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Larger broods in the Northern Goshawk Accipiter gentilis
near urban areas in southern Finland
Tapio Solonen
Solonen, T., Luontotutkimus Solonen Oy, Neitsytsaarentie 7b B 147, FI-00960 Helsinki,
Finland. tapio.solonen@pp.inet.fi
Received 8 May 2008, accepted 4 September 2008
I examined if the distance from densely-built (urban) areas is reflected in the brood size of
the Northern Goshawk near the southern coast of Finland. The data were collected from
70 nesting territories in 1976–2007, including 270 fledged broods. Within an approxi-
mate distance of less than 2.5 km from the nearest urban area, the average brood size was
significantly higher than that in more rural environments. In the most densely populated
urban area, the brood size was significantly higher than in the least urban areas. In general,
urban habitats seemed to provide more stable food and nesting conditions as compared to
rural ones. This was suggested by the long-term stability of relatively many of the most ur-
ban nesting localities, and by the recent apparently increasing tendency of Goshawks in
urban areas to re-occupy earlier abandoned territories and even to establish new ones. For
the Northern Goshawk, the urban life habits may locally provide superior alternatives for
the rural ones. From a conservation point of view, urban and near-urban territories of the
Northern Goshawk might be particularly valuable due to the general scarcity of suitable
nest-sites in urban environments.
1. Introduction
Birds inhabiting environments that are intensively
used by man may be original inhabitants of the
area, or colonists that have adapted to occupy such
modified or new habitats. At present, natural habi-
tats are rare in Europe where rural (agricultural
and silvicultural) and urban environments pre-
dominate (Tucker & Evans 1997, Donald et al.
2000). From the point of view of breeding birds,
man-made rural and urban environments may dif-
fer in many respects other than the direct effects of
human activities (Bird et al. 1996, Marzluff et al.
2001). Such factors, including local food supply,
wintering conditions, bird density (competition,
prey supply) and predation, may considerably af-
fect the occurrence and breeding of birds.
Various studies have shown that some bird spe-
cies, in particular small passerines, produce fewer
eggs and fledglings in urban habitats than they do
in rural ones (Perrins 1965, Berressem et al. 1983,
Cowie & Hinsley 1987, Schnack 1991, Hõrak
1993, Solonen 2001). However, urban habitats
may be superior for some other species such as
birds of prey and corvids because they often are
free from persecution there and have an abundant
year-round food supply (Newton 1986, Jerzak
2001, Vuorisalo et al. 2003, Kelcey & Rheinwald
2005, Rutz et al. 2005, Chace & Walsh 2006,
Solonen & af Ursin 2008). Unlike most passerines,
birds of prey may have home ranges that extend
beyond the urban boundary and therefore do not
need to meet all their ecological requirements
within urban areas.
Ornis Fennica 85:118–125. 2008
Birds of prey are used to be considered as in-
habitants of remote districts far away from human
settlements. This is supposed to be due to avoiding
persecution but also to deterioration of habitats by
various human activities (e.g., Newton 1979). In
more recent years, however, various species have
become established even in the most heavily built-
up areas (e.g., Bird et al. 1996, Marzluff et al.
2001, Kelcey & Rheinwald 2005, Chace & Walsh
2006, Rutz et al. 2006). Feeding and breeding con-
ditions may have improved in urban areas due to
increasing populations of suitable prey while they
may have got worse in the surroundings (e.g.,
Marzluff et al. 2001, Rutz 2006, 2008). Besides
the clear changes in clutch size and brood size, the
food supply may have various (less easily visible)
effects on the reproduction and offspring survival
of birds (e.g., Newton 1998, Byholm & Kekkonen
2008). These include variations in the breeding in-
vestments (such as egg volume and egg quality) or
in the condition and fitness of offspring.
