methods only demonstrate the number of
birds that can be recognized by the recorders. Sightings are
likely influenced by recognizable traits, spatial and temporal
flight patterns as well as and bird calls produced noise. Due
to these issues, the sighting data can only demonstrate the
presence and absence of certain species; presence confidence
should increase with higher numbers because they are less
likely to be mistakes (Table 2; Table 3). The data may also
be used as a rough estimate of urban abundance based on the
count of each species, however the above issues need to be
taken into account (Table 2; Table 3). Additionally, information of the relative amount of raptor urban use and degree
of use from Hager (2009) is included. Table 3 compares
sighting trends across Peterborough, Toronto and Waterloo.
Waterloo is selected due to similarities of population size and
area to Peterborough (Table 1); there are no other urban centers in Ontario similarly sized to Peterborough within eBird’s
database (Statcan.gc.ca 2016a; 2016b). Toronto is included
due to its massive land area and population, and its proximity
to Peterborough (Table 1) (Statscan.gc.ca 2016c).
Comparing the eBird highest counts from Peterborough
with Waterloo and Toronto shows some species trends: the
broad-winged hawk (Buteo platypterus) is the bird with the
highest hawk count for all three regions, with notably higher
counts than the rest of the hawk data (Table 3). However,
only eBird in Peterborough shows high results for the broadwinged hawk (Table 2). The consistently high broad-winged
hawk counts may be the result of sightings during migratory
periods, in which these hawks can be found in flocks with
thousands of individuals (Allaboutbirds.org 2016a). Additionally, the red-tailed hawk (Buteo jamaicensis) has the second
highest hawk count for all three regions (Table 3), and is
the only species (owl or hawk) which has consistently high
counts from all three Peterborough sources (Table 2). Redtailed hawks have been described as “the most seen hawks in
Ontario” by Bezener (2000) which may explain the consistently high counts (p 97). The sharp-shinned hawk is the third
most recorded hawk for Toronto and Peterborough, but neither
of these counts are considerably high (Table 3). The red-tailed
hawk and sharp-shinned hawk are rated as a potential urban
resident with 3/3 points of degree of urban use, whereas the
broad-winged hawk is only rated a regular user with 2/3 points
(Table 2; Hager 2009). All three of these hawks use urban
areas for the winter, migration and breeding (Table 2; Hager
2009). The Cooper’s hawk (Accipiter cooperii) is also a potential urban resident with a 3/3 score of urban use, but does
not have high sighting counts (Table 2; Table 3; Hager 2009).
Also, the gray hawk’s (Buteo plagiatus) 2 sightings in Peterborough from the CBC are likely misidentifications, because
the gray hawk is a tropical species of southern North America
and South America (Allaboutbirds.org 2016b). The 1 Swainson’s hawk (Buteo swainsoni) sighting in Toronto may also be
a misidentification, because its normal distribution, similar to
the gray’s hawk does not extend to Ontario (Allaboutbirds.org
2016c).
The owl species sightings are all below a count of 100,
and do not follow any location trends (Table 2; Table 3). The
absence of barn owl (Tyto alba) sightings from every source
and location except Waterloo (count of 1), is likely indicative
of its endangered status (Ontario.ca 2016). The two owls with
notably higher sightings from the rest of the owls are the great
gray owl (Strix nebulosa; 83 sightings from eBird) and the
great horned owl (Bubo virginianus; 40 sightings from the
CBC) (Table 2). The great gray owl likely has high counts
because it is the tallest and has the greatest wingspan of all
North American owls and is thus relatively easy to identify
(Allaboutbirds.org 2016d). The great horned owl is also a
large owl that is also easy to identify, due to the ear-like
feathered tufts that on its head (Allboutbirds.org 2016e). The
low overall owl sightings may be more due to the difficulties of
studying owl habituation due to their quiet nocturnal activity,
migration and habitat loss (Dykstra et al. 2012; Hager 2009).
2. Literature Review and Discussion:
Urban Raptor Adaptation
2.1 Limiting Factors and Generalists/Specialists
It may seem reasonable that the human disturbances, low
vegetation levels and structures associated with urban centers
would limit the productivity and nesting sites of bird species;
however, many published studies have found that many birds,
including raptors, adapt well to urban habitats (including:
Hager 2009; Dykstra et al. 2012; Chase and Walsh 2006;
Rodewald and Kearns 2011; Stout et al. 2006; Minor et al.
