Productivity, Mortality, and Management of Urban Peregrine Falcons in Northeastern North America

Abstract

The peregrine falcon (Falco peregrinus) population in eastern North America has grown significantly since the early 1980s, especially in urban areas, but few studies have assessed the factors that influence productivity. We reviewed all documented nesting attempts from southern Ontario, Quebec, Massachusetts, New Jersey, and Pennsylvania from 1980 through 2006 to evaluate these factors. Of 801 nesting attempts, 663 were successful, producing a total of 1,613 young. Mean productivity ranged from 1.7 young fledged per nesting attempt in New Jersey to 2.9 in Quebec. Peregrines nesting in quarries or on buildings had higher productivity than those using marsh towers or bridges, but productivity did not differ overall between urban and rural sites. Nests with overhead cover had higher productivity than those without, as did nests in trays or boxes compared to sites without any human-provided nesting aids. Peregrines favored nest sites facing east to south, but productivity did not vary significantly with direction. Several adults have contributed disproportionately to the growth of the eastern population, with just 5 females and males accounting for 8% and 9% of all young fledged, respectively. Of 160 documented mortalities, we identified cause of death for 118, with the most common being collisions with buildings (36%), vehicles (9%), aircraft (8%), and power lines (8%). In many urban areas, grounded fledglings are rescued and returned to higher perches. Of 85 individuals from southern Ontario that were rescued, at least 8 have subsequently bred, producing 65 known offspring. Although peregrines have been thriving in eastern cities, continued management effort may be required for them to maintain their level of success, with key measures including provision of appropriately located nest boxes and rescue of grounded fledglings. © 2014 The Wildlife Society.

Full text

Research Article
Productivity, Mortality, and Management
of Urban Peregrine Falcons in Northeastern
North America
MARCEL A. GAHBAUER,
1
Migration Research Foundation, P.O. Box 10005, Ste-Anne-de-Bellevue, QC, Canada H9X 0A6
DAVID M. BIRD, Avian Science and Conservation Centre of McGill University, c/o 10980 Dunne Road, North Saanich, BC, Canada V8L 5J1
KATHLEEN E. CLARK, New Jersey Division of Fish and Wildlife, 2201 Rt 631, Woodbine, NJ 08270, USA
TOM FRENCH, Massachusetts Division of Fisheries and Wildlife, One Rabbit Hill Road, Westborough, MA 01581, USA
DANIEL W. BRAUNING, Wildlife Diversity Program, Pennsylvania Game Commission, 2001 Elmerton Ave, Harrisburg, PA 17701, USA
F. ARTHUR MCMORRIS, Wildlife Diversity Program, Pennsylvania Game Commission, 2001 Elmerton Ave, Harrisburg, PA 17701, USA
ABSTRACT The peregrine falcon (Falco peregrinus) population in eastern North America has grown
significantly since the early 1980s, especially in urban areas, but few studies have assessed the factors that
influence productivity. We reviewed all documented nesting attempts from southern Ontario, Quebec,
Massachusetts, New Jersey, and Pennsylvania from 1980 through 2006 to evaluate these factors. Of 801
nesting attempts, 663 were successful, producing a total of 1,613 young. Mean productivity ranged from 1.7
young fledged per nesting attempt in New Jersey to 2.9 in Quebec. Peregrines nesting in quarries or on
buildings had higher productivity than those using marsh towers or bridges, but productivity did not differ
overall between urban and rural sites. Nests with overhead cover had higher productivity than those without,
as did nests in trays or boxes compared to sites without any human-provided nesting aids. Peregrines favored
nest sites facing east to south, but productivity did not vary significantly with direction. Several adults have
contributed disproportionately to the growth of the eastern population, with just 5 females and males
accounting for 8% and 9% of all young fledged, respectively. Of 160 documented mortalities, we identified
cause of death for 118, with the most common being collisions with buildings (36%), vehicles (9%), aircraft
(8%), and power lines (8%). In many urban areas, grounded fledglings are rescued and returned to higher
perches. Of 85 individuals from southern Ontario that were rescued, at least 8 have subsequently bred,
producing 65 known offspring. Although peregrines have been thriving in eastern cities, continued
management effort may be required for them to maintain their level of success, with key measures including
provision of appropriately located nest boxes and rescue of grounded fledglings. Ó2014 The Wildlife Society.
KEY WORDS collisions, eastern North America, Falco peregrinus, management, mortality, nesting, peregrine falcon,
productivity, rescue, urban.
The peregrine falcon (Falco peregrinus; hereafter peregrine)
was historically rare in eastern North America, with a
population of only 500 pairs east of the Rockies and south of
the Arctic (Enderson et al. 1995). By the 1950s, a rapid and
severe decline was underway, culminating in the eastern
population being considered extirpated by 1964 (Berger
et al. 1969, Bollengier 1979). Although a few peregrines
began breeding in cities as early as the 1930s, only 6 urban
nest sites were known prior to 1949 (Groskin 1952). The
growth of the urban population did not accelerate until the
1980s, following the release of many captive-bred juveniles in
cities, starting in 1974 (Fyfe 1988). Cities offer peregrines
benefits in the form of abundant prey and potential nest sites,
and scarce predators, but they also pose unique hazards such
as buildings with which peregrines may collide, a hazard that
is especially great for juveniles learning to fly around towers
with reflective glass.
