Changes in the numbers of corvids wintering in Warsaw over years

Changes in the numbers of corvids wintering in Warsaw over years

Jerzak L., Kavanagh B.P., Tryjanowski P. (red.)
Ptaki krukowate Polski [Corvids of Poland]
Bogucki Wyd. Nauk., Poznań 2005
Tomasz D. Mazgajski, Agnieszka Woźniak, Michał Żmihorski
Changes in the numbers of corvids wintering
in Warsaw over years 1974–2004
Zmiany liczebności krukowatych zimujących
w Warszawie w latach 1974–2004
ABSTRACT: The population trend of corvids wintering in Warsaw presented in this paper
are based on a comparison of the birds’ densities observed in city parks. The authors were
able to find reliable data for the last thirty years on this subject from studies of only 7 winters, conducted in two parks located in the centre of the city: Ogród Saski (16 ha) and Ogród
Krasińskich (10 ha).
It was found that the highest number of wintering Rooks was observed at the end of the
1980s. At the beginning of the 1990s, their numbers started to decrease. The number of wintering Jackdaws remained stable over the last thirty years. Populations of Hooded Crows and
Magpies remained at a constant level after a significant increase at the beginning of the 1980s.
The lack of data about the origin of the birds spending winters in the city renders a detailed
analysis of the results obtained impossible. It is not clear if the observed trends in wintering
populations are connected with climate changes, shortening of migration routes or changes
occurring in breeding populations, such as the synurbization process.
KEY WORDS: Corvus cornix, Corvus frugilegus, Corvus monedula, Pica pica, roosting, wintering
STRESZCZENIE: W pracy opisano zmiany liczebności ptaków krukowatych zimujących w
Warszawie porównując zagęszczenia ptaków żerujących w parkach miejskich. Dla okresu
ostatnich trzydziestu lat możliwe było zebranie publikowanych jak i niepublikowanych danych z badań prowadzonych w ciągu 7 zim w dwóch parkach położonych w centrum miasta
– Ogrodzie Saskim (16 ha) i Ogrodzie Krasińskich (10 ha). Stwierdzono, że do połowy/końTomasz D. Mazgajski, Museum and Institute of Zoology, Polish Academy of Sciences, Wilcza
64, 00-679 Warszawa, Poland, e-mail: [email protected]
Agnieszka Woźniak, Michał Żmihorski, Department of Ecology, University of Warsaw, Banacha 2, 02-097 Warszawa, Poland
420
Tomasz D. Mazgajski, Agnieszka Woźniak, Michał Żmihorski
ca lat 80. liczebność zimujących w Warszawie gawronów wzrastała, zaś od początku 90.
nastąpił spadek liczebności tego gatunku, który trwa do chwili obecnej. Liczebność zimujących kawek, mimo pewnych fluktuacji pozostaje na podobnym poziomie. Na początku
lat 80. liczebność zimujących w Warszawie wron siwych i srok dość szybko wzrosła, ale od
tego okresu utrzymuje się na podobnym poziomie.
Brak danych o pochodzeniu ptaków – czy są one przybyszami z innych populacji, czy też
gniazdują w mieście, utrudnia analizę uzyskanych wyników. Nie jest do końca jasne, czy na
stwierdzone trendy liczebności wpływają czynniki związane z klimatem i skracaniem tras
wędrówek tych ptaków, czy też zmiany zachodzące w populacjach lęgowych, m. in. proces
synurbizacji.
SŁOWA KLUCZOWE: Corvus cornix, Corvus frugilegus, Corvus monedula, Pica pica, noclegowiska,
zimowanie
Introduction
There is much data describing the population trends of breeding corvids in Poland.
This is especially true for the Rook Corvus frugilegus (Bereszyński 1987, Hordowski
1995a, Tryjanowski 1996, Biaduń 1998, Kasprzykowski 2001). Some data for other
species, such as the Hooded Crow C. cornix and Magpie Pica pica are also available,
especially in relation to their synurbization process (Górski & Kotlarz 1997,
Luniak et al. 1997, Udolf 2004). Less attention has been paid to changes in the
wintering populations. Large winter corvid roosts in cities, where Rooks and Jackdaws C. monedula predominate, are most frequently studied, and consist of descriptions of roosting behaviour, the sites, as well as an estimation of the numbers at
the winter roosts (Grodziński 1971, Jadczyk 1994, Hordowski 1995b, Winiecki
2000, Jakubiec & Jadczyk 2001). However, studies indicating changes in the numbers of wintering birds are scarce (Hordowski 1995b).
