J Ornithol (2009) 150:39–44 DOI 10.1007/s10336-008-0315-9
ORIGINAL ARTICLE
The response of the Great Reed Warbler Acrocephalus arundinaceus to climate change Andrzej Dyrcz Æ Lucyna Halupka
Received: 26 October 2007 / Revised: 19 March 2008 / Accepted: 13 May 2008 / Published online: 4 June 2008 Ó Dt. Ornithologen-Gesellschaft e.V. 2008
Abstract We examined long-term responses in the breeding performance of the Great Reed Warblers Acrocephalus arundinaceus to climate change. The study took place in various years from 1970 to 2007. During the study period, mean temperatures in the breeding season of the species increased and precipitation decreased significantly. We found evidence for the significant advancement in both earliest and annual median first-egglaying dates. This advancement correlated with temperature increases early in the season. The latest first-egg laying dates, however, remained unchanged. Other breeding statistics: clutch size, nest losses, and production of young per nest, did not change significantly over the study period. Precipitation did not affect any of the analysed measures. It is important to note, though, that during dry seasons, the production of young per successful nest was higher. In contrast to some woodland species, the Great Reed Warbler seems to adapt well to climate change by shifting laying dates. The reason for this is probably to optimise food resources. Keywords Acrocephalus arundinaceus Great Reed Warbler Climate change Laying date Timing of breeding
Communicated by F. Bairlein. A. Dyrcz (&) L. Halupka Department of Avian Ecology, Zoological Institute, Wrocław University, ul. Sienkiewicza 21, 50-335 Wrocław, Poland e-mail:
[email protected]
Introduction The global mean temperature rose by 0.6°C in the twentieth century. The warming process seems to have accelerated (Houghton et al. 2001). For example, the last decade of the twentieth century was the warmest compared to the previous decades. The current decade is even warmer. Such a distinct, sudden, climatic change must be profoundly affecting living organisms. There is growing evidence that organisms do respond to environmental changes (Tryjanowski et al. 2002; Walther et al. 2002; Parmesan and Yohe 2003). For example, many migratory birds react to increased temperatures or to large-scale climatic phenomena such as the North Atlantic oscillation (NAO) by changing the timing of their arrival and departure (Mitrus et al. 2005; Tryjanowski et al. 2005; Sparks et al. 2007; but see also Kanˇusˇcˇa´k et al. 2004). Increased spring temperatures often affect changes in bird breeding performance, especially phenology (Bairlein and Winkel 2000; Both et al. 2004), though substantial differences, both between and within species, can be observed as well (Sanz 2003; Visser et al. 2003, 2004). However, the impact of climate change on population productivity and other fitness-related measures, is restricted to a few, mainly woodland bird species. Examples are the tits and flycatchers (Visser et al. 1998; Bairlein and Winkel 2000; Sanz 2002; Crick 2004; Visser et al. 2004). Data from these species suggest that some populations do suffer from climate change by producing less offspring, and rearing fewer second broods, etc.(Both and Visser 2001; Sanz et al. 2003; Visser et al. 2003). The response of species occupying other habitats, however, may be quite different (see Schaefer et al. 2006). This study examines the breeding parameters of the Great Reed Warblers Acrocephalus arundinaceus, studied in Poland in various years from 1970 to 2007.
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The Great Reed Warbler is a common breeding species found in European reedbeds. It is a long-distance migrant which winters in Africa. The species is a medium size (31 g) insectivorous passerine, feeding on various species of prey collected from both within and outside its territories (Leisler 1991; Cramp 1992). It prefers to occupy edges of reedbeds, and nests over deep water, among thick reed stems. In the population study, the most important reasons for nest losses were predation and adverse weather conditions (Dyrcz 1974, 1981, and unpublished data), as elsewhere (e.g. Beier 1981). The first birds arrive from their wintering grounds in April. They start laying eggs in May, and continue until Junelate July.