1993; Boksakowski and Smith 1997; Campbell 2009).
The ability of a species to successfully adapt to urban areas
is controlled by certain ecological limiting factors. Animals
living in urban areas will typically be affected by different
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 3/12
Table 1. The population and land areas of the City of Peterborough, Waterloo and Toronto, using information from Statistics Canada’s 2011
Census.
Statscan.gc.ca 2016a; 2016b; 2016c respectively.
2011 Census Population
2011 Census Land Area
(km2 )
Peterborough
Waterloo
Toronto
78,698
63.8
98,780
64.02
2,615,060
630.21
limiting factors than in natural areas due to altered ecosystems
and resource availability (Adams et al. 2006). Adams et
al. (2006) suggest that there are generally fewer limiting
factors for wildlife within urban centers than natural habitats
due to an increased abundance of resources including food,
water and shelter, and decreased predator abundance. Raptors
are limited by both the availability of habitat for nesting and
food/prey resources, and may be generalists or specialists with
respect to both these resources (Bird et al. 1996; e.g. Rullman
and Marzluff 2014). A specialist species requires certain prey
to meet its dietary needs or certain habitat needs, while a
generalist is able to use a broader variety of prey/habitats
to meet the same needs. Prey availability is influenced by
prey vulnerability and predatory access (Rullman and Marzluf
2014). These factors shift across urban environments, and
thus shift prey availability (Rullman and Marzluf 2014). Prey
vulnerability is affected by many factors including foraging
behaviour, bird feeders, breeding status/courtship behaviours,
nest selection and overall landscape-based predation access
risks (Rullman and Marzluf 2014). Additionally, the density
of avian nesting is limited by resource density, quality and
by territorial zones from neighbouring birds – which is to say
the amount of food, nesting site availability and competition
(Newton 1986 as cited in Mannan and Boal 2000).
Rullman and Marzluf (2014) observed that prey generalist Cooper’s hawks (Accipiter cooperii) and barred owls
(Strix varia) were able to meet dietary needs within urban and
forested areas with sufficient land-cover. Thus prey access
is likely interlinked with habitat factors. Specialist animal
species are usually limited in their ability to adapt to urban
centers, due to the specific habitat and/or prey requirements
(Gilbert 1989). Campbell (2009) suggests that the potential
for specialist species to colonize greenbelts is affected by the
degree of similarity between greenbelt forest and contiguous
forests, and how recreational activities affect these species.
This would likely also apply to specialist species nesting in
urban landscapes. Thus, the similarity of urban forests to
contiguous forests may affect the degree to which specialist
species can adapt to urban habitats. Generalist species are
more likely to adapt to urban environments with less strict
boundaries. Urban areas contain a relatively low predator
abundance, anthropogenic feeding, high perching availability,
good access to water bodies along with natural land-cover areas (Campbell 2009) and thus likely attract generalist species.
2.2 Habitat/Nesting
The ability of hawks and owls to habituate within urban environments is suggested to be dependent on the populations
of species rather than species alone (Dykstra et al. 2012).
This is to say that a population of a certain raptor species may
be able to thrive within some urban centers and not others.
Thus it is important to remember when reviewing the species’
within the literature that the success of a species population
to colonize an urban area does not mean that all populations
of that species will colonize urban areas (Dykstra et al. 2012).
However, reports concerning local population adaption may
be used to provide a synthesis of avian responses to urban
environments, and more comprehensive ideas can apply to
broader regions.
2.3 Land Cover
A key factor affecting of bird habitat is the land-cover type
(the landscape type on a regional scale: grassland, forested
land, developed land, etc.). The number of bird species associated with certain habitats has been found to be higher
within Ottawa’s urban centers than greenbelts and some contiguous forestland, due to greater land-cover heterogeneity
within the urban areas (Campbell 2009). This is also the case
with raptors. Stout et al. (2006) found that red-tailed hawks
(Buteo jamaicensis) had greater nesting use of heterogeneous
habitats, which are representative of suburban areas, than homogenous habitats representative of highly developed areas.
The heterogeneous nesting habitat had over three times the
amount of unused grassland and woodland land-cover for
nesting (Stout et al. 2006). This may imply that the redtailed hawk may successfully nest within Peterborough due to
the city’s large amount of suburban areas, surrounding green
spaces and inner parks. These areas may provide sufficient
grassland and forested land-cover for red-tailed hawks and
other raptor species.