Nest sites are a limiting factor for peregrines (Ratcliffe
1962) and reproductive success of many species is influenced
by the microclimate at nest sites (e.g., Wiebe 2001, Ardia
et al. 2006). In particular, temperature may affect both the
viability of eggs (Webb 1987) and the survival of nestlings
(Quinney et al. 1986). The choice of a nest site by peregrines
is likely related to a combination of physical and ecological
attributes. Hunt (1988) speculated that for peregrines,
critical nest features may include protection from weather
and predators, availability of a substrate in which a scrape
for eggs can be excavated, and suitable directional exposure.
Nest orientation has been shown to affect nest temperature
for a variety of bird species (Inouye 1976, Hartman and
Oring 2003, Ardia et al. 2006). For some species though,
reproductive success is unrelated to nest orientation (Rendell
and Robertson 1994), suggesting that this topic must be
assessed on a species by species basis.
Received: 3 January 2014; Accepted: 14 August 2014
Pubished: 12 November 2014
1
E-mail: marcel@migrationresearch.org
The Journal of Wildlife Management 79(1):10–19; 2015; DOI: 10.1002/jwmg.803
10 The Journal of Wildlife Management 79(1)

Despite substantial recent growth in the eastern population,
the peregrine remains a species of conservation interest in
much of the region; therefore, biologists are often called upon
to encourage individuals to nest in certain areas, or to lure
them away from undesirable sites (Martell et al. 2000).
Although characteristics of the cliff-nesting population in
New York and New England were described by Corser et al.
(1999), less has been published on the urban population in the
east, and there remains little understanding of the key features
that attract peregrines and contribute most to their nesting
success. Because peregrines show high fidelity to nest sites,
and favored locations are often used by a succession of adults,
understanding what can be done to maximize the suitability
of these and other sites is of great value (Pagel 1989,
Ratcliffe 1993). In the past, efforts have been made to
improve cliff habitat in an attempt to attract peregrines
(Boyce et al. 1982, Pagel 1989). In urban areas, human
assistance has been provided in various forms, including hack
releases, fosters, fledgling rescues, and nest site modifications,
but the effect of these efforts has not been evaluated.
We compiled data on nesting attempts of peregrines from
southern Ontario, Quebec, Massachusetts, New Jersey, and
Pennsylvania to describe aspects of the nesting ecology of the
eastern population, and provide recommendations for its
effective management, with an emphasis on urban sites. Our
objectives were to assess 1) the relative value of nest site
attributes with respect to influencing productivity, 2) causes
of mortality, and 3) effects of human assistance on the
population, including the degree to which such management
affects productivity and survival. Where possible, we
compared data between urban and cliff-nesting populations.
STUDY AREA
Our study area included several northeastern provinces and
states that feature a concentration of urban-nesting pere-
grines: Ontario, Quebec, Massachusetts, New Jersey, and
Pennsylvania. For Ontario, we used data only from the
southern part of the province, as northern records of
peregrines have been previously reported as part of the
Midwest population (e.g., Redig et al. 2007).
METHODS
We included all known nesting attempts in the study area,
from the beginning of the population recovery in 1980
through 2006. In Quebec, monitoring effort was variable,
with only urban sites and easily accessible cliff nests surveyed
in most years, contrasting with extensive searches of
historical and potentially suitable locations in 2002 and
2005. In the other provinces and states, survey effort was
fairly consistent through time, and we believe the vast
majority of nesting attempts were documented. Data were
collected by government agencies, non-profit organizations,
and independent volunteer observers.
Characterization of Site Attributes
We evaluated 8 nest site characteristics (Table 1). Although
habitat and structure were known for all nest sites, other
attributes were missing data for some sites. We evaluated
nest site selection in and around Toronto, Ontario, which
had the most concentrated population of building-nesting
peregrines in our study area. For each of 7 nest sites used
between 2002 and 2006, we used a random number table to
select direction and distance of 4 unused but potential
nest locations within a 1-km radius. Based on nest occupancy
data from throughout the study area, we restricted potential
sites to structures at least 15 m in height and with a
horizontal surface on which nesting could occur. Addition-
ally, to compare characteristics of used and available
urban nest sites on a regional basis, we used a random
number table to select another 28 buildings throughout
Table 1. Definition of ecological and physical attributes used to classify peregrine nest sites in Ontario, Quebec, Massachusetts, Pennsylvania, and New
Jersey from 1980 to 2006.
Attribute Class Description
Habitat Urban (downtown) Downtown core, characterized by a high density of skyscrapers
Urban (other) All other urban or suburban habitat, including quarries in an urban setting
Rural (cliff) Areas away from urban and coastal habitat, including quarries in a rural setting
Rural (marsh) Coastal salt marshes
Structure Building Office towers, apartments, smokestacks
Bridge Bridges over water
Quarry Active or inactive rock quarries
Cliff Natural rock faces
Tower Nesting towers erected for peregrines
Cover Full Nest location with full overhead protection, exposed to snow or rain only when very windy
Partial Nest location with some overhead protection, but still exposed to most precipitation
None Nest location with no overhead cover
Substrate Dirt Soil, accumulated dirt or debris
Gravel Small, loose, movable pebbles
Bare Smooth concrete or metal surface
Aid Box Nest box with sides and a full roof
Tray Nest tray without side walls or a roof
None Nothing provided for nesting
Orientation E, SE, S, SW, W, NW, N, NE Direction faced by the nest, to the nearest cardinal or intercardinal direction
Nest height <50, 51–100, 101–150, >150 Height in m, classified into 4 categories
Distance to water <1, 1–4.9, 5–20 Distance in km to the nearest major water body (ocean, lake, or river >50 m wide)
Gahbauer et al. Urban Peregrine Management 11

Toronto, from a list of all buildings meeting the aforemen-
tioned criteria.
