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Environmental factors affecting the reproductive rate of urban Northern Goshawks


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Successful raptor reproduction requires both a nesting and foraging habitat (Newton 1979). A good nesting habitat reduces the risk of predation (Mainwaring et al. 2014, Anderson et al. 2015), and creates a suitable microclimate for breeding (Robertson 2009). The foraging environment is important for satisfying both parent and nestling food requirements (Reynolds et al. 2006). The reproductive rate of raptors is affected by other environmental factors such as climatic factors (Fairhurst and Bechard 2005), artificial disturbance (Krüger 2002), intraspecific (Bretagnolle 2008) and interspecific competition (Krüger 2002), and predation (Krüger 2004). Therefore, to elucidate factors affecting reproductive rates of raptors, it is necessary to estimate the correlation between reproductive rate and various environmental factors. Increasing urbanization worldwide significantly affects many animal species (Ramalho and Hobbs 2012). Urbanization has increased with human population growth, and the responses from species are gaining more attention from researchers (Bateman and Fleming 2012). Urbanization can bring drastic changes to the behavior and life history of birds (Dominoni et al. 2013). Sometimes it has a deleterious effect, such as the extinction or extirpation of a species or a decrease in population density (Marzluff and Ewing 2001). Conversely, some species have expanded their range into urban areas, not just temporarily, but also to breed (Bird et al. 1996, Boal and Dykstra 2018). Avian responses to urbanization differ according to species and taxonomic group. Urbanization may provide suitable conditions for habitation by some raptors due to reductions in intra- and interspecific competition, and more abundant prey (Chace and Walsh 2006). For example, Eastern Screech-Owls (Megascops asio) breeding in urban areas have higher reproductive rates than those breeding in rural areas (Gehlback 1996). However, increased risk of disease, chemical contamination, collision with buildings and vehicles, and decreased foraging areas have also been reported (Hager 2009). For example, Eurasian Kestrels (Falco tinnunculus) breeding in urban areas have lower reproductive rates than a nearby rural population (Sumasgutner et al. 2014). The varied responses of different species to urbanization underscore the urgent need for more ecological studies of raptors in urban environments (Morrison et al. 2016). The Northern Goshawk (Accipiter gentilis) is a medium-sized raptor that is widespread in the northern hemisphere. Typical goshawk breeding habitat includes remote forested areas that are not subject to human-induced disturbance (Kenward 2006). Earlier studies on the reproductive rate of the goshawk, mainly in Europe and the United States, have been summarized by Kenward (2006). However, goshawks have expanded their range to urban areas in Japan and Europe and have been breeding in urban environments there (Higuchi et al. 1996, Rutz et al. 2006b). Urban goshawks' feeding habits (Würfels 1994, 1995, Altenkamp 2002, Rutz 2003, 2004, Rutz et al. 2006a), home ranges and space use (Rutz 2006), foraging strategies (Rutz 2012b), breeding-site selection (Natsukawa et al. 2017), reproductive parameters (Solonen 2008, Rutz 2012a, 2012b), and colonization history (Rutz 2008) have been investigated, but studies on the determinants of their reproductive rate are limited. Here, we report the results of an investigation of the reproductive rate of a Northern Goshawk population in an urbanized area from 2014 to 2016, in which we examine the relationship between the number of fledglings and environmental factors such as nesting and foraging environments, anthropogenic disturbance, predation risk, and intraspecific competition.
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J. Raptor Res. 53(4):377–386
Ó2019 The Raptor Research Foundation, Inc.
Graduate School of Environment and Information Sciences, Yokohama, National University, 79-1 Tokiwadai,
Hodogaya-ku, Yokohama-shi, Kanagawa-ken, 240-8501 Japan
Wild Bird Society of Japan Kanagawa Branch, 2-8 Sakae-cho, Kanagawa-ku, Yokohama-shi, Kanagawa-ken, 221-
0052, Japan
Faculty of Environment and Information Studies, Keio University, 5322 Endo, Fujisawa-shi, Kanagawa-ken, 252-
0882, Japan
ABSTRACT.—Urbanization has increased with human population growth and the responses from raptor
species are gaining more attention from both researchers and the public. Northern Goshawks (Accipiter
gentilis) now breed in urban areas in Japan and Europe; however, there are few studies examining the factors
that influence their reproductive rate in urban areas. We investigated the reproductive rate (number of
fledglings per nest) of the Northern Goshawk population in an urbanized area of Japan from 2014 to 2016,
and used a binomial mixture model to examine the relationship between the number of fledglings per nest
and environmental factors such as nesting and foraging environments, anthropogenic disturbance,
predation risk, and intraspecific competition. The goshawk nesting success rate from 2014 to 2016 was
71.6%, with an average reproductive rate of 1.7 fledglings per occupied nest. The percentage of canopy cover
of nesting stands had a significant positive effect on fledgling numbers, and the number of adjacent
occupied nests had a significant negative effect on fledgling numbers. The positive effects of canopy
coverage may be explained by the protection offered by canopy against direct sunlight, wind, and rain. The
negative effect of the adjacent occupied nests may result from an increase in the amount of time and energy
goshawks spent in territory defense, and a decrease in available foraging habitat due to intraspecific
KEY WORDS:Northern Goshawk; Accipiter gentilis; breeding success;canopy coverage;density effect;interference
competition;reproductive rate;urban.
RESUMEN.—La urbanizacio´n ha aumentado con el crecimiento poblacional humano. Consecuentemente, se
observa una mayor atencio´ n, por parte del p´
ublico en general y de los investigadores, ala forma en la que las
aves rapaces responden a este feno´ meno. Accipiter gentilis se reproduce actualmente en a´ reas urbanas de
Japo´ n y Europa; sin embargo, son escasos los estudios que examinan los factores que influyen sus tasas
reproductivas en a´ reas urbanas. Entre los a ˜
nos 2014 y 2016 investigamos la tasa reproductiva (n´
umero de
volantones por nido) de una poblacio´n deA. gentilis en un a´rea urbanizada de Japo´ n. Utilizamos un modelo
mixto binomial para examinar la relacio´n entre el n ´
umero de volantones por nido y factores ambientales
tales como los sitios de anidacio´ n y alimentacio´ n, las molestias antropoge´nicas, el riesgo de depredacio´n y la
competencia intraespec´ıfica. El e´ xito reproductor de A. gentilis desde 2014 a 2016 fue 71.6%, con una tasa
reproductiva promedio de 1.7 volantones por nido ocupado. El porcentaje de cobertura del dosel en los
sitios de anidacio´ n tuvo un efecto positivo significativo sobre el n ´
umero de volantones, mientras que el
umero de nidos adyacentes ocupados tuvo un efecto negativo significativo sobre estos. Los efectos positivos
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de la cobertura del dosel pueden explicarse por la proteccio´n que esta ofrece frente a la luz directa del sol, al
viento y a la lluvia. El efecto negativo de los nidos adyacentes ocupados podr´ıa ser resultado del aumento en
la cantidad de tiempo y energ´ıa que los individuos de A. gentilis dedicaron para la defensa del territorio y de
la disminucio´n en la disponibilidad de ha´ bitat de alimentacio´ n debido a la competencia intraespec´ıfica.
[Traduccio´n del equipo editorial]
Successful raptor reproduction requires both a
nesting and foraging habitat (Newton 1979). A good
nesting habitat reduces the risk of predation
(Mainwaring et al. 2014, Anderson et al. 2015),
and creates a suitable microclimate for breeding
(Robertson 2009). The foraging environment is
important for satisfying both parent and nestling
food requirements (Reynolds et al. 2006). The
reproductive rate of raptors is affected by other
environmental factors such as climatic factors (Fair-
hurst and Bechard 2005), artificial disturbance
uger 2002), intraspecific (Bretagnolle 2008)
and interspecific competition (Kr¨
uger 2002), and
predation (Kr¨
uger 2004). Therefore, to elucidate
factors affecting reproductive rates of raptors, it is
necessary to estimate the correlation between
reproductive rate and various environmental factors.