It was found that Rooks breeding on the Russian Plain spend their winters in
Poland. Also, part of the Polish breeding population may not migrate but remain in
its breeding areas (Busse 1969, Jadczyk 1994). Ringing data suggests a tendency
towards shortened migration routes (Busse 1969, Siefke 1994, Busche 2001),
probably connected with climate changes. Lithuanian data indicates that the winters have become warmer (Raudonikis 2001), therefore more and more birds may
be spending winter closer to their breeding grounds. All those aspects may affect
long term changes in the numbers of wintering birds.
The aim of this study was to present the dynamics of corvids wintering in Warsaw based on changes in the densities of birds foraging in two urban parks located
in the centre of the city.
Study area
Wintering corvid flocks consist of Rooks, Jackdaws, Hooded Crows and Magpies. It
was found that these species achieved their highest densities in old urban parks, as
well as in the city centre (cf. Luniak 1981, Luniak et al. 1997, Luniak et al. 2001).
Changes in the numbers of corvids wintering in Warsaw over years 1974–2004
421
Fig. 1. Location of the studied areas in Warsaw, 1975–2004
KRA – Ogród Krasińskich, SAS – Ogród Saski
Ryc. 1. Lokalizacja terenów badań w Warszawie, 1975–2004
KRA – Ogród Krasińskich, SAS – Ogród Saski
By analyzing the existing data, it was possible to follow changes in the numbers
of birds foraging in two parks: Ogród Saski (16 ha) and Ogród Krasińskich (10 ha).
These parks were established in XVI–XVII centuries, and are located in the city
centre in close proximity (Fig. 1). There are many old trees in the parks, but grassy
areas predominate and there are many walks and paths for pedestrians. Pedestrians
(and especially their dogs) can disturb the foraging birds, but they may also be a
source of additional food.
Methods
Published results (Luniak 1981), unpublished materials (Szczepanowski 1984, results of bird counts conducted for “Birds of Warsaw” atlas project – Luniak et al.
2001) and data collected by the authors themselves were used to determine the
population trends of wintering corvids. Calculations based on these data determined the average density (individuals/10 ha) for each park separately, and the average density for each particular species for both parks together. However, the
number of counts made and the duration of the study differed in each study used
for these comparisons, as follows:
– winter of 1974/1975 – 2–3 counts between mid-December and the end of February (Luniak 1981);
– winters of 1982/1983 and 1983/1984 – 14–16 counts from November to the
end of March (Szczepanowski 1984);
– winter of 1987/1988 – 3–4 counts between November and February. These
were carried out by the members of the Warsaw Ornithological Club of the Polish Zoological Society as a part of an inventory of the birds of Warsaw (Luniak
et al. 2001). The data were available as a range of the bird densities. The centre
of the range was used for this study;
– winter of 1995/1996 – 8 counts from November to February (own data);
– winter of 2002/2003 – 6 counts from December to February (own data);
– winter of 2003/2004 – 10 counts from November to February (own data).
422
Tomasz D. Mazgajski, Agnieszka Woźniak, Michał Żmihorski
Results
At the end of the 1970s and during the 1980s, the number of wintering Rooks increased. This was the most numerous species at the end of 1980s, with a slow decrease following in later years to 2004 (Fig. 2A).
The number of wintering Jackdaws in Warsaw was stable, although some fluctuations were observed (Fig. 2B).
After a strong increase at the beginning of 1980s, the number of both Magpies
and Hooded Crows also stablised (Fig. 3A, B).
Fig. 2. Changes in the densities of wintering corvids in Warsaw 1974–2004; A – Rook
Corvus frugilegus, B – Jackdaw C. monedula
Ryc. 2. Zmiany zagęszczeń zimujących krukowatych w Warszawie 1974–2004; A – gawron
Corvus frugilegus, B – kawka C. monedula
Changes in the numbers of corvids wintering in Warsaw over years 1974–2004
423
Discussion
In this paper, we attempt to present long-term changes in the numbers of corvids
wintering in Poland. It is difficult to say if the results reflect actual trends in wintering populations because of the differences in the methods used to count birds in
the studied winters, as well as the relatively small areas used in the calculations of
the densities of birds.