Productivity and Mortality
We documented the number of young fledged for all nesting
attempts. For some locations, data on the number of eggs laid
and hatched were also available, but we did not evaluate
hatching success because these data were not reliably
collected at most sites. Fledging and nesting success,
respectively, refer to the percentage of young that left the
nest successfully and the percentage of nests at which at least
1 juvenile fledged. We defined mean nest productivity as the
average number of young fledged per nesting attempt at each
location. We ranked the total productivity of all nest sites and
compared the top and bottom quartiles to identify differences
in nest site attributes between locations that have made the
greatest and least contributions to the eastern population.
Additionally, to facilitate comparisons over time, we divided
the study period into temporal quartiles based on when each
site was first colonized, with the first 25% of sites from 1983
to 1994, the next 25% from 1995 to 2000, the third 25% from
2001 to 2003, and the final 25% from 2004 to 2006,
reflecting the accelerating expansion of the population to
new locations.
Data on mortality were available only for southern Ontario,
Massachusetts, and Pennsylvania, and were primarily
provided by state or provincial wildlife agencies and local
volunteer organizations monitoring peregrine nests, with
some additional records through band recoveries and satellite
telemetry. All records included age and location of death, and
sex and cause of death were also available for most. We
assessed indirect human influence on peregrine productivity
with respect to the provision of nest boxes or trays (Table 1).
Additionally, direct assistance was occasionally provided
through rescue of fledglings that came to the ground in urban
areas. Fledgling rescues were systematically recorded only for
southern Ontario, and we therefore assessed them only for
that region.
Statistical Analyses
Because the size of the recovering population was relatively
small, and some attributes were not consistently recorded at
all sites, the data were inadequate for reliable multivariate
analyses. Rather, we used Mann–Whitney Uor Kruskal–
Wallis (K–W) tests to perform univariate analyses of each
nest site attribute, as mean nest productivity was in most
cases not normally distributed. We used chi-square
goodness-of-fit tests to compare the frequency of use of
categorical nest site attributes. We used the Rayleigh test to
evaluate orientation of nest sites, and compared orientation
of used and available nest sites with the Watson–Williams
test (Zar 1999). We used SPSS 9.0 (SPSS Inc., Chicago, IL)
to perform all statistical analyses.
RESULTS
From 1980 through 2006, 1,613 young were documented
from 801 known nesting attempts at 152 nest sites in
southern Ontario, Quebec, Massachusetts, New Jersey, and
Pennsylvania (see Table S1, available online at www.
onlinelibrary.wiley.com). The number of nesting attempts
increased over time, from a mean of 9 per year in the 1980s to
a mean of 67 per year between 2002 and 2006.
Nest Site Characteristics and Productivity
Urban nest sites (n¼87) outnumbered rural nest sites
(n¼65). The mean number of young fledged per nesting
attempt was similar between urban (2.03 0.07, n¼416)
and rural (1.99 0.06, n¼385) nesting attempts (Mann–
Whitney U¼79,229, Z¼0.268, P¼0.790). Within urban
areas, fledging success varied strongly by nesting structure
(K–W x2
2¼7:71, P¼0.021); nest sites in quarries had a
mean productivity (2.83 0.44, n¼4) almost twice as great
as those on bridges (1.53 0.19, n¼35), whereas produc-
tivity at building nests was intermediate (2.10 0.17, n¼48;
see Table S1, available online at www.onlinelibrary.wiley.
com). We did not find a difference in productivity between
nests on downtown buildings (2.10 0.20, n¼22) versus
those on buildings in less intensively developed urban habitat
(2.09 0.28, n¼26; Mann–Whitney U¼270.0, Z¼0.332,
P¼0.740). The extent of overhead cover was known for 42
of 48 building nest sites. Sites with full overhead cover had
higher mean productivity (2.34 0.20, n¼30) than those
with only partial cover or no cover at all (1.69 0.40,
n¼12), but results were too variable for the difference to be
significant (Mann–Whitney U¼129.5, Z¼1.41, P¼0.158).
Nesting substrate was identified for 60 of the urban nests.
Productivity was greatest at nests with dirt or debris
(2.72 0.46, n¼6), intermediate at nests with gravel
(1.93 0.16, n¼48), and lowest at nests on bare concrete
or metal ledges (1.32 0.44, n¼6). However, the difference
was not statistically significant (K–W x2
2¼4:12, P¼0.127).
On bridges, fledging success was low (<1.6 young fledged
per nesting attempt) regardless of whether or not aid was
provided in the form of a nest tray or box (K–W x2
2¼0:37,
P¼0.832; see Table S2, available online at www.onlineli-
brary.wiley.com). On buildings, fledging success was
somewhat higher at sites with boxes (2.83 0.11, n¼7)
or trays (2.14 0.34, n¼14) than those without either
(1.87 0.25, n¼24), but results were again somewhat
variable (K–W x2
2¼4:01, P¼0.135), although the mean
productivity at nests with boxes was greater than at all other
locations combined (Mann–Whitney U¼74.5, Z¼1.84,
P¼0.066).