Undeveloped land-cover within urban centers can support rare urban birds (Loss et al. 2009). Thus, undeveloped
land-cover will likely support habitat-specialist raptors. The
availability of natural land-cover patches can also influence
the potential of migratory birds using urban habitats (Konze
2009). For information on managing urban raptor habitats see
Literature Review: Management Implications. Raptors seem
to thrive within high natural and urban land-cover types, as
demonstrated in western Washington (Figure 3) (Rullman and
Marzluf 2014). Thus the ability for raptors to excel within
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 4/12
Table 2. Hawk sightings in Peterborough Ontario from three separate bird sighting counts. Records are from eBird’s highest individual count
records; the Peterborough Field Naturalist’s Christmas Bird Count’s (CBC) totals over the past 20 years; the Atlas of Breeding Birds of Ontario’s
(ABBO) records from 2001-2005. Degree of opportunity is on a scale of 1-3, representing inconsistent use, regular use and potential residency,
respectively (Hager 2009). Urban activity use describes whether the bird uses urban areas for breeding (B), the winter (W) and for migration (M)
(Hager 2009). N/A is included for species that are included by Hager (2009), but are not reported to use urban centers.
Degree of urban activity and urban activity use are modified from (Table 1 as cited in Hager 2009, p 214) (Sightings are from eBird.og 2016b;
Netapp.auborn.org 2016; Birdsontario.org 2016)
Species
eBird
CBC
ABBO
Degree of Ur- Urban
ban Activity
Activity
Use
Hawk Species
Broad-winged hawk (Buteo platypterus)
Cooper’s Hawk (Accipiter cooperii)
Ferruginous hawk (Buteo regalis)
Gray Hawk (Buteo plagiatus)
Northern Goshawk (Accipiter gentilis)
Red-Shouldered Hawk (Buteo lineatus)
Red-tailed hawk (Buteo jamaicensis)
Rough-legged hawk (Buteo lagopus)
Sharp-shinned Hawk (Accipiter striatus)
Swainson’s Hawk (Buteo swainsoni)
362
3
0
2
0
3
10
0
7
0
0
69
0
0
11
0
660
7
51
0
18
1
40
0
3
11
17
0
5
0
3
3
2
1
1
3
3
N/A
2
2
Owl Species
eBird
CBC
ABBO
Degree of Ur- Urban
ban Activity
Activity
Use
Barn Owl (Tyto alba)
Barred Owl (Strix varia)
Boreal Owl (Aegolius funereus)
Eastern Screech-Owl (Megascops asio)
Great Gray Owl (Strix nebulosa)
Great Horned Owl (Bubo virginianus)
Long-eared Owl (Asio otus)
Northern Hawk Owl (Surnia ulula)
Short-eared Owl (Asio flammeus)
Snowy Owl (Bubo scandiacus)
Northern Saw-whet Owl (Aegolius
acadicus)
0
13
1
2
83
5
1
1
1
1
11
0
0
0
3
0
40
0
0
1
1
0
0
3
0
0
0
2
1
0
0
0
0
2
2
N/A
3
N/A
2
1
1
1
3
2
certain land-covers seems to be dependent on the species,
population, and land-cover type.
2.4 Species and Population Traits
Separate species sizes can also associate with separate landcover types. For example, in a study by Campbell (2009),
large birds were more commonly found within contiguous
forests than small birds. This may be due to high tree density
and low human activity (Campbell 2009). Also, generalist
species were not found in the contiguous forest areas in this
study. This may imply that larger species are generally habitat specialists. Campbell (2009) suggests that bird presences
can be well predicted using habitat types, while species presence patterns are better explained by species type. Different
communities have varying success with urban habitats and
nesting. Minor et al. (1993) found that great-horned owls
and red-tailed hawks were able to successfully nest and repro-
BWM
BW
W
B
WM
BW
BWM
N/A
BWM
B
B
BW
N/A
BW
N/A
BW
W
BW
M
W
BWM
duce equally well in urban and natural areas. Similarly, the
barred owl’s nesting habitat usage has been observed to be
unaffected by suburban developments (Dykstra et al. 2012).
This instance may be due to the owls having low interactions
with humans because of their low detectability and nocturnal
activity as mentioned in Primary Report: Sighting Data and
Discussion (Dykstra et al. 2012; Hager 2009). Coleman et al.