Methods Study area The study was carried out during 14 breeding seasons over different year time spans, from the years 1970 to 2007 (1970–1974, 1981–1984, 1997, and 2004–2007). The study took place on fish-ponds near Milicz (SW Poland). Most of the ponds were located within the nature reserve ‘‘Stawy Milickie’’. The size of the ponds ranged from 40 to 180 ha, and water depth was 50–150 cm. The edges of the ponds are overgrown with belts of reed beds 2–160 m wide. Besides common reed Phragmites communis, the emergent vegetation comprises patches of cattail Typha sp., great bulrush Scirpus lacustris, sweet flag Acorus calamus and water manna-grass Glyceria aquatica. The banks surrounding the ponds are overgrown with old trees (mainly oaks) and bushes. Data collection Each year, the study was conducted from the arrival of the birds from their wintering grounds, (late April–early May) until the end of July. The basic routine was to trace males after their arrival and then to find nests in the territories of singing males. During the study period, a total of 893 nests was found, varying from 21 to 134 each year. The onset of laying was assessed directly or recalculated according to hatching date or nestling age. The nests were visited on an average of three times a week. An exception was in 2007, when nests were not visited after the 10th day of nestling life. Breeding density of the studied population differed widely between years from 11.5 to 38 pairs per 10 ha. However, we have not detected any significant trend in the population density throughout the study period (rs = 0.192, P = 0.665).
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Climatic data The meteorological data (temperature and precipitation of spring–summer months) were obtained from the national Institute of Meteorology and Water Management for the study region. In the paper, mean temperatures for single months and longer periods are used. All will be referred to as mean temperatures. Data analysis To detect changes in breeding phenology the following measures were analysed: (1) the earliest laying date, (2) annual median laying date calculated for all nests in the season, including renests after failure and second broods, and (3) the latest laying date. All use of the term ‘‘laying date’’ refer to the date of the laying of the first egg. The analysis of reproductive parameters also included the length of the laying period (the time interval between the date of laying of the first egg in the earliest and the latest nest in a season), clutch size, nest failure (proportion of nests that did not produce any fledgling), and production of fledglings per nest (average number of young that managed to leave the nest successfully). We used non-parametric statistics as not all variables met the requirements necessary to conduct parametric tests (Sokal and Rohlf 1995). All P values refer to two-tailed tests.
Results The years between 1970 and 2007 became significantly warmer and drier. The mean temperatures for each month between April (arrival of Great Reed Warblers from their wintering grounds) and August (fledgling of the young from the latest nests) increased significantly. The most important rise occurred in April (rs = 0.647, n = 38, P \ 0.0001) and May (rs = 0.474, n = 38, P \ 0.0001). The increase amounted to 2.2°C, comparing the mean May–July temperatures (months corresponding with laying period of the species) in the 1970s with those from 2001 to 2007: 15.3 and 17.5°C, respectively (Fig. 1). During the study period, the mean May–July monthly precipitation decreased significantly (rs = -0.339, n = 38, P = 0.038) (Fig. 2). In the study period, egg-laying commenced between 30 April and 25 July. In various years, the earliest layings started between 20 May (1970) and 30 April (2006). The advancement of the earliest laying date was significant over the study period (rs = -0.628, n = 14, P = 0.018). This advancement occurred, though, only during the later years. In the 1970s–1990s, the earliest
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20 Earliest 1st egg (day of May)
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Fig. 3 Earliest first-egg laying date of the Great Reed Warbler Acrocephalus arundinaceus in various study years
Fig. 1 Mean temperature in May–July 1970–2007
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individuals started laying in mid-May (11–20 May), and during this period no tendency for advancement was found (rs = -0.006, n = 10, P = 0.973) (Fig. 3). During the later years from 2004 to 2007, the earliest eggs were laid earlier, in early May or even late April (30 April in 2006) (Figs. 3, 4). Annual median laying dates (20 May–3 June) also advanced significantly, (rs = -0.665, n = 14, P = 0.012). The latest laying date varied considerably between years (15 June–23 July), and we did not find an advancement in this variable across years (rs = -0.033, n = 14, P = 0.916). Likewise, the length of the laying period remained unchanged (rs = 0.302, n = 14, P = 0.288). The last two seasons, however, were exceptionally long (79 and 81 days) compared to previous ones (32–64 days) (Fig. 4). We did not observe any significant changes in clutch size
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Pentads Fig. 4 Changes in first-egg laying pattern of the Great Reed Warbler (percentage of nests in consecutive pentads) between 1970–1974 and 2004–2007. First pentad: 26–30 April, last pentad with data shown: 20–24 July. Number of nests sampled in both study periods were 248 and 451, respectively
(rs = 0.392, n = 14, P = 0.161), the level of nest losses (rs = -0.335, P = 0.263) or the production of young per nest (rs = 0.220, n = 13, P = 0.459). The earliest laying date correlated significantly with the temperature in April (rs = -0.750, n = 14, P = 0.003) (Fig. 5). Annual median laying dates correlated best with temperatures in April–May (rs = -0.565, n = 14, P = 0.038). We did not detect any effect of ambient temperature on clutch size, length of laying period, nest losses and production of fledglings per nest. We found that during years with lower precipitation the production of young per successful nest was higher (rs = -0.602, n = 13, P = 0.031). However, other breeding statistics did not correlate with precipitation.