Although 57% of nest sites on buildings faced east to south
(see Table S1, available online at www.onlinelibrary.wiley.
com), overall nest orientation was statistically random
(Rayleigh test, Z¼1.41, n¼42, P<0.2). Productivity
differed by orientation, being greatest at nests facing
southeast (3.41 0.13, n¼4) and lowest at those facing
northwest (0.72 0.72, n¼2); however, variability was
sufficiently high to preclude a significant result (K–W
x2
7¼11:31, P¼0.126).
We had nest height data for 54 urban sites (see Table S1,
available online at www.onlinelibrary.wiley.com). Produc-
tivity was positively correlated with nest height, with the
mean nearly twice as great at buildings over 150 m high
12 The Journal of Wildlife Management 79(1)

(3.07 0.16, n¼3) compared to those under 50 m
(1.67 0.24, n¼22). However, the overall pattern was
weak due to high variability (K–W x2
3¼5:77, P¼0.123).
All bridge nests were over water, whereas all other nest sites
were set back at least a short distance from water bodies. We
calculated distance to water for all 52 urban quarry and
building nest sites (see Table S1, available online at www.
onlinelibrary.wiley.com). Although nest productivity was on
average 33% higher at nests an intermediate distance from
water (2.74 0.35, n¼18) than those within 1 km
(2.06 0.18, n¼30) or greater than 5 km away
(2.05 0.73, n¼4) there was no overall difference among
distance classes (F
2,49
¼0.085, P¼0.919; K–W x2
2¼0:27,
P¼0.874).
Nest Site Selection in Southern Ontario
Among breeding adults that were hatched and/or bred in
southern Ontario, 36 of 37 from urban nests also chose cities
for nesting, whereas the only 2 adults produced at cliff sites
moved to urban habitat. We examined nest site selection
within an urban environment in the Toronto area. Used sites
did not differ significantly from either adjacent or regionally
available potential nesting locations in terms of building
height or nest height (Table 2). Used sites were on average
almost twice as close to water as randomly selected potential
sites within the region, though the difference was not
significant. Toronto peregrines had a significant preference
for ledges with overhead cover. All used nest sites in the
Toronto area faced east, southeast, or south, whereas both
adjacent and regionally available sites were distributed
relatively evenly in all directions.
Characteristics of Preferred Nest Sites
Between 1980 and 2006, the top quartile of urban nest sites
in southern Ontario, Quebec, Massachusetts, New Jersey,
and Pennsylvania with respect to total productivity
accounted for 70% of all young fledged with a mean
productivity of 2.64 0.17 (n¼21), whereas the bottom
quartile contributed only 2% (0.55 0.16, n¼21; Mann–
Whitney U¼22.0, Z¼5.31, P<0.001) (see Table S3,
available online at www.onlinelibrary.wiley.com). Nest sites
in the top quartile of productivity were located higher on
buildings (96.2 9.9 m) than those in the bottom quartile
(63.6 9.8 m; Mann–Whitney U¼60.5, Z¼1.90,
P¼0.058), but the 2 groups did not differ significantly
with respect to distance from water (Mann–Whitney
U¼206.0, Z¼1.10, P¼0.272), substrate (x2
2¼2:32,
P>0.2), or overhead cover (x2
2¼3:20, P>0.2). However,
nest boxes or trays were used at 76% of nest sites in the top
quartile of total productivity, compared to just 23% of those
in the bottom quartile (x2
2¼12:58, P<0.002).
The northeastern peregrine population expanded greatly
between 1980 and 2006, with much of the growth
concentrated around Toronto, Montreal, Boston, and other
urban centers (Fig. 1). Among urban nest sites, half of the
known locations were occupied for the first time only since
2001 (see Table S4, available online at www.onlinelibrary.
wiley.com). Over time, the meanheight of the newly occupied
urban nest sites declined by 39%, from 93.6 11.1 m at the
first 21 nests colonized between 1983 and 1994 to
57.5 11.5 m for the most recent 21 nests established
between 2004 and 2006 (K–W x2
3¼7:54, P¼0.057).
Conversely, the mean distance of nests from water increased
over time from 0.3 0.1 km at nests established between 1983
and 1994 to 2.1 1.0 km at those initiated between 2004 and
2006 (K–W x2
3¼6:30, P¼0.098). We did not find a
difference over time in selection of sites with respect to
overhead cover because full cover has been preferred
consistently. However, we found a significant difference
over time in substrate selection (x2
6¼13:08, P<0.05), with
all nests established from 1983 through 1994 using gravel, but
only half of those started from 2004 to 2006 using gravel,
compared to 33% in dirt. Use of nest aids was also highest
early on, with 75% of the first quartile of nest sites in boxes or
trays, whereas 82% of the latest quartile of nests lacked boxes
or trays (x2
6¼22:88, P<0.001).
Some individual adults contributed disproportionately to
the growth of the population (see Table S5, available online
at www.onlinelibrary.wiley.com). Of over 350 banded or
otherwise distinctive breeders, just 5 (<3%) females and 5
(<3%) males accounted for 8% and 9% of all young fledged,
respectively. All of these individuals nested at urban sites
with a gravel substrate. Just 1 site lacked overhead cover, and
all individuals except the pair in Toronto used a nest tray or
box. Of 8 sites used by these 10 adults, 5 faced either east or
southeast. Nest height was variable, but distance to water was
short (1.1 km or less) for all but 1 site.
Table 2. Comparison of attributes of used and potential urban nest sites for peregrines in Toronto, Ontario (mean SE). Adjacent sites were randomly
selected from within a 1-km radius of used nest sites and regional sites were randomly selected from all buildings of at least 15 m height in the Toronto area.