The Northern Goshawk Accipiter gentilis is a
common, widely but sparsely distributed year-
round inhabitant of various forested areas of the
temperate and boreal climatic zones (Cramp &
Simmons 1980, Kenward 2006). In Finland, the
main habitat of the species constitutes of old and
large spruce forests far away from human settle-
ments but due to the lack of alternative options it
also accepts small fragments of suitable habitats
even in the vicinity of built-up areas (e.g., Solonen
1993). A suitable nesting tree is an essential pre-
requisite for breeding that is otherwise largely
governed by the adequate availability of food (e.g.,
Lindén & Wikman 1983, Bijlsma & Sulkava 1997,
Kenward 2006, Rutz et al. 2006). The Finnish
Northern Goshawk population has long shown a
slow but steady decline (e.g., Bijlsma & Sulkava
1997). During recent decades, the living condi-
tions of Fennoscandian Northern Goshawks have
changed dramatically because nesting habitats and
food supply in forests have remarkably deterio-
rated due to intensive forestry (Widén 1997,
Solonen 2003, Selås et al. 2008).
In this paper, I examine if the distance to urban
areas relates the brood size of Finnish Northern
Goshawks (cf. Rutz 2008, Selås et al. 2008). I pre-
dicted that due to abundant anthropogenic food
supply the brood size is higher near human settle-
ments (Diermen 1996, Rosenfield et al. 1996,
Boal & Mannan 1999, Millsap & Bear 2000, Rutz
et al. 2006). In addition, I expected a negative rela-
tionship between the distance from urban habitats
and the frequency of territory occupancy, and a
positive relationship between the brood size and
the number of successful nesting attempts, sug-
gesting that the most suitable localities were urban
and that they also were occupied most frequently
(cf. Sergio & Newton 2003).
2. Material and methods
2.1. Study area
The study was conducted in 1976–2007 in Uusi-
maa, southern Finland (60°N, 25°E). The main
study area was situated in the municipalities of
Helsinki, Espoo, Vantaa, and Sipoo, but some data
from the surrounding Nurmijärvi, Tuusula, Hy-
vinkää, Mäntsälä, Pornainen, Askola, and Porvoo
were also included (cf. Solonen 1993). On the ba-
sis of the geographical situation and general habi-
tat characteristics, the study area was broadly sub-
divided into four sub-areas (Fig. 1): a) the urban
south-western part, b) the rural western part (sur-
Solonen: Brood size in urban and rural Northern Goshawks 119
Fig. 1. Schematic map of the distribution of main
urban habitats (thin lines) and the nesting territories
of the Northern Goshawk from where the data were
derived (black dots) in the study area of the Hel-
sinki district and its surroundings, near the southern
coast (heavy line) of Finland. The sub-areas
(south-western & western, eastern, and northern)
are broadly indicated by the clusters of data points
delimited by a dashed line.
rounding the sub-area a), c) the mainly rural east-
ern part, and d) the largely rural northern part.
The south-western main study area of more
than 500 km2consisted of the capital district of
about one million inhabitants, the most urban area
in the country (sub-area a), and the nearby rural
habitats of mixed fields and forests (sub-area b).
Even in the sub-area a the built-up areas were,
however, quite fragmented because urban sprawl
is relatively recent, and still largely surrounded by
more or less rural habitats. Really urban, densely
built-up city environments covered relatively re-
stricted areas near the southern coast. Pressed by
various urban areas there were several small city
parks and some larger forest tracts mainly used for
recreation (e.g., Solonen 2001). The sub-area b
was characterized by relatively productive fields
and forests in the vicinity of the most urban sub-
area a. In the eastern sub-area c, the proportion and
density of built-up areas were low, while they were
relatively high in the northern sub-area d, where
the rural habitats were relatively barren.
2.2. Hawk monitoring
Territories and nests of Northern Goshawks were
localized mainly by listening for calling birds,
checking the known potential nest-sites, and
searching for new ones in suitable habitats
(Forsman & Solonen 1984). Nest-sites found were
monitored annually, but the monitoring often
lasted only short periods of years due to the com-
mon destruction of nest-sites, usually by intensive
forestry. Alternative and new replaced nest-sites
were sought, but often there were no suitable habi-
tats available in the vicinity of the lost ones or they
seemed to be too close to the nest-sites occupied by
neighbouring pairs (see Solonen 1993). So, the an-
nually monitored locations as well as the monitor-
ing periods of single nest-sites varied consider-
ably. In general, however, Northern Goshawks oc-
cupied the study area relatively evenly (Solonen
1993), the main gaps in the distribution map (Fig.