Increasing urbanization worldwide significantly
affects many animal species (Ramalho and Hobbs
2012). Urbanization has increased with human
population growth, and the responses from species
are gaining more attention from researchers (Bate-
man and Fleming 2012). Urbanization can bring
drastic changes to the behavior and life history of
birds (Dominoni et al. 2013). Sometimes it has a
deleterious effect, such as the extinction or extirpa-
tion of a species or a decrease in population density
(Marzluff and Ewing 2001). Conversely, some
species have expanded their range into urban areas,
not just temporarily, but also to breed (Bird et al.
1996, Boal and Dykstra 2018). Avian responses to
urbanization differ according to species and taxo-
nomic group. Urbanization may provide suitable
conditions for habitation by some raptors due to
reductions in intra- and interspecific competition,
and more abundant prey (Chace and Walsh 2006).
For example, Eastern Screech-Owls (Megascops asio)
breeding in urban areas have higher reproductive
rates than those breeding in rural areas (Gehlback
1996). However, increased risk of disease, chemical
contamination, collision with buildings and vehicles,
and decreased foraging areas have also been
reported (Hager 2009). For example, Eurasian
Kestrels (Falco tinnunculus) breeding in urban areas
have lower reproductive rates than a nearby rural
population (Sumasgutner et al. 2014). The varied
responses of different species to urbanization
underscore the urgent need for more ecological
studies of raptors in urban environments (Morrison
et al. 2016).
The Northern Goshawk (Accipiter gentilis)isa
medium-sized raptor that is widespread in the
northern hemisphere. Typical goshawk breeding
habitat includes remote forested areas that are not
subject to human-induced disturbance (Kenward
2006). Earlier studies on the reproductive rate of the
goshawk, mainly in Europe and the United States,
have been summarized by Kenward (2006). Howev-
er, goshawks have expanded their range to urban
areas in Japan and Europe and have been breeding
in urban environments there (Higuchi et al. 1996,
Rutz et al. 2006b). Urban goshawks’ feeding habits
urfels 1994, 1995, Altenkamp 2002, Rutz 2003,
2004, Rutz et al. 2006a), home ranges and space use
(Rutz 2006), foraging strategies (Rutz 2012b),
breeding-site selection (Natsukawa et al. 2017),
reproductive parameters (Solonen 2008, Rutz
2012a, 2012b), and colonization history (Rutz
2008) have been investigated, but studies on the
determinants of their reproductive rate are limited.
Here, we report the results of an investigation of the
reproductive rate of a Northern Goshawk popula-
tion in an urbanized area from 2014 to 2016, in
which we examine the relationship between the
number of fledglings and environmental factors
such as nesting and foraging environments, anthro-
pogenic disturbance, predation risk, and intraspe-
cific competition.
Study Area. We studied urban goshawks within 793
in the Kanagawa Prefecture, central Japan,
which includes Kawasaki City, Yokohama City,
Yamato City, Zama City, Ebina City, Ayase City,
Fujisawa City, Chigasaki City, and Samukawa Town
(Fig. 1; see Natsukawa et al. 2017 for coordinates).
The landscape is generally flat, with rolling hills and
a mean altitude of 159 masl. The climate is mild, with
an average monthly temperature of 15.88C (mean
monthly temperature range: 5.9–26.7). Rainfall is
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Figure 1. Eastern portion of Kanagawa Prefecture, Japan, study area for urban-breeding Northern Goshawks. Black
shading indicates forested area (13.0%), grey indicates open land (11.8%), crosshatch indicates water (2.5%), and white
indicates built-up areas (72.7%).
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high in summer and autumn and lower in winter.
Annual precipitation averages 1689 mm (maximum
monthly average precipitation 234 mm, minimum
monthly average precipitation 55 mm). Land-cover
types in the study area included 13.0% forest, 11.8%
open land (farmland, rice field, and grassland),
2.5% water, and 72.7% built area (paved road,
residential area). Approximately 80% of this area
was classified as Densely Inhabited Districts (DIDs).
A DID refers to a block area (delineated by roads,
rivers, etc., and ranging in size from 3000 to 5000
) with a population density of 4000 people/km
or a similar-sized district of small neighboring
sectors having a total population of 5000. DIDs
are used for distinguishing between urban and rural
areas in Japan.
Surveys of Breeding Goshawks. In 2014–2016, we
investigated all forests in the study area on foot to
find occupied nests of goshawks. We defined
occupied nests as nests with the confirmed existence
of a pair nest-building, copulating, egg-laying, or
rearing young. Following Murase et al. (2015), we
classified nests located within 400 m of a nest from
the previous year as the same breeding site.
To determine the number of fledglings in an
occupied nest, we visited each occupied nest 8 to 12
times, and we used binoculars (123) and a spotting
scope (25–503) to observe the nests. Time per
observation varied from 10 to 40 min, and distance
from the observer to the nest tree varied from 60 to
148 m. We did not conduct these surveys on rainy
days, as we anticipated that visibility would be
compromised. We regarded nestlings that reached
80% of the average age of first flight as fledglings
(Steenhof and Newton 2007), and we calculated
reproductive rate as the number of fledglings per
occupied nest. We defined breeding success as the
proportion of occupied nests at which one or more
nestlings fledged.
Measurements of Potential Covariates. Local
and landscape factors. Following James and Shugart
(1970), we measured canopy coverage within a
radius of 11.3 m from the nest tree. We visually
estimated the canopy coverage in 10% increments.
We determined that a plot of 11.3-m radius was an
appropriate size because our previous surveys of
these forests showed that the vegetation structure
(tree size, number of understory trees and shrubs,
etc.) in the area up to this distance from the nests
clearly differed from other places in the same forest
(see Natsukawa et al. 2017).
We measured landscape-level factors using a land-
cover map (resolution 10 m310 m) of the study area
published by Japan Aerospace Exploration Agency
jpn.htm), which reflected average land cover from
2006 to 2011. The land-cover map was subdivided
into categories: forest, open land (farmland, rice
fields, and grassland), water, vegetated built-up
lands (total of block areas that have 30% small-
scale vegetation of 900 m
), and built-up lands
(total of block areas that have 30% small-scale
vegetation of 900 m
We plotted a circle of radius 2 km, approximately
the size of a goshawk home range in Japan (Kudo et
al. 2005), around each nest and determined the
percentage of land-cover types in each. Goshawks
inhabiting the surveyed area tended to forage along
forest edges (Natsukawa et al. 2017). Therefore, we
measured the tangent length of forest and open
area, and the tangent length of forest and vegetated
built-up land. In addition, as an index to distur-
bance, we measured the distance from each nest to
the nearest house. For these measurements, we used
GIS software ArcMap 10.3.
Age of female breeders. Molted feathers of goshawks
can be reliably used for sex identification, ageing,
and individual identification without capture (Op-
dam and M¨
uskens 1976). Three age classes (first-
year, second-year, and third-year or older) can be
distinguished by feather shape, coloration and
patterning. We used this method to identify the
age of female breeding birds. In addition, we also
assessed the age of the breeding female bird by
observing the belly feathers with binoculars or scope.
Like molted feathers, this method can be used to
classify ages into three categories (first-year, second-
year, and third-year or older; Morioka et al. 1995).
Intra- and interspecific factors. To investigate the
effect of intraspecific competition, we tallied the
number of known occupied adjacent nests within a
2-km radius of each nest (Table 1). For three nests in
2014, four nests in 2015, and three nests in 2016 that
were on the margin of the surveyed area, it was
possible that there were adjacent occupied nests
outside the study area. Therefore, we did not
measure the potential covariates at these nests.