Used (n¼7) Adjacent (n¼28) Regional (n¼28) Used vs. adjacent Used vs. regional
Mean building height (m) 96.4 9.8 107.0 9.9 85.4 7.5 U
a
¼94.5, Z¼0.145, P¼0.885 U¼65.0, Z¼1.363, P¼0.173
Mean nest height
b
(m) 89.3 9.8 95.5 8.8 75.0 6.2 U¼97.0, Z¼0.041, P¼0.967 U¼60.0, Z¼1.57, P¼0.116
Ledges present? 7 of 7 (100%) 14 of 28 (50%) 18 of 28 (64%) x2
1¼5:93, P<0.02 x2
1¼3:50, P<0.1
Full overhead cover? 6 of 7 (85%) 8 of 28 (29%) 11 of 28 (39%) x2
1¼7:62, P<0.01 x2
1¼4:83, P<0.05
Orientation 4 E, 2 SE, 1 S 2 E, 1 SE, 2 S,
1 SW, 4W, 1 NW,
2 N, 1 NE, 14 n/a
3 E, 1 SE, 3 S, 3 SW,
2 W, 1 NW, 3 N, 10 n/a
Watson–Williams test:
F
1,33
¼3.05, P<0.1
Watson–Williams test:
F
1,33
¼1.71, P>0.1
Mean distance to water (km) 3.9 0.9 3.5 0.4 7.1 1.0 U¼83.5, Z¼0.60, P¼0.549 U¼69.5, Z¼1.18, P¼0.240
a
Mann–Whitney U-test.
b
Randomly selected from available ledge heights for adjacent and regional sites, or equivalent to building height in cases where the roof provides the only
nesting opportunities.
Gahbauer et al. Urban Peregrine Management 13

Mortality
In total, 160 mortalities were reported from Ontario,
Massachusetts, and Pennsylvania, of which 11 involved
young that died in the nest prior to fledging. Mortality was
greatest for young birds, with half of all recorded deaths
occurring within the first month of flight, including 24
(15.0%) on the first day of flight, and another 20 (12.5%) in
the first week. In southern Ontario, fewer females (23 of 102,
22.5%) than males (32 of 89, 36.0%) died within their first
month of flight (x2
1¼4:17, P<0.05).
Figure 1. Pattern of expansion of occupied peregrine nest sites in Ontario, Quebec, Massachusetts, Pennsylvania, and New Jersey over 4 time periods from
1980 to 2006.
14 The Journal of Wildlife Management 79(1)

The key causes of mortality differed by age group (Table 3).
Collisions with buildings were overwhelmingly the leading
cause of mortality for fledglings, but much less common for
older birds. Collectively, collisions represented the greatest
source of mortality for juveniles, but stationary objects such
as buildings and power lines accounted for 54% fewer deaths
than aircraft and other vehicles. Territorial battles were the
most frequent cause of death for adults but were not recorded
for younger birds. Conversely, predation or attacks by other
raptors were a minor cause of mortality for fledglings and
juveniles but not reported at all for adults. Although
comprehensive mortality data were not available for New
Jersey, loss of nestlings to trichomoniasis has been such a
serious problem at some urban nests that young at 3 locations
are routinely administered medication at 1 and 3 weeks of
age.
Human Assistance
Human assistance in the form of nest trays or boxes was
provided at 30 of 152 nest sites. In urban habitat, the mean
total number of young fledged over the study period was
almost 3-fold higher at nest sites with trays or boxes
(17.1 2.7, n¼30) than those without (5.9 0.9, n¼57;
Mann–Whitney U¼474.0, Z¼3.41, P¼0.001). A more
direct form of assistance is the rescue of fledglings that have
come to the ground on one of their early flights, often putting
them at risk from traffic or other urban hazards. Statistics on
the frequency of fledgling rescues were available only for
southern Ontario. Of the 193 young fledged in southern
Ontario between 1995 and 2006, 85 (44%) were rescued a
total of 109 times. Eight (9.4%) of these individuals are
known to have subsequently survived and bred successfully,
producing a total of 65 young. In comparison, 10 of 108
(9.3%) individuals that did not require rescue as fledglings are
known to have bred successfully, producing 63 offspring.
DISCUSSION
Our survey of 152 eastern nest sites revealed no difference in
mean productivity between urban and rural habitat, nor
between downtown areas and less intensively developed parts
of cities. Most urban nests were on buildings or bridges, with
productivity on average 37% higher on buildings. Among
urban nests, productivity tended to be higher at those with
overhead cover, dirt or gravel substrate, and with nest boxes.
Although a majority of nests faced east, southeast, or south,
we did not find a consistent relationship between orientation
and productivity. Productivity was also unrelated to distance
to water, but was positively correlated with nest height. In
addition to providing nest boxes and trays, human assistance
includes rescue of fledglings from hazardous ground
conditions in cities, and individuals assisted in this manner
accounted for half of the next generation offspring produced
by Ontario peregrines. Despite this assistance, young
peregrines in cities have high rates of mortality, with
building collisions taking the biggest toll on fledglings, and
other hazards such as vehicles and power lines becoming
greater concerns after the first few weeks of flight.