1) being due to gaps in the data available for the
present study.
Because of practical difficulties and probably
higher risks of harming nesting success when
checking the clutch size, the investment to repro-
duction was measured by the brood size. In vari-
ous species of birds, the brood size largely follows
similar patterns of variation than the clutch size
(e.g., Solonen 2005, but see Byholm 2005).
The nests found were usually climbed to estab-
lish brood size when ringing the nearly fledged
young. In a few cases also a reliable record of the
number of fledged young was accepted. The data
included 270 successful broods (at least one young
fledged). The mean size of successful broods from
70 nesting territories (sensu Newton 1979) was in-
cluded in the analyses.
2.3. Explaining variation
The habitats of Northern Goshawks were broadly
divided into densely built-up urban areas and less
intensively used rural ones. Densely built-up habi-
tats were characterized by urban-type land use,
and included traffic routes, industrial areas, com-
mercial houses, and groups of buildings of at least
200 inhabitants not allowing any gaps of typically
more than 200 meters, as defined by Statistics Fin-
land. The gaps were usually parks, recreational fo-
rests, and small plots of more or less natural open
habitats. Other, larger agricultural, silvicultural
and wilderness areas were defined as rural. The
approximate distance (to the nearest km) of nest-
sites from the nearest densely built-up area served
as an indicator of the urban effect on the brood
size. Based on this distance, the territories were di-
vided into two categories separated by a value of
2.5 km, approximating the general nearest-neigh-
bour distance of the species in the district (Solonen
1993). In addition, the mean data for the most ur-
ban capital district were compared with those of
other, less urban parts of the study area (Fig. 1). As
the real habitat use of the foraging hawks was not
known, the above broad classification of habitats
seemed justified and adequate for the present pur-
pose.
Groups were compared by t-test, Mann-Whit-
ney rank sum test, or by one way analysis of vari-
ance. When the data compared did not meet the re-
quirement of normality, ln-transformation was
used. Relationships between variables were exam-
ined by Pearson product moment correlation. P
values higher than 0.05 were considered non-sig-
nificant. Calculations were performed by Sigma-
Stat 3.1 statistical software.
120 ORNIS FENNICA Vol. 85, 2008
3. Results
On average, the 270 successful broods of the
Northern Goshawk produced 2.71 (± 0.86 SD,
range 1–4) fledglings. The territorial means (n=
70) averaged 2.75 (± 0.55 SD). The mean brood
size of the nesting territories in the vicinity (< 2.5
km) of urban environments (2.95 ± 0.48 SD, n=
19) was significantly higher than that further away
from the densely built-up areas (2.67 ± 0.56 SD, n
= 51) (t68 = 2.06, P= 0.043, ln-transformed data).
The mean brood size differed significantly be-
tween the most urban capital district and the least
urban study areas eastwards (Mann-Whitney test,
T27,31 = 973.5, P= 0.006; medians 3.0 and 2.5, re-
spectively). There were expected kinds of differ-
ences also between the four minor sub-areas (a–d)
but they were not significant (Table 1).
The number of successful broods in the nesting
territories showed pronounced variation (Fig. 2).
Relatively many of the most frequently occupied
nesting territories were situated near to urban areas
but the relationship, in general, was not significant
(r= – 0.168, P= 0.164, df = 68). There were no
significant relationship either between the brood
size and the distance from urban habitats (r=
0.042, P= 0.731, df = 68) or between the brood
size and the frequency of territory occupancy (r=
– 0.077, P= 0.529, df = 68).
4. Discussion
The present study suggests that rural and urban
habitats differed in some perceptible respects con-
cerning the reproductive success of Northern Gos-
Solonen: Brood size in urban and rural Northern Goshawks 121
Table 1. Mean brood size of the Northern Goshawk (averages for means of nesting territories) in the four
sub-areas of the present study (Fig. 1). The differences are not significant (one-way analysis of variance,
F= 1.305, P = 0.280).