As an index to the number of potential predators
of goshawk eggs or young, we counted the number
of Large-billed Crows (Corvus macrorhynchos), Carri-
on Crows (C. corone), and Black Kites (Milvus
migrans) within the study area by conducting a spot
census near each nest stand (within 100 m) during
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the time period from incubation (April) to early
nestling-rearing (May). In the spot census, we
counted the number of these species that were seen
within 100 m of the fixed point within 30 min. We
did not conduct the spot census surveys on rainy
days, as we anticipated that visibility would be
Weather factors. Climate factors such as tempera-
ture and rainfall influence the reproductive rate of
goshawks (e.g., Fairhurst and Bechard 2005).
Therefore, we calculated average temperature and
average total rainfall in the incubation period
(April) and the nestling period (May–July) of each
year from the meteorological data of the Japanese
Meteorological Agency (
no¼&year¼&month¼&day¼&view¼). For the weath-
er information, we used the observation data of the
observatory nearest to each nest.
Statistical Analysis. To help explain the environ-
mental factors affecting reproductive rate (here, the
number of fledglings), we analyzed the data using a
binomial mixture model (Royle 2004). In this
model, it is possible to simultaneously estimate the
number of individuals (the number of fledglings in
this study) and the factors affecting the detection
probability as a function of the covariates. There-
fore, the model provides a powerful framework for
correcting the observation error caused by false
negatives (exist but not observed) and estimating
unbiased true states (Ke´ry and Royle 2016). This
model consists of two equations; a state model
expressing the latent true state and an observation
model expressing measured values including obser-
vation errors. We assumed that the state model
follows a Poisson distribution and the observation
model follows a binomial distribution. The covari-
ates of the state model were local and landscape
factors, age of adult birds, intra- and interspecific
factors, and weather factors. The covariates of the
observation model were the observation time per
each survey and the distance from the survey site to
the nest tree. All covariates used for analysis were
normalized to 0 mean and 1 standard deviation. In
this study, the regression coefficients were estimated
by the maximum likelihood method and models of
combinations of all covariates were created and
ranked based on Akaike information criterion
(AIC). Then, the model with the smallest AIC value
was taken as the best-ranked model (Burnham and
Anderson 2002). To avoid multicollinearity, we
calculated correlation using a combination of all
covariates, and did not use any model including a
combination of covariates with jrj.0.7. The effect
of the covariates was considered significant when 0
was not included in the 95% confidence interval
(Arnold 2010). We used statistical software R version
3.1.1 for all statistical analysis and the package
‘‘unmarked’’ (Fiske and Chandler 2011) to create
the binomial mixture model. In this study, we used
data obtained by pseudo-repeated sampling from
the same breeding site and year. Therefore, we
Table 1. Potential covariates of Northern Goshawk reproductive rate, measured for urban nests in Japan. Sample n¼85
nests (2014–2016).
Distance from nest tree to human residence m 90.5 (57.1) 14.4–201.4
Canopy coverage
% 80.1 (30.6) 20.0–100.0
Forest coverage
% 14.9 (11.2) 1.5–50.3
Open (field, rice paddy and grass) coverage
% 20.3 (13.0) 1.1–46.1
Vegetated built-up land coverage
% 20.9 (12.6) 2.0–55.1
Built-up land coverage
% 39.1 (18.8) 9.8–85.9
Tangent length between forest and open land
km 5.8 (5.4) 0.0–30.5
Tangent length between forest and vegetated built-up land
km 4.7 (3.3) 0.1–15.6
Area of forests with nests
0.2 (0.2) 0.0–1.0
Number of adjacent occupied nests
nest 0.7 (0.9) 0.0–3.0
Number of predators bird 14.6 (9.8) 2.0–32.0
Average temperature during the incubation period (April) 8C 14.3 (1.5) 13.3–15.7
Average temperature during the nestling-rearing period (May–July) 8C 22.4 (4.2) 18.9–25.9
Total precipitation during the incubation period (April) mm 144.2 (16.6) 88.0–154.5
Total precipitation during the nestling-rearing period (May–July) mm 176.2 (38.2) 103.8–198.2
Local-scale variable (11.3-m radius from the nest tree)
Landscape-scale variable (2-km radius from the nest tree).
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tested whether there was an effect of territory and
year using a generalized linear mixed model with a
log-link function and Poisson distribution. The
maximum number of fledglings detected at each
breeding site in each year was used as the response
variable, the environmental factors used in the
binomial mixture model were covariates, and
territory and year were included as random vari-
ables. The random variables were assumed to follow
a normal distribution with a mean of zero and a
standard deviation of s or r. The results of this
analysis indicated that the effects of the random
variables were smaller than those of the covariates,
and the results did not differ greatly from those of
the binomial mixture models. Therefore, we did not
consider pseudo-replication in the binomial mixture
Reproductive Rates. We located 31 occupied nests
in 2014, 33 in 2015, and 31 in 2016, for a total of 95
from 2014–2016. All nests in the study were detected
early in the breeding season and goshawks laid eggs
in all occupied nests. Breeding success on the basis
of occupied nests was 71.0% (n¼31) in 2014, 72.7%
(n¼33) in 2015, 71.0% (n¼31) in 2016, and 71.6%
from 2014 to 2016 (n¼95). The average number of
fledglings per occupied nest was 1.7 60.2 in 2014
(mean 6SE, n¼31), 1.7 60.2 in 2015 (n¼33), 1.6
60.3 in 2016 (n¼31), and 1.7 60.3 from 2014 to
2016 (n¼95). The minimum and maximum
number of fledglings per nest in all years was 0
and 4, respectively.
Factors Affecting Reproductive Rate. The 10 nests
on the margin of the surveyed area were excluded
from the analysis as described above, so 85 nests were
used for analysis (Table 1). Most (83 of 85) breeding
females were 3 yr old; the remaining two birds were
2-yr-old females. As a result of model selection, in the
best-ranked model, the state model included canopy
coverage of the nest stand and the number of
adjacent occupied nests within a 2-km radius, and
there were no covariates included in the observation
model (Table 2). Both covariates of the state model
had statistically significant effects (Table 2). Greater
canopy coverage had a large positive effect on the
number of fledglings, while a greater number of
adjacent occupied nests had a small negative effect
(Fig. 2, Table 2). In contrast, no factors affecting the
detection probability could be specified.
We found that canopy coverage of the nest stand
and the number of the adjacent occupied nests were
significantly related to reproductive rate (Fig. 2,
Table 2). In addition, our analytical technique
considered the false negative errors often found in
ecological survey data by explicitly modeling the
detection probability. Therefore, the results of this
study should be less biased than those of studies
applying traditional statistical methods that do not
consider detection probability (Ke´ry et al. 2013).
The positive effect of canopy coverage of the nest
stand may have been due to the protection it offers
nestlings and adults from rainfall and increased
temperature due to direct sunlight. Maximum
temperature (Reynolds et al. 2017) and precipita-
tion (Fairhurst and Bechard 2005) had negative
effects on the reproductive rate of goshawks
elsewhere. A closed canopy is thought to create a
microclimate that is more favorable for breeding
than an open canopy (e.g., McGrath et al. 2003).
Peregrine Falcons (F. peregrinus; Anctil et al. 2014)
and Cape Vultures (Gyps coprotheres; Pfeiffer et al.
2017) also have high reproductive rate in locations
with physically protected nests. The early to middle
portions of the goshawk nestling-rearing period
(May–June) in the study area has the most rainfall,
whereas the later part of the nestling-rearing period
(the beginning of July) has the highest temperatures
of the year, with strong direct sunlight. Nest
locations that minimize the adverse effects of rain
Table 2. Determinants of breeding success analyzed using the binomial mixture model. Model with the smallest AIC is
shown. The estimate is the intercept or regression coefficient. Lower is the 95% confidence interval lower limit value.