Nest Site Selection and Productivity
The use of nest structures we documented in the east differs
somewhat from that in the Midwest, as summarized by
Tordoff et al. (2003) for the period 1998–2002. Buildings
represented a smaller proportion of nest sites in the east
(32%) than the Midwest (44%), whereas bridge use was more
frequent in the east (23% vs. 10%). Cliff use was similar in
both areas, accounting for 33% of eastern sites and 27% of
Midwestern sites. The biggest differences were with respect
to smokestacks, used at just 2 eastern locations but at 19% of
Midwestern nests, and 2 habitat types used only in the east,
coastal marshes (7%) and quarries (5%). Nesting on
smokestacks has been encouraged in the Midwest through
the provision of nest boxes, whereas this has rarely been done
in the east. All quarry nests in our study were in Quebec, but
use of such sites has previously been documented in Britain
(Ratcliffe 1993), Ireland (Moore et al. 1997), Australia
(White et al. 1988), and Alaska (Ritchie et al. 1998). White
et al. (1988) postulated that these were secondary quality
nests, primarily used by inexperienced individuals unable to
compete for superior territories. In contrast, the mean of 2.79
young fledged per nesting attempt at quarry nests was far
higher than on any other structure in our study.
In southern Ontario, we found that 97% of peregrines
hatched in urban habitat also bred in cities, consistent with
earlier results showing that 91% of individuals from eastern
Canadian releases returned to habitat similar to that in which
they were raised or released (Holroyd and Banasch 1990).
Only 2 cliff-hatched peregrines nested in southern Ontario
during our study, but both did so at urban locations.
Although the sample size is too small to draw any
conclusions, the result is consistent with the net movement
of eastern peregrines into urban habitat reported by Cade and
Bird (1990) and the dispersal of coastally hatched peregrines
in California to nearby cities (Kauffman et al. 2003). The
growth of the urban population therefore appears to be more
than just a consequence of habitat imprinting on young
released in cities, as had been proposed by Temple (1988).
Tordoff et al. (1998) suggested that the best nest sites are
colonized first, and as the population grows, secondary
quality sites are eventually settled. To an extent this was
evident in the east, as productivity was positively correlated
Table 3. Causes of death reported for peregrines in southern Ontario,
Massachusetts, and Pennsylvania, 1988–2006 (percentage of known causes
of mortality within each age group shown in parentheses).
Fledgling
(<1 month)
Juvenile
(>1 month)
Adult
(>1 year)
Collision: building 38 (61%) 2 (8%) 3 (16%)
Collision: vehicle 4 (6%) 5 (19%) 2 (11%)
Collision: aircraft 1 (2%) 8 (31%) 1 (5%)
Collision: power lines 4 (6%) 4 (15%) 1 (5%)
Territorial battle 7 (39%)
Other raptor/predation 5 (8%) 1 (4%)
Drowning 4 (6%)
Other 6 (10%) 6 (23%) 5 (26%)
Unknown 18 12 12
Total 80 38 31
Gahbauer et al. Urban Peregrine Management 15

with nest height, and the mean height of newly colonized
nests declined significantly over time. Distance to water also
increased over time, suggesting that prime closer sites may
have been occupied first. Similar changes are evident in
Alaska, where peregrines formerly nested only along rivers
but have expanded to more landlocked cliffs as well as to
urban areas (White 2006). However, peregrines appear to
not always evaluate site quality accurately, as over 25% of the
first 22 urban nest sites colonized in the east were abandoned
after 4 or fewer nesting attempts, and some of the more
productive locations were not settled until later.
In a review of the early years of urban colonization by
peregrines in the east, Cade and Bird (1990) found that 78%
of the 19 known nest sites were on buildings that were among
the tallest in their respective cities, averaging 30 stories, or
approximately 120 m. Redig and Tordoff (1994) likewise
noted that peregrines are attracted to the tallest structures on
the landscape, and will tend to occupy them if suitable nest
sites are available, a pattern similar to that traditionally
observed with cliffs (Tordoff et al. 1999). In comparison, we
found urban peregrine nests to have an average height of
79 m, and using the Toronto population as an example,
concluded that peregrines did not select exceptionally tall
buildings on either a local or a regional basis. Other nest
site attributes may therefore be of greater importance to
peregrines than height.
Cade and Bird (1990) also reported that 84% of the first 19
urban nest sites following recovery were within 800 m of
water. This tendency has remained fairly consistent over
time, with 79% of the 152 sites in our review falling in
the same range. Peregrines in the Midwest also show a
distribution strongly linked to the shorelines of large lakes
(Tordoff et al. 1996). However, we found no evidence that
productivity was correlated with proximity to water for
eastern peregrines. Although cliff-nesting peregrines may
benefit from greater concentrations of prey in association
with water bodies, this may be of less importance to urban-
nesters as their prey base tends to be more terrestrial, with
feral pigeons and other urban birds often dominating (Cade
and Bird 1990).
Site suitability can also be assessed with respect to
productivity. In terms of habitat, we found mean productivity
to be 2.0 young per nesting attempt at both rural and urban
nests, consistent with the lack of difference in the success rate
of rural and urban releases during the early part of the
Canadian recovery program (Fyfe 1988). However, it reflects
an improvement in the relative success of urban sites
compared to the 1.7 young per nesting attempt at urban sites
and 1.9 at rural sites reported by Cade and Bird (1990).
Within urban areas, we documented significantly greater
productivity on buildings than bridges. Tordoff et al. (1998)
reported that as of 1998, productivity in the Midwest was
higher on smokestacks and buildings than on cliffs and
bridges, but not to a significant degree.
