Bird Study Do White Storks Ciconia ciconia always ...

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Do White Storks Ciconia ciconia always profit from an early return to their breeding grounds?: Capsule Arrival date strongly influenced date of breeding and breeding success. P. Tryjanowski , T.H. Sparks , J. Ptaszyk & J. Kosicki Published online: 29 Mar 2010.

To cite this article: P. Tryjanowski , T.H. Sparks , J. Ptaszyk & J. Kosicki (2004) Do White Storks Ciconia ciconia always profit from an early return to their breeding grounds?: Capsule Arrival date strongly influenced date of breeding and breeding success., Bird Study, 51:3, 222-227, DOI: 10.1080/00063650409461357 To link to this article: http://dx.doi.org/10.1080/00063650409461357

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Bird Study (2004) 51, 222–227

Do White Storks Ciconia ciconia always profit from an early return to their breeding grounds? P. TRYJANOWSKI1*, T.H. SPARKS2, J. PTASZYK1 and J. KOSICKI1 1Department

of Avian Biology & Ecology, Adam Mickiewicz University, Fredry 10, PL-61-701 Poznan´, Poland for Ecology and Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire and PE28 2LS, UK

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2Centre

Capsule Arrival date strongly influenced date of breeding and breeding success. Aim To check our hypothesis that in years with low April temperatures, i.e. when storks started to breed, the relationship between timing of breeding and success would be absent, meaning that migrants would have no advantage by returning early to their breeding area. Methods We collected data in Poznan´ province, western Poland during the period 1983–2002. Based on local climatological data we selected eight cold spring years and compared them with the 12 ‘normal’ years. We analysed how weather affected the timing and arrival pattern of White Stork. Results The two groups of years did not differ significantly in population size, but in normal years the arrival date of both parents was earlier. Arrival date strongly influenced (was positively correlated with) date of breeding and (negatively correlated with) breeding success. Conclusion The slopes describing the above relationships did not differ significantly between the two groups of years. Therefore, we speculate that natural selection strongly favours birds that return early from the wintering grounds.

In migratory birds, individuals that are the earliest to arrive on their breeding grounds typically occupy the best territories and consequently have the highest reproductive success (Potti 1998, Kokko 1999, Forstmeier 2002). Moreover, in the White Stork Ciconia ciconia which builds large, ‘perennial’ nests and where the number of potential nest-sites is sometimes limited (Creutz 1985, Profus 1991), advanced arrival increases the opportunities to occupy good nests and save the energy needed for nest construction. Early arrival also provides the opportunity for an earlier start to egg-laying (Møller 1994, Brown & Brown 2000, but see Both & Visser 2001). This may have population consequences because in temperate bird populations reproductive success declines as the breeding season progresses (reviewed in Svensson 1997, including the White Stork, Profus 1991, Goutner & Tsahlidis 1995). Therefore, a strong selection is expected for early arrivals from wintering grounds located in tropical Africa (more general models in Kokko 1999 and Morbey & Ydenber 2001). However, birds that arrive early on their breeding *Correspondence author. Email: [email protected]

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grounds can pay the costs, even on some occasions by death (Whitmore et al. 1977, Brown & Brown 2000), mainly due to the influence of and/or changes in the food supply. It may be that the costs of arrival date would always be greater for individuals in poorer condition (Møller 1994, Kokko 1999, Forstmeier 2002). Unfavourable weather conditions on the migration route, as well as on the breeding grounds, can affect not only individuals, but also the entire population. The influence of bad weather conditions on White Storks at their breeding grounds is well known. Schulz (1998) reported than during so-called ‘disturbance years’ (years with adverse weather and low food supply) a large proportion of the birds did not breed or started breeding very late, resulting in very low breeding success and a high proportion of non-breeding birds. For example, in 1997, a pronounced disturbance year (Grieschon-Pflieger 1997), only 20% of the Eastern population returned at the normal date. The majority of the Eastern population arrived after at least 4–6 weeks’ delay due to extremely cold weather in the Middle East. As a consequence, the reproductive success of the breeding population was extremely low. While such events may affect White Stork, it should be


Do early Storks benefit?

remembered that it is a long-lived bird (up to 25 years; Schulz 1998), and one poor year may not be detrimental to lifetime reproductive success. We suspected that productivity was affected by changes in the timing of breeding, as well as by changes in the length of the interval between arrival date and start of reproduction. If first arrivals pay the costs of early migration, we would anticipate that the regression line of breeding success on arrival timing (and timing of breeding) in cold years would be less steep in comparison with the regression in favourable years. Because snow and restrictions in the food supply will mainly affect individuals that migrated too early, natural selection should favour individuals arriving later in cold years (Zajac 1995, Brown & Brown 2000). Here we consider some questions concerning the arrival pattern of White Storks in relation to differences in spring conditions. Does the population size differ between the two types of years – and which could affect performance in a density dependent way? Is there a difference in arrival date between the two groups of years? What is the relationship between arrival date and the start of breeding? What are relationships between arrival date, breeding date and breeding success? MATERIALS AND METHODS