Upper is the 95% confidence interval upper limit.
State (Intercept) 0.34 1.19 0.52
State (Canopy coverage) 1.42 0.44 2.39
State (Number of neighbor nests) 1.13 1.66 0.59
Detection (Intercept) 4.01 3.26 4.76
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and warm temperatures may enhance reproductive
rate. In Japanese urban areas, because of the decline
of forest management, canopy coverage is increas-
ing, providing more suitable nesting habitats.
Furthermore, trees that may hinder the flight of
goshawks are also increasing in potential nest stands.
Goshawks in urban areas prefer nest stands in forest
environments that do not hinder flight, rather than
areas of high canopy coverage (Natsukawa et al.
2017). In the current study, some goshawks used
nesting environments where the canopy coverage
was low, even though these areas were associated
with lower reproductive rates.
The significant negative effect of the number of
adjacent occupied nests on reproductive rate suggests
density effects, such as a decrease in the number or
size of foraging sites and an increase in the time-cost
for territory defense due to an increase in individual
interference (intraspecific competition). This latter is
termed the interference competition hypothesis and
is considered to be the main factor linked to the
density effect (Both 1998). In fact, three pairs of
goshawks that we observed extensively, stayed around
their own nests until sunset after the retreat of an
invading individual goshawk (n¼8 occurrences of
this behavior among three nest sites). During those
periods of vigilance, the nestlings were not fed at all
(H. Natsukawa unpubl. data). Goshawk populations
breeding in urban areas generally breed at a higher
density than populations breeding in rural areas in
Europe (Rutz et al. 2006b). Breeding goshawks are
highly territorial and the nearest neighbor distance
(NND) has little variability in areas where the
environment is uniform (Rutz et al. 2006b). However,
the forests in urbanized areas tend to be fragmented,
and NND has greater variability. Goshawks inhabiting
the study area select breeding sites where there is
both forest and open space (Natsukawa et al. 2017),
and NND can be as low as approximately 700 m in
such sites. Urbanization of the study area is increas-
ing, and forest and open areas each make up
approximately 10% of the study area (see Methods).
In urban areas where nesting and foraging sites are
limited, density effects may occur because goshawk
pairs are already breeding at a high density. The
frequency of individual interference in raptors is
closely related to NND (Newton 1979). Other raptors
such as Ospreys (Pandion haliaetus; Bretagnolle et al.
2008) and White-tailed Eagles (Haliaeetus albicilla;
Heuck et al. 2017) have decreased reproductive rates
due to intraspecific interference competition.
In addition to direct interference as a cause of the
density effect, the habitat heterogeneity hypothesis
may also explain decreasing breeding success as
density increases. This hypothesis posits that breed-
ing density and breeding success are not directly
related (Rodenhouse et al. 1997), but instead high-
quality breeding sites are occupied first according to
Figure 2. Relationships between the reproductive rate (number of fledglings per occupied nest) and covariates of the
best-ranked binomial mixture model for Northern Goshawks in an urbanized area from 2014 to 2016 (n¼85 nests). (A)
indicates the effect of the number of neighboring nests, and (B) shows the effect of canopy coverage. Black lines are fitted
values. Grey lines represent 95% confidence intervals. Variable descriptions in Table 1.
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the ideal despotic distribution (Fretwell and Lucas
1969), with lower-quality breeding sites occupied
later. As breeding density increases, the number of
individuals breeding in low-quality breeding sites will
increase, and thus overall reproductive rate will
decrease. It has been suggested that the habitat
heterogeneity theory explains, at least in part, the
reproductive rate of goshawks (Kr¨
uger and Lind-
stro¨m 2001) and other raptors such as Eurasian
Sparrowhawks (A. nisus; Newton 1991) and Spanish
Imperial Eagles (Aquila adalberti; Ferrer and Dona´zar
1996). However, in our study, the number of
fledglings produced in all nests located in areas
with particularly high density (e.g., seven nests in the
southwestern part of our study area with an average
NND of about 1.2 km) was either one or zero per
nest. This result, which differs from that of Kr ¨
and Lindstro¨m (2001), suggests that interference
competition, rather than habitat heterogeneity,
influences the reproductive rate of goshawks.
Our study suggested that the high canopy coverage
had a significant positive influence on the reproduc-
tive rate of goshawks and the density effect (likely due
to interference competition) had a negative influ-
ence on the reproductive rate of goshawks. We were
unable to investigate the relationship between food
availability and reproductive rate, as has been studied
elsewhere (e.g., Salafsky et al. 2005), due to a lack of
prey data. We acknowledge that food availability is
likely an important variable in the analysis of factors
influencing reproductive rate, as it is in other raptor
species (e.g., Terraube et al. 2012, Therrien et al.
2014), and we encourage other researchers to
investigate this aspect of urban goshawk ecology.
Studies on urban-breeding goshawks are much less
common than studies of populations breeding in
conventional habitats (Rutz et al. 2006b), and
additional data are needed to strengthen the
comparison among populations in different habitats
(Rutz 2006).
We are deeply grateful to the editors and referees for
carefully reading our manuscript and for giving useful
comments. This study was supported by a research study
support project in 2014 by Bird Research (an NPO; http://
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... Hooded Vultures, Necrosyrtes monachus (Temminck, 1823) (Accipitriformes: Accipitridae), for instance, achieve a higher reproductive success near human habitations thanks to the protection from traditional customs [58], and human disturbance can help reduce the number of predators such as snakes [59], thus reducing the stress from nest feeding. In addition, as a companion species, the BCLT shows a certain degree of compatibility with human activity in the selection of nesting areas during the breeding period [17]. Meanwhile, some studies have shown that roads may pose a threat to large mammals or amphibian reptiles to some extent, but the impact on species, especially on small mammals or birds, is not always negative and sometimes can be neutral or positive [60]. ...
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Understanding the impact of human disturbance on the breeding habitat selection of endangered species is critical to improving their conservation. Blue-crowned Laughingthrush Pterorhinus courtoisi (Ménégaux, 1923) syn. Garrulax courtoisi (Passeriformes: Leiotrichidae) is an endangered species in China. To explore the nesting habitat selection of the Blue-crowned Laughingthrush and its response to human disturbance during the breeding period, we conducted a field survey at its nesting sites in Wuyuan County and Dexing City, Jiangxi Province, from March to July 2021 and in June 2022. At the home range scale (200 m) the results of a principal component analysis (PCA) showed that this species had a clear preference for infrastructure construction, grassland, farmland and bare land. At the microhabitat scale (12.26 m), we compared the ecological parameters of a nest plot and control plot using a Kruskal–Wallis H test and found that there were significant differences in the vegetation coverage, crown breadth, shrub coverage, herb maximum height, herb average height and herb species number of the nesting area between the two groups. Ensemble species distribution models showed that the suitable habitat of the Blue-crowned Laughingthrush covered an area of 108.63 km2. Distance to waterways, deciduous broadleaved forests and roads were the main factors impacting the habitat distribution of the Blue-crowned Laughingthrush. Our study suggests that (1) it is highly important to improve the protection of breeding sites and suitable living areas close to the settlements and preserve a certain nesting habitat selection space for the Blue-crowned Laughingthrush during the breeding period; and (2) it is necessary to continue to monitor the potential suitable breeding habitat. This study provides a scientific basis for the protection of the Blue-crowned Laughingthrush by local forestry bureaus and conservation departments in the future.
... Recently, arti cial structures in cities have attracted attention as novel bird nesting sites. For example, some raptors utilize urban buildings for nesting (Mak et al. 2021;Natsukawa et al. 2019), and bird nesting in arti cial structures may have implications for the interaction between people and nature in cities. ...