Quick re-occupancy by peregrines of vacated territories has
long been observed (Hickey 1942), suggesting that certain
sites are particularly attractive. Our comparison of used and
available sites in the Toronto area indicated that the only
features strongly selected by peregrines are the presence of
recessed ledges and overhead cover. Cold weather, especially
accompanied by rain, reduces nesting success for some birds
(Kostrzewa and Kostrzewa 1990, Boal et al. 2005), and has
specifically been implicated in nest failure of peregrines
(Redig and Tordoff 1992). Conversely, peregrines may also
be at risk of nest failure from very hot weather in early
summer at exposed nests (Redig and Tordoff 1996). As such,
we were not surprised to find productivity to be higher at nest
sites with full overhead cover. The preference for recessed
ledges may also reduce the probability of disturbance from
routine maintenance activities (Cade and Bird 1990).
Although shelter from above offers the best protection
from the elements, nest orientation can also have an impact
on temperature regulation. Sullivan et al. (2003) reported a
significant preference for southeast-facing nests among a
cliff-nesting population of American kestrels (F. sparverius).
Hooge et al. (1999) found that woodpecker nests facing east
warmed up more quickly each morning and had greater
reproductive success. Similarly, tree swallows (Tachycineta
bicolor) favor nest boxes facing east or southeast
(Lumsden 1986, Rendell and Robertson 1994); however,
Ardia et al. (2006) reported similar results only in the first
half of the breeding season, suggesting that orientation
toward the morning sun may hold a significant advantage
only to early nesters. As peregrines typically begin nesting in
early spring, they would be expected to benefit from having
nests exposed to the morning sun. Among the first 19 urban
nests in eastern North America, 42% faced east (Cade and
Bird 1990). This has remained the preferred orientation,
though the frequency has dropped to 29%; southeast and
south are the next most frequent orientations, and also likely
to benefit from warmer mornings. However, local factors
such as the direction of prevailing winds or shadows from
other structures may cause other orientations to be optimal at
some nest sites. To some degree, the results may reflect the
disproportionate placement of nest trays and boxes on east or
southeast facing ledges, making it impossible to differentiate
whether peregrines are picking a site because of orientation
or nesting aids.
We found productivity at eastern urban nests to be
positively correlated with nest height. At cliff nests, the
opportunity to hunt prey directly from high nests has been
associated with greater hunting success (Jenkins 2000). This
may hold true to an even greater extent in cities, as visibility
from lower ledges could be limited by taller adjacent
buildings.
As in the Midwest (Tordoff et al. 1997), the adults in the
east with the greatest total lifetime production also tended to
have higher than average annual productivity. Most of these
eastern birds nested at sites that had attributes associated
with high productivity. However, the degree to which their
success is associated with the features of the nest site or the
birds themselves cannot be determined. This reflects the
challenge of attempting to assess productivity in relation to
nest site features, without being able to account for variation
in the fertility and parenting abilities of the adults using
them. Relatively few of the contrasts we documented were
16 The Journal of Wildlife Management 79(1)

statistically significant because of the high level of variability
in data, much of which may be related to the biology and
behavior of the individuals involved.
Urban Advantages and Disadvantages
Although we found no difference in productivity between
rural and urban nests, the urban population has grown to
account for the majority of the eastern population, indicating
an attraction of peregrines to such habitat. Given that
peregrine populations are generally considered to be limited
by the availability of nest sites (Ratcliffe 1962, Tordoff
et al. 1998), this may simply reflect peregrines taking
advantage of the substantial increase in the availability of
urban nest sites compared to the historically limited options
on eastern cliffs. For example, peregrines in recent decades
expanded into areas such as Ohio where no historical
breeding records exist because of a lack of suitable natural
nesting habitat (Redig and Tordoff 1996).
Peregrines are also probably attracted to cities by the
scarcity of predators, abundance of prey throughout the year,
and variety of high perches and potential nest ledges that fit
the role of natural cliffs, albeit often providing superior
shelter from weather (Cade and Bird 1990, Redig and
Tordoff 1996). Not only is the prey base in cities sometimes
more abundant than in rural areas, but artificial lights permit
peregrines to hunt at night if necessary (Wendt et al. 1991,
DeCandido and Allen 2006). Though rare, polygyny has
been documented at least a few times at urban sites in the
Midwest (Redig and Tordoff 1994) and for at least 1 male in
southern Ontario (M. A. Gahbauer, Migration Research
Foundation, personal observation), a behavior perhaps
enabled by abundant resources in cities.
Collisions with buildings and vehicles are the most obvious
additional sources of mortality in urban areas compared to
rural habitat, and account for a majority of deaths by urban-
hatched peregrines in the east, as in the Midwest and in New
England (Sweeney et al. 1997, Tordoff et al. 2000, Faccio
et al. 2011). But as common as such collisions are, only 20%
of urban-hatched peregrines in southern Ontario, Massa-
chusetts, and Pennsylvania are confirmed to have died in
their first year. In New Jersey, trichomoniasis has also been
an important cause of mortality among nestlings, but it
appears to be much less prevalent through the rest of the
study area. Shooting was among the other causes of death
identified in Pennsylvania (Katzner et al. 2012). High first-
year mortality is typical among raptors (Newton 1979), with
estimates of 57% (Brown and Amadon 1968), 60% (Tordoff
and Redig 1999), and 70% (Enderson 1969) for peregrines.
No doubt many additional mortalities were sustained by
urban-hatched peregrines in our study that were not
documented, and our results may have been somewhat
biased toward locations and causes of mortality from which a
carcass would be readily discovered (e.g., predation may be
under-represented), but the reported frequency of collisions
alone does not provide evidence of a higher mortality rate in
cities.