(2002) found no relationships between the quality of habitats
and raptor productivity, implying that cover type may be a
stronger parameter than habitat quality. Additionally, for an
urban area to be a bird nesting source, it is important to have
high reproductive success, nest site re-occupancy, and population recruitment, as observed in Milwaukee, Wisconsin, by
Stout et al. (2007).
Some species have greater selectivity for certain nesting
land-cover types. The sharp-shinned hawk (Accipiter striatus)
has been observed to nest within coniferous trees in habitats
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 5/12
Table 3. The highest recorded counts of hawk and owl populations from eBird in Peterborough, Toronto, and Waterloo, Ontario (eBird.org 2016b;
2016c; 2016d respectively).
Hawk Species
Toronto
Waterloo
Peterborough
Broad-winged hawk (Buteo platypterus)
Cooper’s Hawk (Accipiter cooperii)
Ferruginous Hawk (Buteo regalis)
Gray Hawk (Buteo plagiatus)
Northern Goshawk (Accipiter gentilis)
Red-Shouldered Hawk (Buteo lineatus)
Red-tailed hawk (Buteo jamaicensis)
Rough-legged hawk (Buteo lagopus)
Sharp-shinned Hawk (Accipiter striatus)
Swainson’s Hawk (Buteo swainsoni)
7060
18
0
0
0
40
750
1
646
1
1200
9
0
0
1
8
110
30
15
0
362
3
0
2
0
3
10
0
7
0
Owl Species
Toronto
Waterloo
Peterborough
Barn Owl (Tyto alba)
Barred Owl (Strix varia)
Boreal Owl (Aegolius funereus)
Eastern Screech-Owl (Megascops asio)
Great Gray Owl (Strix nebulosa)
Great Horned Owl (Bubo virginianus)
Long-eared Owl (Asio otus)
Northern Hawk Owl (Surnia ulula)
Northern Saw-whet Owl (Aegolius
acadicus)
Short-eared Owl (Asio flammeus)
Snowy Owl (Bubo scandiacus)
0
2
2
6
2
6
10
1
4
1
2
1
37
1
38
10
1
2
0
13
1
2
83
5
1
1
1
15
5
5
17
1
1
of low coniferous composition (Coleman et al. 2002). Also,
the red-tailed hawk has been observed to nest specifically in
well-covered and mature stands, however, this may be due to
predation threats in other land-cover areas rather than being
due to preferential habitats (Coleman et al. 2002). Former
nesting sites also influence raptor habitat, for some raptors
will use the nests of separate species. The barred owl was
observed by Dykstra et al. (2012) to frequently nest in recently
used nests of red-shouldered hawks (Buteo lineatus) within
suburban areas (Dykstra et al. 2012). Time may also be an
influencing factor, for the successful nesting sites of Cooper’s
hawks was also observed to have rates of reoccupy of periods
of over 2 generations (Stout et al. 2007). Additionally, hawks
have been suggested to behave with greater efficiency in new
nesting areas, with peak improvements occurring within the
first 1-2 years (Mannan and Boal 2000).
2.5 Fragmentation
Birds living within urban environments have been observed
to favour fragmented habitats. Great horned owls (Bubo virginianus) nesting in Connecticut, northern New Jersey, and
southeastern New York were observed to select complex fragmented habitats rather than communities in rural habitats
(Smith et al. 1999). Great horned owls were even attracted
to rural fragmented land-covers due to human developments
(Smith et al. 1999). Some raptors are known to excel within
fragmented landscapes, including forest edges and human
developed landscapes (Rullman and Marzluff 2014). This
attraction towards urbanized fragmented landscapes has been
suggested to be due to fragmented forests having greater nesting choices and forest openings than contiguous forests, and
greater prey items supplemented by urbanization (Smith et al.
1999). Similarly, sharp-shinned hawks have been reported to
nest within close proximity to forest openings associated with
human activity within and surrounding the city of Montreal
(Coleman et al. 2002).
2.6 Timescales
The age of neighbourhoods within certain landscapes may also
influence nesting. For example, old suburban neighbourhoods
(> 75 years) with dense populations have been observed to
include old trees that are likely to include cavities of sufficient
size for nesting of species such as the barred owl (Dykstra
et al. 2012). Also, old neighbourhoods in Chicago, Illinois,
have been observed to have high degrees of undeveloped landcover (Loss et al. 2009) which may provide specialist habitat.