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Human–nature interactions are gaining attention as evidence accumulates on the health and well-being benefits of interacting with nature. Since public interest in the species affects support for biodiversity protection, it is important to elucidate and quantify the relevant factors in conservation efforts. However, the factors that affect public interest are highly abstract proxies for the distance between humans and species. Therefore, more detailed variables that represent familiarity with humans (e.g., habitat type) and their relationship with public interest need to be identified. In the present study, we examined the factors influencing public interest in 391 bird species in Japan using Wikipedia pageviews as a proxy. Daily view data from July 1, 2015, to December 31, 2020, were obtained to explore statistical models explaining Wikipedia pageviews for birds using a generalized linear model and Akaike information criterion. Our results revealed that species that inhabit urban areas, nest on artificial structures, are designated as game birds, and have a large body size attract high public interest. Furthermore, species related to an urban habitat type and food use were particularly associated with high public interest. The results showed that people were highly interested in birds that were physically and culturally close to humans. Although cities are historically novel environments, a variety of species have successfully utilized the urban environment, and cities are potentially new interaction sites between people and wildlife. Therefore, planning for a highly biodiverse city could increase public interest in species and generate broad public support for conservation.
... Higher testosterone is also sometimes associated with increased aggression and more exaggerated signals of dominance among species [37,38]. Species occurrence and success in cities can be limited by competitive interactions [9,39,40], and therefore dominance signals and aggressive behaviour are likely to be relevant to persistence in urban habitats in some species. Although biodiversity is generally lower in urban areas, the species that thrive there tend to occur at higher densities than in non-urban environments; thus, intraspecific competition for resources may be heightened [11,12,41]. ...
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As cities expand across the globe, understanding factors that underlie variation in urban tolerance is vital for predicting changes in patterns of biodiversity. Endocrine traits, like circulating hormone concentrations and regulation of endocrine responses, might contribute to variation in species' ability to cope with urban challenges. For example, variation in glucocorticoid and androgen concentrations has been linked to life-history and behavioural traits that are associated with urban tolerance. However, we lack an understanding of the degree to which evolved differences in endocrine traits predict variation in urban tolerance across species. We analysed 1391 estimates of circulating baseline corticosterone, stress-induced corticosterone, and testosterone concentrations paired with citizen-science-derived urban occurrence scores in a broad comparative analysis of endocrine phenotypes across 71 bird species that differ in their occurrence in urban habitats. Our results reveal context-dependent links between baseline corticosterone and urban tolerance, as well as testosterone and urban tolerance. Stress-induced corticosterone was not related to urban tolerance. These findings suggest that some endocrine phenotypes contribute to a species' tolerance of urban habitats, but also indicate that other aspects of the endocrine phenotype, such as the ability to appropriately attenuate responses to urban challenges, might be important for success in cities.
... Heat and drought may have directly influenced the breeding success (Reichert et al., 2012) or indirectly by changing the behavior (Buchholz et al., 2019) of interacting species, e.g., an altered pattern of predator activity. Moreover, consideration of intraspecific competition known to affect reproductive output in other bird species (Natsukawa et al., 2019) was beyond the scope of this study. ...
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The lack of suitable nesting sites is one key driver behind the farmland bird crisis in Europe. Winter cereals become impenetrable for ground‐breeding birds like the Eurasian Skylark (Alauda arvensis), curtailing breeding time. Stable Skylark populations depend on multiple breeding attempts per year; thus, the widespread cultivation of winter cereals has strongly contributed to their tremendous decline. Crop diversification is thought to be a potential measure to counteract this development. Therefore, we explored how individual Skylarks respond to the decreasing suitability of winter cereals as nesting habitats in heterogeneous but otherwise conventionally managed farmland. Our study focused on: (i) the degree to which Skylarks prematurely cease nesting activity, switch nesting habitats, or breed on linear structures like tramlines. Additionally, we analyzed: (ii) if nest success decreases throughout the breeding season and (iii) how often Skylarks make a successful breeding attempt per year. We radio‐tagged 28 adults in a German population during April 2018 and 2019, tracked half of them for more than 3 months, and measured their breeding success. Additionally, we monitored nests of untagged pairs, resulting in 96 nests found. None, except one tagged individual, stopped breeding activity before July 1st. Home ranges were mainly stable, but Skylarks switched nesting habitats away from winter cereals to crops like sugar beet or set‐aside. High‐risk nesting sites like corn and linear structures played a minor role in breeding. Overall, Mayfield logistic regressions revealed no seasonal decrease in nest success, and tagged Skylarks had sufficient time to make 1.5–1.8 breeding attempts, of which 0.8 were successful. We suggest that heterogeneous farmland in our study area, which enabled diversely composed home ranges, prevented a curtailment of the breeding season. Thus, our study reinforces the need for crop diversification which gives Skylarks a chance to survive in modern farmland. Dense‐growing crop vegetation in modern farmland is thought to curtail the potential time for breeding of Eurasian Skylarks (Alauda arvensis), thus contributing to the dramatic population decline in Skylarks in Europe. Contradicting this, we found no time constraints for successful reproduction in our Central European study population after following radio‐tagged Skylarks during the breeding season. Our findings are most likely a result of the heterogeneous composition of the study area, which guaranteed the constant availability of accessible and safe nesting habitats. Thus, our work reinforces the need for crop diversification that gives Skylarks a chance to survive in modern agricultural landscapes. Credits: Lisa Dumpe.
... However, her study used lower-resolution weather data from a single, central weather station, possibly contributing to the lack of an observed effect. Natsukawa et al. (2019) also found no effect of weather, but their study was limited to 3 yr of observations. ...
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Weather is thought to influence raptor reproduction through effects on prey availability, condition of adults, and survival of nests and young; however, there are few long-term studies of the effects of weather on raptor reproduction. We investigated the effects of weather on Northern Goshawk (Accipiter gentilis; henceforth goshawk) breeding rate, productivity, and fledging date in south-central Idaho and northern Utah, USA. Using data from 42 territories where we found evidence of breeding attempts in !1 yr from 2011-2019, we analyzed breeding rates using 315 territory-season combinations, analyzed productivity for 134 breeding attempts, and analyzed fledging date for 118 breeding attempts. We examined 35 predictor variables from four categories: precipitation, temperature, wind, and snowpack. Of the variables we evaluated, April precipitation, previous year's April-July precipitation, April-May mean temperature, and March-May mean temperature were related to measures of goshawk reproduction. Greater April-July precipitation in the previous year and lower April precipitation in the current year were associated with higher breeding rates. Years with warmer average April-May temperatures were associated with increased goshawk productivity. Years with greater April-July precipitation during the previous year and lower mean March-May temperatures were associated with later fledging dates. Based on these relationships, we considered projected changes in weather in the northern Great Basin over the next 50 yr as a result of climate change (without directly accounting for habitat changes caused by climate change), and predicted that climate change will: (a) have no significant effect on goshawk breeding rate, (b) have a positive effect on goshawk productivity, and (c) cause a shift toward earlier goshawk breeding. Our results indicate that weather is significantly related to goshawk reproduction in the northern Great Basin, and we suggest that the relationship between raptor breeding and weather be further investigated to enable higher resolution predictions of how changes in the climate may influence their populations, particularly changes that may not have been captured by our study.
... Successful breeding is important in maintaining the ecosystem balance, insofar as it affects the population size (Komdeur, 1996;Pearce-Higgins et al., 2012;Zhao et al., 2019) and reflects the impact of any local environmental change (Natsukawa et al., 2019). Therefore, studies focusing on the relationship between wind farms and the reproduction of birds would further reflect the long-term impact of such farms on avian populations (Dahl et al., 2012;Flaspohler et al., 2001;Hatchett et al., 2013;Pearce-Higgins et al., 2009). ...