The study was conducted during 1983–2002 in the agricultural landscape of western Poland, near Poznan´ (52°N, 16°E). In this area of 1227 km2, arable fields are interspersed with meadows, pasture, human settlements and small woods (for details see Ptaszyk 1994). In the study area, White Storks build nests mainly on the roofs of farm buildings, electricity pylons and trees, thus making the nests extremely conspicuous; they are also used year after year. The dates of arrival of White Storks to their nests were recorded on special forms by farmers living near occupied nests and sent direct to the authors (details in Ptaszyk et al. 2003). In an earlier paper (Ptaszyk et al. 2003), we reported no evidence of a weekend bias to this phenological recording and we found no effect of population size on recorded arrival date. We used information on the timing of both partners returning to a nest, although here we concentrate mostly on the data of the second partner (mostly females; Creutz 1985), assuming that the decision on the timing of breeding (and thus breeding success) is more influenced by the female. The timing of breeding is defined when the first egg

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is laid in the nest (first-egg date or FED) and has been estimated from information on ringing cards. The age of nestlings was established using bill measurements because, in contrast to other body measurements, the White Stork bill increases linearly for at least the first 70 days of the nestling’s life (Kania 1988). Because White Stork hatching is asynchronous within a nest, the growth of nestlings may differ. We calculated the age of the oldest chick and added the incubation period (31 days; after Creutz 1985, Schulz 1998) and hence we estimated FED. Therefore, data on FED were only available for pairs with breeding success. The population size and breeding success of the local population were established by standard methods used during the International Census of White Storks (Creutz 1985, Schulz 1998). Monthly air temperature data for the period 1983–2002 were obtained from the Poznan´ meteorological station. The station recorded daily maximum and minimum temperatures, amount of precipitation and other variables. Based on conditions known to cause White Stork mortality (Creutz 1985), we targeted periods of ≥ 7 days in length with daily maximum temperatures ≤ 5°C and with snow or heavy rain. We searched the climatological data set for the frequency of these occurrences during the period 15 March to 15 April and divided years into two groups: eight years that experienced ‘returned’ winter events (hereafter ‘cold’ years: 1983, 1986, 1987, 1993, 1995, 1996, 1997, 2000) and the remaining 12 years (hereafter ‘normal’ years). The phenology of the spring migration and FED were computed using Julian dates. Because migration timing and FED differed between seasons, the median for each year was subtracted from both dates so that the investigation focuses on relative, rather than absolute, earliness in each year. Throughout the text, values are reported as means ± se. Calculations were conducted using the SPSS for Windows package. All basic statistical analyses were applied according to the recommendations of Zar (1999). RESULTS Population size

The number of breeding White Stork pairs in the study area varied between 51 and 76, and the two year groups did not differ significantly in population size (means ± se: 65.4 ± 3.2 cf. 64.6 ± 2.4 pairs during cold and © 2004 British Trust for Ornithology, Bird Study,

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normal years, respectively, t = 0.20, P = 0.84). However, population size increased significantly during the 20-year study by about one pair a year (slope = 1.054, r2 = 53.9%, P < 0.001, details in Ptaszyk et al. 2003). We examined the residuals from the regression line of population size on year to confirm that the two year groups did not significantly differ from this trend (mean residual 0.87 ± 2.18 versus –0.58 ± 1.64 during cold and normal years, respectively, t = 0.54, P = 0.596). There was also no difference in the changes in population size from one year to the next (–0.25 ± 3.10 versus –0.82 ± 1.96 pairs during cold and normal years, respectively, t = 0.16, P = 0.873).


Arrival dates in cold and normal years

The median arrival date of the first partner from a pair in cold years was 7 April (± 0.7 days) and 3 April (± 0.5 days) in normal years; these differed significantly (Mann–Whitney U-test, z = –4.88, P < 0.0001). Similarly, the median dates of the return of the second partner differed between the two year groups (12 ± 0.6 versus 9 ± 0.5 April in cold and normal years, respectively, Mann–Whitney U-test, z = –3.92, P < 0.0001). In cold years, the first bird arrived at the nest on average five days before its partner, and in normal years on average six days earlier; differences between the two year groups were statistically significant (Mann– Whitney, z = –2.69, P = 0.007). Breeding success in cold and normal years