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Earliest first-egg date (day of May)
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Fig. 5 The relationship between April temperature and the earliest first-egg laying date. Values on Y axis represent the day of May, 0 is 30 April
Discussion In 1970–1997, the beginning of laying of Great Reed Warblers always started in mid-May, and varied little (10 days) throughout the years. For that period no tendency towards earliness was detected. Only after 2000, when April temperatures increased more than previously (Mann– Whitney U test, U = 3, P = 0.012, n1 = 10, n2 = 4), did the earliest laying dates advance. Most likely the beginning of laying is associated with the stage of vegetation development, which is generally temperature-dependant (Hawkins and Sweet 1989; authors’ unpublished data). This may be important for two reasons. First, vegetation both within the reedbed and on the banks of the pond hosts potential foods (Dyrcz 1979), and insect development is often correlated with temperature-related changes in vegetation (Buse and Good 1996; Hodkinson and Bird 2006). Although in our study area potential food resources are rich throughout the season (Dyrcz and Zdunek 1996) and the species feeds its nestlings with a variety of foods, few prey types dominate in the diet (Dyrcz 1979). Hence it may be important for the species to synchronise the outbreak of most important food types with peak food requirements of the nestlings (Lack 1954; Perrins 1970). Second, reedbed-nesting species often require proper concealment of their nests to reduce risk of predation (Borowiec 1985; Schulze-Hagen et al. 1996; Bata´ry et al. 2004). In the study area, reed growth rate is temperaturedependant (authors’ unpublished data), as elsewhere (Dykyjova et al. 1970). For this reason, mean April temperature, which affected reed-height in early May, correlated well with the laying dates of the earliest
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individuals. Likewise, reed-height in late May was probably influenced by temperatures in both April and May, which correlated with annual median laying dates (occurring in late May). Apart from the relationship between vegetation and insect development, higher spring ambient temperatures correlate positively with insect activity and abundance (Taylor 1963; Bergman et al. 1996). Therefore, periods of higher insect activity (and hence available food for incubating parents and nestlings) probably occurs earlier, which enhances an earlier start of breeding. Concerning the latest laying dates and the length of the laying period, the species shows huge differences between years. Both variables tended to be affected by precipitation and temperature in late season. Low temperatures and heavy rains were associated with short seasons (unpublished data). It has been demonstrated that the species is very sensitive to weather conditions, especially rain, which has a strong influence on nestling mortality (Dyrcz 1974, 1981; Schaefer et al. 2006). We also found that, during seasons with heavy precipitation, fledgling production per successful nest was lower, due to higher partial losses. Therefore, it is not surprising that in years with a rainy and cold July, the latest laying dates tended to be advanced and the laying period shorter. Although so far we have not detected a significant increase in the length of the breeding season, the two last seasons were exceptionally long. They were longer than all the earlier ones. This resulted from the advancement in the beginning of the laying but not the end of laying. It may, therefore, be expected that the laying period will become significantly longer, as it was found in the Reed Warbler Acrocephalus scirpaceus, studied in the same area (Halupka et al. 2008). We found several differences in response to climate change between our population of the Great Reed Warbler and the one from Bavaria, investigated by Schaefer et al. (2006) between 1973 and 2003. Both populations tended to advance their earliest and annual laying dates. The Bavarian population, however, tended to also advance its latest laying dates, but ours did