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The use of wind turbines for energy generation has complex impacts on ecosystems, especially for flying wildlife. Given the importance of birds in many ecosystems and food webs, they are often used as ecological indicators to evaluate the direct (e.g., collision mortality) and indirect (e.g., habitat degradation and loss, low food available) impact of wind farms. To test the effect of wind farms on the nest distribution of magpie (Pica pica), a common bird species in agroforestry systems on Chongming Island, China, the differences in magpie nest density variables and character variables were compared between quadrats inside and outside a wind farm. The relationship between magpie nest character variables and density variables in each quadrat was also examined and the impacts of landscape variables on magpie nest density variables were clarified in each quadrat. From March to December 2019, nest density variables (including total and in-use nest density) were recorded in quadrats inside and outside a wind farm in an agroforestry system on Chongming Island, Shanghai. Three nest character variables and five landscape variables were also recorded in each quadrat. The total nest density and nesting height in the quadrats outside were significantly higher than those inside the wind farm. There was a significant negative correlation between the average nest size in the quadrats and the total nest density, and a significant positive correlation between the average nesting height in the quadrats and both nest density variables. Moreover, farmland shelterbelt network cover and the distance to the nearest wind turbine were significantly positively related to the total nest density, whereas the farmland shelterbelt network cover was significantly positively related to the in-use nest density. These results indicate a negative effect of wind turbines on the nest density of magpies, which could be addressed by the provision of more farmland shelterbelt networks inside the wind farms located in agroforestry systems on Chongming Island, China.
... All field surveys were carried out over an area of 793 km 2 in eastern Kanagawa (see Natsukawa et al., 2019; 2020 for coordinate information and figure). This area was selected based on administrative districts, within which the entire areas of the Kawasaki, Yokohama, Yamato, Zama, Ebina, Ayase, Fujisawa, and Chigasaki cities, as well as the Samukawa town, were targeted. ...
With ongoing rapid urbanization, urban biodiversity conservation is becoming a primary issue for the maintenance of urban ecosystem functions, and given that birds play valuable ecosystem roles, their preservation is a matter of both ecological and social importance. Although it is generally considered that urban areas have low biodiversity and ecosystem value, these areas can serve as important wintering grounds for both resident and migratory birds. However, as comprehensive surveys of such biota are time- and resource-consuming, it would be useful to establish efficient conservation surrogates that could be used to identify areas of conservation importance. In this study, I evaluated the efficacy of raptor habitats as a conservation surrogate for high avian taxonomic and functional diversities in urban ecosystems in the wintering season. From December 2019 to February 2020 (wintering season), avian species that appeared within 500 m of 30 Northern Goshawk (Accipiter gentilis) breeding sites in eastern Kanagawa Prefecture, Japan, were investigated using point counts. I subsequently measured species richness and dendrogram-based functional diversity (FD) as indicators of avian taxonomic diversity and the ecosystem functions provided by birds, respectively, and performed the same procedures at 30 randomly selected points in areas similar to goshawk breeding habitats. Thereafter, using generalized linear models, I compared species richness and FD at sites with and without goshawk nests. I accordingly found that the presence of goshawk breeding sites is significantly positively associated with avian species richness and FD, explaining most of the variability in these data (species richness: 66.4%, FD: 66.0%). In conclusion, these findings indicate that goshawk breeding sites could be used as a conservation surrogate to identify and preserve habitat hotspots for wintering birds in urban ecosystems. These results can provide a basis for implementing appropriate conservation measures to maintain avian diversity in urban ecosystems.
... Email: abundance, whereas urban land cover and rainfall negatively affect it (Bionda & Brambilla 2012). Urban Northern Goshawks Accipiter gentilis fledge more young in nesting stands with more canopy cover (Natsukawa et al. 2019). ...
Both habitat and weather can strongly influence reproductive rates of birds. We measured reproductive rates of suburban and rural Red‐shouldered Hawks Buteo lineatus in southern Ohio, USA, from 1997 to 2016, and then tested how weather conditions and habitat in the areas surrounding the nest‐sites were related to two measures of reproductive rate. Reproductive rates of Red‐shouldered Hawks did not differ between the suburban and rural study areas, and were relatively stable from year to year. Suburban Red‐shouldered Hawks produced 2736 young in 1773 nesting attempts (i.e. nest with eggs) at 302 territories (1.54 young per nesting attempt), while rural‐nesting hawks produced 996 young in 640 nesting attempts at 108 territories (1.56 young per nesting attempt). Annual nesting success averaged 58.9 ± 1.1% for suburban birds and 58.9 ± 2.1% for rural birds. Several factors influenced Red‐shouldered Hawks’ reproductive rates, measured as either the number of young per nesting attempt or the percentage of nesting attempts that were successful. Warmer air temperatures during May (the nestling period) and increasing amounts of coniferous forest were associated with higher reproductive rates, while increasing April air temperatures and increasing amount of grassland cover had the opposite effect. Land cover variables associated with suburban landscapes, such as the amount of residential development, did not influence the number of young per nesting attempt or nesting success, suggesting that Red‐shouldered Hawks are well‐adapted to human‐dominated landscapes in southern Ohio. Our study also illustrates the value of long‐term datasets for improving our understanding of factors that affect raptors’ demographic parameters.
... The study area covered 793 km 2 in eastern Kanagawa, central Japan (see Natsukawa et al., 2019 for details). The area included Kawasaki City, Yokohama City, Yamato City, Zama City, Ebina City, Ayase City, Fujisawa City, Chigasaki City, and Samukawa Town. ...
Preserving biodiversity and ecosystem functions in rapidly increasing urban areas has become a focal issue of conservation science. With limited budgets and human resources, a potentially efficient conservation strategy is to prioritize important areas in urban ecosystems, based on raptor habitats. Here, northern goshawk (Accipiter gentilis) breeding sites in urban Kanagawa, Japan were used to explore the effectiveness of raptor habitats as conservation surrogates for biodiversity and ecosystem function. The presence of birds was recorded within 500 m of 37 goshawk breeding sites through point counts in the breeding season of 2019. Species richness and dendrogram-based functional diversity (FD) were then determined as measures of biodiversity and ecosystem function, respectively. As controls, 50 sites with landscape compositions similar to goshawk breeding sites were also evaluated. The analysis involved two generalized linear models using species richness and FD as response variables and the presence/absence of goshawk breeding sites as covariates. Goshawk breeding sites had a significant positive effect on species richness and FD, explaining most of their variance (species richness: 63.0%, FD: 56.9%). In conclusion, this study confirmed that goshawk breeding sites that maintain high urban biodiversity and ecosystem function can be used to prioritize and preserve important areas for conservation.
... We studied urban-dwelling goshawks within a 793 km 2 area in Kanagawa Prefecture, central Japan ( Fig. 1; see Natsukawa et al., 2017Natsukawa et al., , 2019. This area included Kawasaki City, Yokohama City, Yamato City, Zama City, Ebina City, Ayase City, Fujisawa City, Chigasaki City, and Samukawa Town. ...
Land cover change accompanying large-scale and rapid urbanization has caused major losses to biodiversity, with urban planning that incorporates biodiversity conservation being urgently needed. Understanding how land cover impacts the distribution and dynamics of breeding sites of biodiversity indicator species could facilitate the creation of long-term, landscape-scale urban planning to enhance biodiversity. Here, we identified land cover factors that influence the distribution and dynamics of the breeding sites of goshawks, a biodiversity indicator species, in the urban areas of Kanagawa, Japan. We analyzed how breeding site presence and absence data were correlated with land cover factors using a logistic regression model. To evaluate the dynamics in breeding site distribution, a meta-population dynamics model was used to analyze the relationship between breeding site occupancy history and land cover factors over 6 years. Forest cover, open land, and highly vegetated urban cover positively affected the distribution of goshawk breeding sites. However, the relative importance of highly vegetated urban cover was not as high as forest cover or open land. Forest cover negatively affected local extinction probability, while forest cover and open areas positively affected local colonization probability. In comparison, highly vegetated urban cover had low relative importance for the distribution of breeding sites, with no relationship being detected for breeding site distribution dynamics. The importance of forest cover and open land for both the distribution and dynamics of goshawk breeding sites emphasizes the importance of these land cover types for the breeding of urban-dwelling goshawks. Therefore, managers should focus on maintaining forests and open land to conserve goshawk breeding sites in urban areas to facilitate their long-term viability. Highly vegetated urban cover was considered to be less important for the long-term viability of goshawk breeding sites.