Sub-area Habitat Number of territories Mean brood size (SD)
South-western (a) Urban 18 2.94 (0.50)
Western (b) Near urban 18 2.73 (0.44)
Eastern (c) Mainly rural 22 2.68 (0.68)
Northern (d) Largely rural 11 2.57 (0.50)
Fig. 2. The distribution of
the number of successful
broods in the urban
(black) and rural (white)
Northern Goshawk nest-
ing territories.
hawks. As hypothesized, in the vicinity of urban
areas, brood size seemed to be higher than else-
where. It may have been difficult to prove con-
vincingly, however, due to the considerable indi-
vidual (territorial) variation in the brood size
(range 1–4) and to obviously pronounced smaller-
scale variation in the habitat quality within both ur-
ban and rural environments. So, the real effects of
urbanization may have been poorly characterized
by the broad habitat criteria used. In spatially and
temporally heterogeneous data, slight differences
will probably emerge clearly only in large data sets
or within a long run. In these respects, the present
data seemed to be somewhat too scanty.
4.1. Advantages of living
in an urban environment
Some additional observations from the study area
suggest that, at present, urban environments might
provide particularly suitable breeding conditions
for Northern Goshawks. In general, near-urban
habitats seemed to offer nesting conditions and
food supply that are stable as compared to those of
the rural environments of the district for the fol-
lowing reasons. Firstly, the intensive harvesting of
rural old forests has led to continuous or frequent
re-establishment of territories (Solonen 1993)
while the recreational forests near densely built-up
areas have often been treated more moderately.
Thus, some of the most urban nesting localities of
the study area have been occupied, or at least re-
mained in usable condition, throughout the study
period. Secondly, concentrations of Feral Pigeons
Columba livia domestica, corvids, and other bird
species preferring urban areas seemed to be a
tempting alternative to the nowadays generally
scanty food supply of more rural and forested ar-
eas (cf. Lindén & Wikman 1983, Selås 1997, Rutz
et al. 2005, Rutz 2006, Byholm et al. 2007, By-
holm & Kekkonen 2008). This is also suggested
by the recent and apparently increasing tendency
of hawks to re-occupy old, long unused urban ter-
ritoriesandeventoestablishnewones(T.Solo
-
nen, unpubl.). These observations coincide with
those recently reported from various more urban-
ized areas (Bijlsma & Sulkava 1997, Chace &
Walsh 2006, Rutz et al. 2006).
The production of young in birds is largely de-
termined by food-related factors (Newton 1980,
1998). It might be affected not only by the amount
but also by the quality of available food. The local
food supply in turn is largely determined by rela-
tively predictable local habitat factors. The out-
come of the habitat selection in birds seems to be a
compromise between profits and disadvantages
encountered. The primary habitats of most species
can be found in rural, or rather in wilderness areas.
Increased urbanization has led to local increases in
avian density and biomass (e.g., Millsap & Bear
2000, Sorace 2002), and high densities of potential
prey have attracted various birds of prey to hunt
and consequently breed in urban environments
(e.g., Bird et al. 1996, Marzluff et al. 2001, Kelcey
& Rheinwald 2005, Chace & Walsh 2006, Rutz
2008). Also pairs nesting in rural habitats in the vi-
cinity of urban areas may have benefited from us-
ing urban food resources (Solonen 1993).
In the case of the Northern Goshawk, the urban
life seemed to be locally a good, if not even a supe-
rior alternative to the rural one (Rutz et al. 2006,
Rutz 2008, this study). Similar results have been
reported also for some other species of birds of
prey (Botelho & Arrowood 1996, Diermen 1996,
Gehlbach 1996, Boal & Mannan 1999, Millsap &
Bear 2000). However, there are pronounced dif-
ferences in European and American Northern
Goshawks and their use of urbanized areas (Ken-
ward 1996, Bosakowski 1999, Morrison 2006).
European hawks have been subject to intensive
human presence and development for a very long
time and apparently have adapted to use many ur-
ban areas. American Northern Goshawks, on the
contrary, are not found to breed near urban settle-
ments.
4.2. Variation in the number
of successful nesting attempts
The expected relationship between the distance
from urban habitats and the number of successful
nesting attempts was not significant in the present
data. Probably this resulted largely from the fact
that the number of successful nesting attempts
showed pronounced variation due to the differ-
ences in the long-term stability (frequency of oc-
cupation and preservation of habitats) of the nest-
ing territories examined and in the length of the ob-
122 ORNIS FENNICA Vol. 85, 2008
servation period. Additional explanations can be
derived from habitat alterations and the causes of
unsuccessful breeding attempts (e.g., Byholm &
Nikula 2007).