Based on counts of Rooks foraging in two urban parks, it was found that their
highest abundance occurred at the end of the 1980s, and decreased slightly over
Fig. 3. Changes in the densities of wintering corvids in Warsaw 1974–2004; A – Hooded
Crow C. cornix, B – Magpie Pica pica
Ryc. 3. Zmiany zagęszczeń zimujących krukowatych w Warszawie 1974–2004; A – wrona
siwa C. cornix, B – sroka Pica pica
424
Tomasz D. Mazgajski, Agnieszka Woźniak, Michał Żmihorski
time. This result was confirmed by data describing the entire assemblage of Warsaw corvids (about 80% Rooks). Estimations of the number of birds flying to communal roosts revealed that the highest number, ca. 220 thousand birds, occurred
during the winter of 1988/89, whereas it decreased to 180 thousand in the next
winter and in 1995/96, only ca. 125–130 thousand birds were estimated
(Mazgajski & Szczepanowski 2005).
At least three, partially related factors could explain the population trends of
wintering corvids in Warsaw described in this paper. First, the trend may be connected to climate changes. Milder winters may result in shorter migration distances, with the birds remaining closer to their breeding grounds. Second,
long-term trends in the breeding populations may affect the number of wintering
birds. Third, changes in habitat selection and the birds’ distribution in wintering
areas may have taken place. However, all these explanations are rather speculative,
and there is insufficient information to verify them.
The small amount of data for this paper – 7 winters over 30 years – makes a reliable correlation of corvid population dynamics with climatic factors, such as
changes in winter air temperature or snow cover, impossible.
It is also not possible to determine if the trends observed reflect any changes in
the breeding population and/or shortening of migration routes, because many recoveries of ringed birds are needed to make such comparisons. Only 5 ringed
Rooks have been found In Warsaw in the winter (4 from the Russian Plain and 1
from Latvia). There are no data for Jackdaws and Hooded Crows, which would allow an evaluation of their migratory or sedentary characteristics. More than 200
Rooks and Jackdaws that had died in the roost were inspected during two winters
(1994/95 and 1995/96), but no ringed birds were found (data: T. Mazgajski, A.
Kędra, P. Tykarski).
However, it may be possible to analyze the observed trends by only using ringing data, especially when considering the shortening of migration routes as a factor affecting wintering population dynamics. This was postulated a number of
years ago (Busse 1969), but evidence for it was only found in the case of the
Hooded Crow in a more recent analysis (Siefke 1994, Busche 2001). It is possible
that the rapid increase in numbers of wintering Rooks in the 1980s was connected with such a process, when masses of these birds remained in Central Europe. Thus, the decrease in the wintering population at the beginning of 1990s
observed both in Poland (Hordowski 1995b, this study) and Central Europe
(Unger & Bauer 2001) could be related to the process of shortening migration,
and a significant proportion of Russian Rooks may be wintering closer to their
breeding grounds.
There are no data establishing the origin of the Jackdaws and Hooded Crows
wintering in Warsaw. We found that the population of wintering Jackdaws was stable over the last 30 years (Fig. 2B). Luniak et al. (2001) states that most of the Jackdaws breeding in Warsaw, spend the winters in the city. The increase of Hooded
Crow and Magpie numbers in the early 1980s could be explained by progress in the
synurbization process (Luniak et al. 2001). This phenomenon is connected with
the cessation of migration (e.g. Konstantinov et al. 1982), thus the stable popula-
Changes in the numbers of corvids wintering in Warsaw over years 1974–2004
425
tion of Hooded Crows wintering in Warsaw, observed after that time suggests that
most birds breeding in the city spend the winter season there also.
Unfortunately, the lack of ringing data and the use of only a limited number of
winter bird counts makes it impossible to verify the hypotheses on the observed
population trends of wintering corvids. To do so requires long-term monitoring
and ringing recoveries. This would enable us to determine if the observed changes
are related to urban breeding birds or true winter visitors.
Acknowledgements
We are grateful to Department of Ornitology of PAS for data for ringings and recoveries of corvids in Warsaw.
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