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The Nearctic northern goshawk (Accipiter gentilis atricapillis) is a resident of conifer, broadleaf, and mixed forests from the boreal to the southwestern montane regions of North America. We report on a 20-year mark-recapture investigation (1991–2010) of the distribution and density of breeders, temporal and spatial variability in breeding, nestling sex ratios, local versus immigrant recruitment of breeders, breeding age structure, age-specific survival rates, and rate of population change (l) of this species on the Kaibab Plateau, a forested sky island in northern Arizona, USA. We used an information-theoretic approach to rank models representing alternative hypotheses about the influence of annual fluctuations in precipitation on the annual frequency of goshawk breeding and fledgling production. We studied 125 goshawk breeding territories, representing approximately 87% of an estimated 144 total territories based on a mean distance of 3.8 km between territory centers in a 1,728-km2 study area. The salient demographic feature of the population was extensive annual variation in breeding, which manifested as large inter-annual variation in proportions of pairs laying eggs, brood sizes, nest failure rates, and fledgling production. The percent of territories known in a prior year in which eggs were laid in a current year ranged from 8% to 86% (�x¼37%, SE ¼4.51), annual mean nest failure rate (active nests that failed) ranged from 12% to 48% (overall �x¼23%, SE¼2.48), and mean annual brood size of successful nests (fledged �1 fledgling) ranged from 1.5 young to 2.5 young (overall �x¼2.0 young, SE¼0.03). Inter-annual variation in reproduction closely tracked inter-annual variation in precipitation, which we hypothesize influenced primary forest productivity and bird and mammal prey abundance. The best breeding years (1992–1993, 77–87% of pairs laid eggs) were coincident with a record-long El Ni~no-Southern Oscillation (ENSO) wet period and the worst breeding year (2003; 8% of pairs laid eggs) was the last of a 3-year record drought. Overall breeding success was 83% with most failures occurring during incubation; once eggs hatched, goshawks tended to fledge young. The pooled 20-year nestling sex ratio did not differ from unity (53% M; n ¼410M, 366 F) but was significantly male-biased in 2 years and female-biased in 1 year. Nonetheless, the overall greater production of male fledglings followed a strong trend of greater male production in other goshawk populations, suggesting that breeders might have been adaptively adjusting their offspring sex ratio, perhaps to produce more of the rarer (male) sex. Annual recruitment of new individuals into the breeding population averaged 43% during the study. Study area recruitment rate of hawks locally born (in situ) and banded was 0.12. Both sexes had equal tendencies to return to the Kaibab Plateau to breed (no differences in philopatry) and there were no differences in natal dispersal distances (natal to first breeding site) between the sexes. During the final years of study (1999–2010), an estimated 46% of breeding recruits were locally born and 54% were immigrants from distant forests. Minimum age at first breeding was 2 years and mean age at first breeding by known-age hawks (banded as nestlings or aged on plumage at first breeding) was 3.7 years for males and 3.5 years for females. Mean lifespan (yr from first banding as nestling to last resighting) of known-age goshawks was 6.9 years for both sexes. Mean minimum apparent lifespan of breeders aged �4 years based on plumage at first capture was 6.5 years for both sexes. Average age of goshawks at their first detection was 3.9 years old, at which time apparent survival was estimated at 0.77 for both sexes, which was just slightly less than the peak survival of 0.78 as a function of age. Age-specific survival estimates showed a steady decline after 9 years old and approached 0 at 20 years of age. Estimates of l for breeding adults (M, 0.94, SE ¼0.037; F, 0.98, SE ¼0.038) provided only weak evidence for a population decline during the study. Although sex was not in the top survival model, models including age þsex were competitive, evidencing lower male than female survival, a finding corroborated by the occurrence of sex effects in the top l model. Lower male survival may result from higher mortality associated with hunting agile prey in vegetation-filled environments during long breeding seasons when they are the primary forager. Lower survival may be compensated by the more frequent production (53%) of male fledglings. High-severity crown fire was an existential threat to the population. In addition to 4 large high-severity fires that burned roughly 3,770 ha (equal to 3 goshawk territories) in the 30 years preceding 1991, 6 high-severity fires burned another 30,945 ha during our study and killed most (>64%) of the forests in 8 known territories and possibly another 2 that were burned before we completed surveys. Based on a lack of any recent demographic perturbations in age structure, a relatively high and time-constant annual adult survival rate, confidence intervals around adult l estimates overlapping 1.0, and a study area saturated with territories, we surmise that the goshawk population on the Kaibab Plateau was stable during the 20-year study. Nonetheless, uncertainty remains regarding the population’s future status because of a declining trend in breeding frequency, uncertain status (dead, alive, emigrated) of non-breeding adults, extensive temporal and spatial variation in breeding, and high frequency of immigrant recruits to the breeding population on the Kaibab Plateau. If the century-long decline in precipitation persists, especially at the increased rate seen since 1980, and manifests as deeper droughts, diminished wet periods, and weaker pulses in forest productivity, then the Kaibab Plateau goshawk population would be expected to show unambiguous evidence of decline. Evidence would include reduced local and regional goshawk reproduction and survival, reduced frequency of immigration, and further habitat loss to catastrophic fire.
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Understanding the mechanisms that shape density-dependent processes and population dynamics is often essential for species conservation. Two key mechanisms of density-dependent reductions in reproductive performance are a limited access to foraging habitats (the habitat heterogeneity hypothesis) and territorial aggression towards conspecifics (the interference competition hypothesis) at high population densities. Disentangling the relative importance of these mechanisms within populations below their carrying capacity is important for the evaluation of the success of conservation measures. However, relatively few studies have attempted to quantify the relative importance of both mechanisms for the reproductive performance of a population. Many raptor populations are ideal model systems to investigate density-dependent effects, since they are currently recovering from human-induced reductions during the last decades. Using a 14-year data set, we combined analyses of individual reproductive performance with a mechanistic population model in order to investigate early signs of density-dependent regulation in a population of White-tailed Eagles Haliaeetus albicilla in north-east Germany. We found a negative effect of the number of neighbouring breeding pairs and a positive effect of water surface area (as a proxy for the availability of favourable foraging habitat) on breeding success and on the average number of nestlings. The mean nearest neighbour distance between breeding pairs has decreased, and the mean distance of nests to the nearest water body has increased over the last 14 years. Moreover, the population model indicates that even though the population is still growing, carrying capacity could be reached at about 500 to 950 territorial pairs. These results suggest that the selection of nesting sites is determined by a trade-off between the distance to favourable foraging habitat and the distance to neighbouring breeding pairs. To avoid increasing competition with conspecifics, due to continued population growth, breeding pairs seem to select increasingly suboptimal habitats. Therefore, our results suggest that the habitat heterogeneity and interference competition hypotheses are not necessarily mutually exclusive as mechanisms of density-dependent population regulation, but can determine the reproductive performance of a raptor population simultaneously. Thus, a future decline in breeding success does not necessarily reflect a decrease in habitat quality, but may rather be a consequence of density-dependent mechanisms. This information may be useful for the interpretation of population trends and for the development of appropriate management strategies for recovering raptor populations. This article is protected by copyright. All rights reserved.