Besides forestry, the construction of buildings
and traffic routes considerably diminished the
amount of suitable habitats and potential nesting
locations for the Northern Goshawk in the study
area. The direct human interference was, however,
only seldom proved to be the primary cause of
nesting losses. Other predators such as the Pine
Marten Martes martes, Eagle Owl Bubo bubo,and
corvids probably were responsible for some nest
losses (cf. Byholm & Nikula 2007). Nest-sites
may also have been abandoned due to competition
with other species such as Pine Marten and Com-
mon Buzzard Buteo buteo using similar nests for
resting or nesting. Several nests had fallen down
due to weak supporting branches of the nesting
tree. Some territories were occupied most proba-
bly by non-breeding individuals. Finally, some al-
ternative nests may have been missed, or nesting
locations may have remained undetected due to in-
sufficient field work.
4.3. Conclusions
The urban environments of the study area appar-
ently provided better conditions for breeding
Northern Goshawks than did the other compared
habitats. The brood size in the urban habitats might
be larger than in the rural ones due to a high and
stable prey supply. From a conservation point of
view, the urban and semi-urban Northern Gos-
hawk territories seemed to be of particular value.
Therefore, I suggest that urban planners take care
of the even and continuous availability of parks
growing mature trees and natural kinds of forests
(see also Sergio & Bogliani 2000). Maintaining
Northern Goshawks in urban habitats could yield
the additional advantage of potentially lowering
the populations of bird species considered by some
as harmful; examples include gulls, pigeons and
corvids.
Acknowledgements. The field work was helped by many
people, not least by my family members. I also wish to
thank Antero Ahola, Johan Bäckström, Eero Haapanen,
Sami Kiema, Seppo Kuusela, Arto Leinonen, Eeva Leppä-
lä, Heikki Lokki, Rolf Michelsson, Esa Pienmunne, Petri
Piisilä, Jari Pynnönen, Matti Päiviö, Rainer Salo, Henrik
Silfverberg, Klaus Silfverberg, Jukka Simula and Jorma
Turunen for their contribution. Marcus Walsh kindly
checked the English. Patrik Byholm, Fabrizio Sergio and
anonymous referees read a draft of the manuscript and
made useful suggestions.
Kanahaukan poikuekoko kaupunki-
ja maaseutuympäristöissä Etelä-Suomessa
Tutkin, vaikuttaako pesäpaikan etäisyys kaupun-
kimaisesta asutuksesta kanahaukan poikue-
kokoon. Vuosina 1976–2007 koottu aineisto on
peräisin 70 pesimäpaikalta yhteensä 270 poiku-
eesta. Korkeintaan n. 2.5 km:n päässä kaupun-
kiympäristöstä kanahaukan keskimääräinen poi-
kuekoko oli merkitsevästi suurempi kuin maaseu-
tumaisemmissa ympäristöissä. Pesyekoko oli
kaikkein kaupunkimaisimmalla alueella merkitse-
västi suurempi kuin vähiten kaupunkimaisella.
Kaupunkiympäristöt näyttäisivät yleisesti olevan
ravinto- ja pesintäolosuhteiltaan maaseutuympä-
ristöjä vakaampia.
Tähän viittaavat myös monien kaupunkirevii-
rien pitkäikäisyys, viime aikoina yleistynyt taipu-
mus asuttaa uudelleen aikaisemmin ilmeisesti kau-
pungistumisen seurauksena hylättyjä pesimäpaik-
koja, sekä asuttaa aivan uusiakin kaupunkiympä-
ristöjä. Kaupunkiympäristö voi paikallisesti olla
kanahaukalle maaseutuympäristöä edullisempi
vaihtoehto. Tuotteliaat kaupunkilaisreviirit voivat
myös elvyttää maalaisympäristöjen heikentynyttä
kanahaukkakantaa. Sopivien pesäpaikkojen niuk-
kuus tekee kaupunkilaisreviireistä erityisen arvok-
kaita.
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