Various other hazards are unique to the urban environ-
ment, including open chimneys and narrow courtyards with
high vertical walls that may trap young birds with weak flight
skills. Peregrines are also at risk of indirect poisoning
through the ingestion of pigeons (Columba livia) contami-
nated with toxins intended to control their population (Cade
and Bird 1990). But conversely, some risks are faced
primarily by rural peregrines. Predation by other raptors,
especially great horned owls (Bubo virginianus), can be a
considerable threat at cliffs (Herbert and Herbert 1965). The
risk of starvation is also greater for young that do not learn to
hunt quickly, as prey may be scarcer than in cities, and can
decrease more rapidly as prey species flock for migration.
Weighing these factors against the distinct benefits offered
by urban habitat, Tordoff et al. (1999) argued that perhaps
cliffs have become second-class nest sites. If this is the case,
the strongest adults may concentrate in cities over time,
perhaps leading to mean productivity in urban habitat
exceeding that in rural areas.
Human Assistance
Tordoff et al. (1996) noted that in the Midwest, nest failure
is so high at urban nests lacking nest boxes (largely because of
heavy rain) that peregrines would likely be unable to persist
in cities without the provision of sheltered nest sites, and
Altwegg et al. (2014) concluded that nest boxes increased
nesting success in a growing urban population in Cape
Town, South Africa. Our results suggest that nest boxes are
not equally critical in eastern North America; boxes were
present at only a minority of buildings, yet overall, urban
productivity was high. Nonetheless, eastern data agree that
boxes on buildings provide a significant advantage, whereas
nest trays alone do not, likely because they lack additional
protection from the elements provided by boxes. Unlike in
the Midwest, where boxes also improved nesting success on
bridges (Tordoff et al. 2003), boxes conferred no advantage
to bridge-nesting peregrines in the east, suggesting that their
low rate of success may be related to factors other than
shelter, such as disturbance or low height above water, which
increases the risk of drowning by fledglings. Only a small
minority of new nest sites in recent years have been at boxes,
but this may be more a reflection of most such existing sites
already being occupied than a reduced interest in them.
Many young peregrines are unable to maintain altitude well
when they first take flight, and in an urban environment,
perching options at lower levels may be limited, leading some
individuals to reach the ground (Cade and Bird 1990). Only
on 1 occasion in Ontario did we observe an adult descend to
provide food to a grounded youngster; more typically they
circle above, encouraging the fledgling to fly back up, but the
lack of stepping stone perches on skyscrapers makes that
nearly impossible for those that have only begun developing
their flight skills. The majority of grounded fledglings would
likely die of starvation (or collisions) if not rescued and
returned to higher perches. In southern Ontario, almost half
of fledglings were rescued, and these individuals in turn
accounted for half of the known next-generation offspring
during the study period; 1 male hatched in Etobicoke,
Ontario in 1998 and nesting in Toronto from 2000 through
2014 has now eclipsed all but 1 of the other records for males
Gahbauer et al. Urban Peregrine Management 17

in northeastern North America, with 44 young produced.
Although similar statistics were not available for other
regions, if the Ontario data are at all typical, rescues seem to
have a significant impact not only on survival of urban
offspring, but also on the continued growth of the
population. Similarly, in New Jersey, the provision of
medication to protect nestlings against trichomoniasis at 3
urban sites likely improves survival significantly.
MANAGEMENT IMPLICATIONS
Urban development has created an abundance of new
potential nest sites for peregrines, thereby increasing the
potential carrying capacity of the eastern population. Both
rural and urban numbers have increased steadily over the past
3 decades, suggesting that a source–sink relationship does not
exist between the 2 habitat types, although some individual
sites have been particularly productive. As the urban peregrine
population continues to expand, individuals will increasingly
attempt to nest at sites of secondary quality. Their potential
for nesting successfully is likely to be improved if they are
provided nest boxes. Local considerations may take prece-
dence, but as general guidelines we recommend that boxes be
placed on recessed ledges of tall buildings facing east or
southeast because these are the conditions that have been
most consistently favored by eastern peregrines. Considering
that productivity was similar between downtown and
suburban areas, but the concentration of hazards to peregrines
is typically far greater in the city core, it may be advisable to
focus box placement on suitable buildings in moderately
developed areas with less traffic and fewer building collision
hazards. We also encourage the continued documentation of
both urban mortalities and rescues, to allow for assessment of
their importance to the urban population over greater
temporal and geographic scales.
ACKNOWLEDGMENTS
We thank the many individual volunteers and organizations
that have actively contributed to monitoring, rescuing, and
reporting data on nesting peregrines over the past 3 decades
in Ontario, Quebec, Massachusetts, New Jersey, and
Pennsylvania, especially the Canadian Peregrine Founda-
tion, Ottawa Field-Naturalists’ Club, Hamilton Naturalists’
Club, Regroupement Que
´becOiseaux, and Conserve Wild-
life Foundation of New Jersey. Thanks also to B. Frei for
assistance with preparation of the figure, and to J.W. Grier
and an anonymous reviewer for valuable input that improved
the final text. This manuscript is based on a chapter of
Gahbauer’s dissertation (2008). M. A. Gahbauer was
supported in part by a Natural Sciences and Engineering
Research Council (NSERC) graduate scholarship.
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Associate Editor: Marc Bechard.
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Gahbauer et al. Urban Peregrine Management 19