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Raptors increasingly live and nest successfully in urban areas. In the urban landscape of Hartford, CT, red-tailed hawks established home ranges in large green spaces such as parks, golf courses, and cemeteries but also nested successfully in the commercial district of downtown and in densely built urban and suburban neighborhoods. Data collected from 11 radio-tagged breeding adult hawks indicated that year-round home ranges averaged 107.7 ha, much smaller than home ranges reported for hawks inhabiting rural areas. Most hawk home ranges had multiple core areas that were usually associated with favored perches or larger patches of ‘usable’ green space, defined as patches ≥0.25 ha in size, and home range size was positively associated with larger usable green space patches in core areas. Most nests were located in the largest core area and were within a larger patch of green space within the largest core area. Rather than just the amount or size of green space patches, the value of urban green spaces for these hawks likely also varies with the number and proximity of suitable perches such as buildings or tall trees, types and density of prey, and amount of human activity in and adjacent to these spaces. Territoriality and intraspecific competition may also influence home range size and dispersion of red-tailed hawks nesting in Hartford. In this urban area, mortality due to ingestion of rodenticides and collisions with vehicles affected hawk reproductive success.
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1. The concept of site-dependent population regulation combines the ideas of Ideal Free Distribution-type of habitat settlement and density dependence in a vital rate mediated by habitat heterogeneity. The latter is also known as habitat heterogeneity hypothesis. Site-dependent population regulation hypothesis predicts that increasing population density should lead to inhabitation of increasingly poor territories and decreasing per capita population growth rate. An alternative mechanism for population regulation in a territorial breeding system is interference competition. However, this would be expected to cause a more even decrease in individual success with increasing density than site-dependent regulation. 2. We tested these ideas using long-term (1975-99) population data from a goshawk Accipiter gentilis population in Eastern Westphalia, Germany. 3, Goshawk territory occupancy patterns and reproduction parameters support predictions of site-dependent population regulation: territories that were occupied more often and earlier had a higher mean brood size. Fecundity did not decrease with increasing density in best territories. 4. Using time-series modelling, we also showed that the most parsimonious model explaining per capita population growth rate included annual mean habitat quality, weather during the chick rearing and autumn period and density as variables. This model explained 63% of the variation in per capita growth rate. The need for including habitat quality in the time-series model provides further support for the idea of site-dependent population regulation in goshawk.
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Capsule: Variation in reproduction between territories was strongly influenced by intra- and interspecific competition, phenotype, levels of rainfall and human disturbance. Aims: To assess the relative importance of competition, food, habitat quality, weather and phenotype as predictors of variation in reproduction in Buzzard. Methods: Annual breeding data were collected from 1989 to 1996 in a 300-km2 study area and related to 35 independent variables through multivariate regression models. Results: Significant predictors of variation in reproduction between territories (78% variance explained) were variables describing intra- and interspecific competition, plumage morph, laying date, precipitation levels and anthropogenic disturbances in the breeding territory. Competition and plumage morph seem to be particularly important, as these variables explained a high level of variation (81%) in the 1996 reproduction of this population. Conclusion: The potential importance of competition, plumage morph and precipitation for reproduction in this Buzzard population is emphasized.
Raptors are an unusual success story of wildness thriving in the heart of our cities—they have developed substantial populations around the world in recent decades. But there are deeper issues around how these birds make their urban homes. New research provides insight into the role of raptors as vital members of the urban ecosystem and future opportunities for protection, management, and environmental education. A cutting-edge synthesis of over two decades of scientific research, Urban Raptors is the first book to offer a complete overview of urban ecosystems in the context of bird-of-prey ecology and conservation. This comprehensive volume examines urban environments, explains why some species adapt to urban areas but others do not, and introduces modern research tools to help in the study of urban raptors. It also delves into climate change adaptation, human-wildlife conflict, and the unique risks birds of prey face in urban areas before concluding with real-world wildlife management case studies and suggestions for future research and conservation efforts. Boal and Dykstra have compiled the go-to single source of information on urban birds of prey. Among researchers, urban green space planners, wildlife management agencies, birders, and informed citizens alike, Urban Raptors will foster a greater understanding of birds of prey and an increased willingness to accommodate them as important members, not intruders, of our cities.
We determined breeding-site selection of Northern Goshawks (Accipiter gentilis) by comparing 33 breeding sites to 40 nonbreeding sites at the small (nest tree and vegetation structure within an 11.3-m radius of the nest) and the large scale (biomass of prey species and landscape structure within a 500-m radius from the nest) in the urbanized area of Kanagawa Prefecture, Japan. Goshawks selected primarily Japanese cedar (Cryptomeria japonica) trees over 50-cm diameter at breast height (DBH) as nest trees. Trees smaller than 35-cm DBH were avoided. We used univariate logistic regression to assess vegetation structure of the nesting area. At the small scale, canopy trees in the nest plots were larger and taller, with more canopy cover, than those in non-nesting plots, and nests were farther from human habitations. In addition, nest plots had fewer understory trees than non-nest plots. In multivariate logistic regression, the DBH of canopy trees, number of understory trees, and distance to human habitation were highest in importance. At the large scale, nest plots had a greater percentage of forest area, smaller percentage of built area, more forest edge facing open land, and more forest edge facing PCDRS (parks, cemeteries, developed land, recreational fields, and small-scale vegetated areas <0.01 km2 in a matrix of ≥30% residential areas). In the multivariate model, the length of forest edge facing open land and the percentage of built area were of highest importance.
Applied Hierarchical Modeling in Ecology: Distribution, Abundance, Species Richness offers a new synthesis of the state-of-the-art of hierarchical models for plant and animal distribution, abundance, and community characteristics such as species richness using data collected in metapopulation designs. These types of data are extremely widespread in ecology and its applications in such areas as biodiversity monitoring and fisheries and wildlife management. This first volume explains static models/procedures in the context of hierarchical models that collectively represent a unified approach to ecological research, taking the reader from design, through data collection, and into analyses using a very powerful class of models. Applied Hierarchical Modeling in Ecology, Volume 1 serves as an indispensable manual for practicing field biologists, and as a graduate-level text for students in ecology, conservation biology, fisheries/wildlife management, and related fields. Provides a synthesis of important classes of models about distribution, abundance, and species richness while accommodating imperfect detection Presents models and methods for identifying unmarked individuals and species Written in a step-by-step approach accessible to non-statisticians and provides fully worked examples that serve as a template for readers' analyses Includes companion website containing data sets, code, solutions to exercises, and further information
The breeding success of endangered colonial nesting species is important for their conservation. Many species of Gyps vultures form large breeding colonies that are the foci of conservation efforts. The Cape Vulture is a globally threatened species that is endemic to southern Africa and has seen a major reduction in its population size (≥50% over 48 years). There is evidence that breeding colonies are prone to desertion as a result of human disturbance. Factors that influence the occupancy and breeding success of individual nest sites is not fully understood for any African vulture species. We investigated cliff characteristics and neighbour requirements of the Msikaba Cape Vulture colony, a major breeding colony in the southern node of the population in the Eastern Cape, South Africa, together with their nest site occupation and breeding success over 13 years. In total, 1767 breeding attempts were recorded. Nest sites that had a higher elevation, smaller ledge depth, greater total productivity and were surrounded by conspecifics were more likely to be occupied, although the amount of overhang above the nest was not an important predictor of occupancy. In accordance with occupation, nest sites with a smaller ledge depth had higher breeding success; however nests with a greater overhang were also more successful and height of the nest site was not an important predictor of breeding success. The breeding success of a nest site in a given year was positively influenced by the number of direct nest neighbours, and nests in the middle of high density areas had greater breeding success. This suggests that maintaining a high nest density may be an important consideration if declines of reproducing adults continue. Breeding success declined over the study period, highlighting the effects of a temporal variation or observer bias. Our results identified optimal nest site locations (ledge depths of 1 m, and at a height of 180 m) and their effects on breeding success. This information can be used for planning reintroduction efforts of the endangered Cape Vulture and for their ongoing conservation. This article is protected by copyright. All rights reserved.