However, population shifts have also been observed in aging
neighbourhoods, including bird species richness decreasing
with neighbourhood age, along with community composition
changes from originally native and migratory birds to exotic
and non-migratory birds (Loss et al. 2009). Thus older neighbourhoods in Peterborough may support greater diversities of
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 6/12
Figure 1. The richness of raptors within a gradient of urban and natural land-cover types within western, Washington, United states. The gray
boxes represent species presence. The species are listed from highest site frequency.
(Fig. 2. as cited in Rullman and Marzluf 2014, p 7)
owl and specialist species, however, species habitation may
also depend on the size of the neighbourhood.
2.7 Human-Built Structures
Human-built structures also effect raptor populations. Generally, developed fences, hedges and vegetative edges have
been suggested to improve the hunting ability of raptors for
prey items (Rullman and Marzluf 2014). For management
purposes, these strategies may be implemented in cases where
local raptors are detrimentally limited by prey. On a smaller
scale, it has been suggested raptor species have greater reproductive rates within human-made artificial nests than natural
nests (Chace and Walsh 2006).
2.8 Migratory Sites
The City of Toronto released a “Literature Review and Data
Assessment” of migratory birds in 2009, that likely has implications for migratory birds in Peterborough. In a broad sense,
birds may use nesting areas that have been routinely used
for generations, as suggested by this review (Konze 2009).
Additionally, many temperate and neotropical birds migrate
past Toronto on their migration routes. The proximity of Peterborough to Toronto may indicate that Peterborough is also
within the migratory range of these species. Toronto was also
suggested to be a nesting stop for migratory birds that cross
Lake Ontario; similarly Peterborough may be a reliable stop
for birds traveling along the Otonabee River (Konze 2009).
2.9 Prey/Diet
2.10 Prey Abundance
The ability of a raptor to find prey is of great importance.
Chace and Walsh (2006) state that urban areas can positively
influence the abundance of raptor prey, which then positively
influences raptor abundance (Chace and Walsh 2006). Redtailed and Swainson’s hawks (Buteo swainsoni) are examples
of hawks that likely consume these high abundances of urban
prey (Chace and Walsh 2006). Additionally, small prey are
specifically abundant in urban centers, and thus the raptors
that consume small prey are not likely to be limited by diet
in urban centres (Chace and Walsh 2006). Similarly, urban
Florida burrowing owls (Athene cunicularia floridana) have
been observed with significantly different diets from rural
Florida burrowing owls, including with high insect diets and
more variable prey items (Mrykalo et al. 2009). However, the
study only recorded from one site in both urban and rural areas, so this information may not be due to habitat differences
(Mrykalo et al. 2009). Examples of raptors which consume
small prey are the burrowing owl and screech owl (Megascops). The ferruginous hawk (Buteo regalis) and rough-legged
hawk (Buteo lagopus) are examples of raptors that consume
larger prey (Chace and Walsh 2006). Therefore, information
on rodent size and species found within Peterborough may
be useful to further the understanding of the potential of each
bird to adapt to developed landscapes.
2.11 Rodent Abundance
Hindmarch and Elliot (2015) observed rat prevalence as positively related to urban-land area at barred owl nesting sites
within southwestern British Columbia. Barred owl rat predation was so great within urban environments that rats were
rated as the most consumed prey item (Hindmarch and Elliot 2015). However, the consumption of rodents in urban
areas can put raptors at an increased risk of rodenticide exposure (see Literature Review: Urban Hazards: Disease and
Poisoning) (Hindmarch and Elliot 2015). Raptor size can
also limit preying ability within in certain developed areas.
Scavenging raptors in the Rio Negro and Neuquén provinces
of Argentina are known to use roads for scavenging, while
the large hunting raptors do not appear to use the roads. This
is suggested to be due to the relatively slow takeoff of large
raptors, which can increase the threat of roadside collisions
and hunting along roads (Lambertucci et al. 2009). In addition to this abundance of prey, urban centers typically have an
abundance of anthropogenic waste and supplementary feeding
stations which habituated animals can use as food resources
(Adams et al. 2006). Food resources in urban centers are
often distributed in concentrated locations, which results in
the distribution of animals concentrating in these areas. This
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 7/12
clumped distribution pattern is similar to natural environments
(Adams et al. 2006).
2007). This may be a major urban predation threat because of
the commonly high populations of raccoons.