not. Therefore, in Bavaria, the whole breeding season seemed to be shifted, but not in Poland. Great Reed Warblers from Germany significantly increased their clutch size. Such an increase was not found in Poland. In Germany, increased clutch size resulted in higher breeding success (increased number of young produced per nest). A similar relationship was not found in our study: both production of fledglings per nest and per successful nest remained fairly constant across years. In contrast, we can observe more similarities with a population of a relative species, the Reed Warbler, studied in the same area over the same time period (Dyrcz 1981; Borowiec 1994; Halupka and Wro´blewski 1998; Halupka
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et al. 2008). The Reed Warbler, however, is much more sensitive to climate change and all trends are more distinct. Both species advanced their earliest and annual median laying dates, but did not advance their end of laying. In the Reed Warbler, it resulted in a much longer laying period. Neither species increased clutch size in spite of the advancement of laying. Likewise, their fledgling production per nest did not change. In spite of this, the Reed Warbler seems to benefit from climate change by increasing its renesting potential (Halupka et al. 2008). So far, we have no evidence that this is also true in the case of the Great Reed Warbler. However, the extremely long seasons of 2006 and 2007 may suggest a similar response in the Great Reed Warbler in the future. This is quite plausible considering that the Great Reed Warbler apparently started to respond to climate change only during the last few seasons. This is late compared to the Reed Warbler. In summary, it seems that so far the Great Reed Warbler has adapted well to climate change by shifting the timing of breeding, but not changing other parameters of breeding biology. The studied population does not benefit from climate warming (as was found in Bavaria), but apparently does not suffer. Hence, the results of this study do not confirm the prediction of Bairlein and Winkel (2000) that long-distance migrants would suffer due to climate change. A comparison of our data with that from the Bavarian population provides evidence that different populations of the same species (even though not very distant) can adapt in different ways to climate change. This was also previously found for woodland species (Visser et al. 2002; Sanz 2003). This suggests an urgent need for more studies investigating the effect of climate warming on avian populations.
Zusammenfassung Die Reaktion des Drosselrohrsa¨ngers Acrocephalus arundinaceus auf den Klimawandel Den Bruterfolg des Drosselrohrsa¨ngers haben wir bezu¨glich einer Langzeitreaktion auf den Klimawandel untersucht. Die Untersuchung wurde in verschiedenen Jahren zwischen 1970 und 2007 durchgefu¨hrt. Im Untersuchungszeitraum erho¨hte sich die Durchschnittstemperatur in der Brutzeit signifikant, wa¨hrend sich die Niederschlagsmenge reduzierte. Wir konnten nachweisen, dass sich sowohl das Legedatum des fru¨hesten, als auch das des Median des zuerst gelegten Eies signifikant verfru¨hte. Diese Verfru¨hung korrelierte mit einem Temperaturanstieg fru¨h in der Saison. Die spa¨testen Legedaten fu¨r das erste Ei vera¨nderten sich jedoch nicht. Andere Brutstatistiken, wie Gelegegro¨ße, Nestverluste und die Anzahl der Jungen pro Nest
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vera¨nderten sich im Untersuchungszeitraum nicht signifikant. Die Niederschlagsmenge hatte auf keinen der untersuchten Parameter einen Einfluss. Dennoch bleibt zu erwa¨hnen, dass in trockenen Jahren die Anzahl an Jungen pro erfolgreiche Brut ho¨her lag. Im Gegensatz zu einigen Waldarten scheint sich der Drosselrohrsa¨nger durch eine Verschiebung des Legedatums gut an den Klimawandel anzupassen. Diese dient vermutlich einer Verbesserung der Nahrungssituation. Acknowledgments We would like to thank Wanda Zdunek for field assistance. The paper benefited from remarks of two anonymous reviewers. The study was supported by The Polish Scientific Committee for Scientific Research (KBN).
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