2.12 Indications of High Prey Abundance
In southeastern Arizona, Estes and Mannan (2003) observed
the prey delivery rate of Cooper’s hawks to urban nests to be
about twice as much as in rural nests. A high rate of prey
delivery to nests may indicate an abundance of prey within the
respective habitat (Estes and Mannan 2003). An abundance
of readily available prey items can also be indicated by hawks
that continually deliver prey items to nestlings, while able to
vary their hunting effort to meet nestling diet requirements
(Estes and Mannan 2003). When there is a high abundance of
prey in a given site, hawk hunting locations will likely become
dependent on the location of their nests (Mannan and Boal
2000).
3. Urban Hazards
2.13 Predation on raptors
Raptor species that prey upon other raptors will likely affect
species abundance and distribution. For example, the Cooper’s
hawk is a known predator of the sharp-shinned hawk (Coleman et al. 2002). Thus, urban areas with a high abundance
of raptor-predating species such as the Cooper’s hawk may
cause reductions in the numbers of the preyed-upon raptors
such as the sharp-shinned hawk (Coleman et al. 2002). This is
not specifically relevant to the available sighting data for these
two species within Peterborough, as the sharp-shinned hawk
has higher sighting counts than the Cooper’s hawk from 2 out
of 3 of the sighting sources within Table 2 and higher sighting
counts within every city in Table 3. Nest depredation is also
relevant, for raptors within urban centers have been observed
as more likely to prey on avian nests in rural areas than urban
areas (Rodewald and Kerns 2011). Similarly, Rodewald et
al. (2011) observed separations of interrelated connections
between breeding birds and nest predators living in urban
areas. This may be due to the abundance of additional prey
and food resources for predators within urban environments.
This separation may increase successful nesting potential in
urban environments, for nestlings likely have a higher survival
potential with a lack of predation.
Supplementary food from bird feeders in particular can
attract avian wildlife to specific locations. Some raptors such
as the Cooper’s and sharp-shinned hawk may prey upon the
birds that are attracted to feeders, and thus are also attracted
to these feeders (Allaboutbirds.org 2016f). However, this is
not always the case: Roth et al. (2008) found no significant
difference in predation from Cooper’s hawks at bird feeders.
Feeders can also be detrimental to the birds that use them
and raptors (see Literature Review: Urban Hazards: Feeding
Stations).
Nests within urban areas may also encounter predation
from other urban adapted animals such as raccoons. Raccoons
may be an undocumented predator to raptor species and their
eggs, as they use similar habitats to Cooper’s hawk nesting
sites and prey upon similarly sized eggs and birds (Stout et al.
3.1 Collisions
While urban centers seem to be beneficial in terms of prey
and habitat, they also affect mortality rates and can be hazardous to resident raptor species. Potential hazards include
vehicular and window collisions, built structures, artificial
lighting, roads and diseases. In a comprehensive review of
86 articles, Hager (2009) discusses urban hazards to raptor
populations. The main hazards were window and vehicle collisions and electrocutions; the collisions and electrocutions are
substantial hazards in both urban and natural environments.
Lighting from human developments can also cause mortalities. In 1996, the Fatal Light Awareness Program (FLAP)
stated that Toronto had an annual mortality rate of 732 deaths
due to artificial lighting (Ogden 1996). Light towers are also
a major cause of bird mortalities, 6.8 million tower-related
bird kills annually are estimated within United States and
Canada (Longcore et al. 2012). Thus lit antenna towers and
other structures within Peterborough may cause mortalities.
In addition, rural land changes such as land-expansion can put
pressure on wildlife by limiting or removing habitats (Neave
et al. 2000). Developing forested land-cover within urban centers would have the same negative effects on nesting raptors;
positive changes would likely occur with park restoration, or
forest planting.
3.2 Roads
Raptors use roads and their surrounding structures for foraging road kills, hovering and hunting prey (Hager 2009).
Roads can also disturb bird populations through noise levels,
visual disturbance and prey concealment (Bautista et al. 2004).
However, in terms of vehicular collisions, Hager (2009) found
that moralities were not significantly different between urban
and natural environments. Road disturbance to raptor species’
may occur on weekly cycles depending on traffic variations,
because scavenging raptor road-occurrence has been observed
to decrease during weekends with high traffic loads (Bautista
et al. 2004). This may have implications for Peterborough
during summer weekends, when tourism is likely highest.
3.3 Feeding Stations
Bird feeding stations can cause issues with urban-living bird
species. Although these stations provide a concentrated and
consistent source of food, the feeders can directly and indirectly cause mortalities. Feeders located near windows can
attract birds and can cause window collision mortalities (Klem
1990). Raptors are also susceptible to window collisions for
they may prey on birds located at these feeders (Boal and
Mannan 1999). Hunters have also been found to use feeding
stations to attract raptor species; this may occur at rates higher
than currently recorded if the hunting is illegal (Boal and
Mannan 1999). Also, feeders have the potential of exposing
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 8/12
birds to Trichomoniasis (Davidson and Nettles 1988 as cited
in Boal Mannan 1999) which may transfer to raptors that feed
on these birds (see Literature Review: Urban Hazards: Disease and Poisoning below) (Friend and Franson 1999; Boal et
al. 1998).
3.4 Disease and Poisoning
Urban adapted raptors may be required to cope with various diseases, including Trichomoniasis. Trichomonas gallinae (Trichomoniasis) is an endemic parasite of which doves
(columbidae) are the foremost host (Stabler 1954 as cited in
Estes and Mannan 2003; Lawson et al. 2011). This can be
passed on to raptor species that prey upon small birds such
as doves. Ultimately, the spread of Trichomoniasis depends
on the degree to which the hosts are present in a raptor’s
diet (Boal and Mannan 1999). Another danger is the infusion of seeds with organophosphates as a poison control of
feral pigeons; this similarly puts raptors that consume these
pigeons at risk of organophosphate poisoning (Boal and Mannan 1999). In addition, as noted above (Literature Review:
Prey/Diet: Rodent Abundance), there can be high abundances
of small prey such as rats within urban environments (Chase
and Walsh 2006; Hindmarch and Elliot 2015). This abundance
would increase the risk of raptors becoming exposed to rodenticides (such as anticoagulant rodenticides [AR]) through
secondary exposure; rats were the main pathway for AR to
barred owls exposure from a study in southwestern British
Columbia (Hindmarch and Elliot 2015).
3.5 Human Communities
Raptors within urban and suburban environments can be noticed and appreciated by neighborhood communities (Boal
and Mannan 1999). However, there have been instances of
individuals being intentionally hit by Cooper’s hawks in Tucson, Arizona, due to nest protection and defensive behaviours
(Boal and Mannan 1999). Aside from concerns of injuries,
the potential threats of these nests within human communities
can cause humans to mistrust raptor nests (Boal and Mannan
1999). Thus, educating the communities with an ecological
understanding of this behaviour should positively influence
local opinions (Boal and Mannan 1999).
4. Management Implications
4.1 Land-cover
To effectively manage for each species, the biology of the
species, its population and limiting requirements should be
understood. In terms of land-cover, it is recommended by
Campbell (2009) to plant a variety of forest densities for
human tolerant and non-tolerant bird species. Similarly, Coleman et al. (2002) recommends that forest stands within urban
centers should be considered raptor habitats and thus be conserved. Additionally, fragmented forest patches within an
urban landscape benefit generalist raptors, and thus should
be conserved (Rullman and Marzluf 2014). For species with
low human tolerance and/or specialist requirements, a sufficient density of undeveloped patches can be estimated with
an understanding of each species’ average flight and alert
distances (Campbell 2009). Additionally, fenced corridors
between undeveloped patches may increase the potential of
colonization of species with low human tolerances (Campbell 2009). Shrub patch size and proximity to roads are also
important land features to consider when planning because
of their relevant consequences of human activity (Campbell
2009). Loss et al. (2009) suggest that natural land-cover
patches can increase avian richness within populations over 2
km away on average, and within urban landscapes composed
of less than 20% natural land-cover patches (including park
and undeveloped land) (Loss et al. 2009). There are 102
parks with varying sizes and levels of forest cover distributed
throughout Peterborough, which are generally within 1-3 km
of each other, and thus may increase suitable raptor habitat
(Figure 4).
4.2 Species size
Urban forests and parks can be managed to provide habitat
for larger birds that are not normally associated with urban
areas. These birds will likely require large areas of contiguous
forests (Campbell 2009). Large raptors sighted in Peterborough include the great horned owl, the great gray owl (Strix
nebulosa) and the red-tailed hawk. Larger birds may require
isolation within these areas, this can be achieved with high
density forests and by fencing off certain areas (Campbell
2009).
4.3 Diseases
In managing for bird feed related diseases such as Trichomoniasis, Mannan and Boal (1999) recommend the regular cleaning of feeders, “dove-proof feeders”, and the general avoidance of attracting doves to the feeders (p 82). There are
available treatments for Trichomoniasis, however, they usually result in bacterial infections or mortalities even when
successful, and thus euthanizing infected individuals has been
recommended (Boal and Mannan 1999).
A potential alternative of rodenticide use is to use raptors as a natural pest control (Hungeryowl.org 2016). This is
also a potential alternative to the use of organophosphate poisons. For example, owls may be encouraged to act as natural
rodent pest management by building owl boxes for nesting
(Hungryowl.org 2016). Community groups and organizations
within Peterborough could advertise and create incentives to
implement these within Peterborough. However, in order for
this to be effective, poison use would first need to be reduced
to prevent exposure, and raptor habitats may require proper
management of land-planning.
4.4 Planning
Management should focus on specific specialist species, for
generalist species are less likely to require management due to
their ability to adapt to a greater range of habitats. Within generalists and specialists, it seems that individual species have
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 9/12
Figure 2. Park locations within the City of Peterborough, Ontario, Canada. (For an enlarged map, see Peterborough.ca 2016)
Urban Raptors: Owl and Hawk Adaptation to Urban Centers — 10/12
their own requirements for habitat and prey. Therefore, urban managers focused on raptors should manage for specialist
species that are affected by the most limiting factors. Additionally, planners may focus on increasing the available habitat
for endangered species such as the barn owl, however, this
would expose them to potentially threatening urban hazards
(see Literature Review: Urban Hazards) (Ontario.ca 2016).
Managing for specialist species may create an umbrella of protection, by providing a greater amount of available resources
for less limited specialist species and generalist species. In
other words, managing for specialists would likely improve
resource availability for low abundance raptor populations.
However, there is insufficient sighting information for Peterborough to determine which species are of high or low urban
abundance. Therefore a systematic evaluation of urban avian
wildlife abundance and distribution within the city would be
useful for this purpose.
5. Conclusion
The ability of raptor species to adapt to urban environments
seems to be dependent on resource availability, commonly in
terms of habitat and diet requirements and their generalist or
specialist requirements with respect to these resources. Generalist species are commonly better able to adapt to urban
environments, while some specialist species can adapt with
the availability of specific habitat, and prey. Urban center
resources can differ from natural areas through factors such
as supplementary feeding and prey abundance (Smith et al.
1999; Adams et al. 2006; Hindmarch and Elliot 2015), builtstructure influences (Hager 2009; Rullman and Marzluff 2014;
Longcore et al. 2012), decreased contiguous forest cover,
natural predators (Campbell 2009; Adams et al. 1006) and
increased land-cover heterogeneity (Campel 2009).
The Peterborough sighting counts (Table 2 and 3) are limited in their ability to accurately demonstrate distribution or
abundance; however, they are important tools for understanding the presence of specific species. The broad-winged and
red-tailed hawks had the highest counts of all recorded species,
while the great gray and great horned owls had slightly higher
counts than other owls. Additionally, there was only one
sighting of the endangered barn owl (Ontario.ca 2016). It
would be useful to have an intensive record of bird species
abundance and distribution in addition to these sightings for
Peterborough and other urban centers.
With respect to the reviewed literature, many studies have
been done in the United States on raptors in urban centers,
however, few have been conducting regarding Canadian urban
centers. Also, many of the studies focus on how specific
species responded to urban habitat/prey availability. This is
generally useful information, however, the degree to which
these species will show similar trends between separate populations and other urban centers is uncertain. This uncertainty
arises because the degree to which a species’ response to urban factors may ultimately depend on the traits of a specific
species population rather than the traits of a species in general
(Dykstra et al. 2012).
This report demonstrates general responses of raptors to
urban areas, and it would be useful for other studies to include
broader implications on urban responses to seek out more
generalized trends. Local results are useful in respect to the
local management of species but may limit the usability of
their results on a broader scale. A generalized understanding
of raptor response to urban areas is also useful, for it provides
resources to regions distant from the original study locations.
The available information on raptor adaptation should be used
by wildlife managers, researchers and policy makers to improve and continue our shared use of urban centers with these
species.
Acknowledgments
This paper was originally for David Beresford’s Environmental Seminar Course (ERSC-4730).
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