The number of fledglings per breeding White Stork pair did not differ significantly between the two year groups (2.58 ± 0.10 and 2.76 ± 0.09 chicks during cold and normal years, respectively, log transformed before analysis, t = –1.28, P = 0.22). In contrast to predictions, the percentage of pairs failing to breed successfully did not differ significantly between the two year groups (25.6 ± 2.9% versus 21.9 ± 2.1% of pairs attempting breeding during cold and normal years, respectively, with percentages arcsin transformed before analysis, t = 1.04, P = 0.31). Arrival date versus date of breeding

After the arrival of the second partner to the nest, birds spent on average 12 days (range: 1–30) in cold years and 13 days (0–48) in normal years (Mann–Whitney U-test, z = –0.959, P = 0.337) before FED. The arrival date of both the first and the second © 2004 British Trust for Ornithology, Bird Study,

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partner from a pair did correlate with FED, in both cold and normal years (r between 0.19 to 0.56, P < 0.05 in all cases, Figs 1a & 2a). Slopes of FED on arrival date of both the first (0.67 ± 0.07 versus 0.38 ± 0.11 in cold and normal years respectively) and second (0.76 ± 0.07 versus 0.57 ± 0.10 in cold and normal years respectively) partners showed evidence of differences between year groups (respectively, P = 0.062 and P = 0.036). Arrival date, date of breeding and breeding success

Breeding success was negatively correlated with arrival date (both parents) and FED, both during cold and normal years (all correlations significant at P < 0.04, except the relationship between arrival date of the first partner and breeding success in cold years (r = –0.09, P = 0.17; Figs 1b & 2b, 1c & 2c). Moreover, in contrast to expectation, we did not find significant differences between cold and normal years in the slope of the regression of breeding success on either arrival date or FED (P > 0.6) in all comparisons, although slopes of breeding success on the arrival date of the first bird differed marginally at P = 0.069). DISCUSSION

Like some other species (Tryjanowski et al. 2002) the White Stork has changed aspects of its spring arrival date during the study period. Considered over a longer time period White Storks in Poznan´ province tended to arrive markedly earlier (Ptaszyk et al. 2003). Series of 20 years’ duration are still relatively short to detect effects and, in some circumstances, may have relatively low statistical power. Hence some of the statistically marginal effects detected in this study may become more pronounced as further years of data are collected. We have concentrated on the dates of the arrival of the second bird on the assumption that this is the more important event in our study area. In other areas where nest-sites may be limiting, the arrival of the first bird (male) to guard a nest-site may be more important. In contrast to predictions and to results reported from other study areas (Lack 1966, Bairlein 1991), we did not find differences in the breeding population size or percentage of pairs with breeding failure between cold and normal years. As elsewhere (Creutz 1985, Schulz 1998) White Storks delayed arrival in cold years, but this did not influence breeding success. In cold years, the return to breeding areas was more synchronized within

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First-egg date Figure 1. Relationships in normal years between (a) arrival date and first-egg date, (b) between arrival date and breeding success and (c) between first-egg date and breeding success. Arrival date is that of the second partner to the nest using standardized Julian date where 0 = median date in each year; breeding success is number of fledglings per pair; first-egg date using standardized Julian date where 0 = median date in each year.

pairs than in normal years, and birds had a different relationship between arrival date and egg-laying. However, the regression slopes of breeding success on FED in the two year groups were very similar. In general, White Stork pairs that started to breed early had better

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productivity, in agreement with theories on timing of breeding reported for many bird species (Svensson 1997). This suggests that, independently of weather conditions, birds in good condition (or in premium sites) start reproduction earlier. Unfortunately we do not have data on the condition of breeding pairs, but strong correlations between arrival date, FED and productivity were present, both in cold and in normal years. © 2004 British Trust for Ornithology, Bird Study,

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These findings suggest that the costs of an early return to breeding grounds are relatively low or are masked by large profits. We suspect that this should lead to direct and/or indirect selection for early arrival, although the amount of variation in breeding success explained by arrival date is low. This is also supported by other observations. Recently White Storks from the Western European population (sensu Bairlein 1991) have started to overwinter in their breeding areas (Schulz 1998, Mata et al. 2001). Moreover, some birds from the Eastern population have recently begun to halt their migration in southern Europe, notably in Bulgaria (Nankinov 1994), rather than continuing to their traditional wintering areas in sub-Saharan Africa (Berthold et al. 2001). Hence, at least some individuals have a smaller migration distance and can return to their breeding areas earlier. However, the relationship is more important for the earliest returning birds that are most likely to be exposed to winter weather. The second partners from pairs (mainly females; Creutz 1985) return later and the influence of bad weather is less important, because White Storks are capable of surviving low temperatures for periods of up to four weeks (Mata et al. 2001). From this point of view, it is interesting why so many White Storks continue to migrate to tropical Africa. In Europe, as well as in North America, there have been marked trends to detect earlier migration in many bird species (reviewed in Walther et al. 2002, Parmesan & Yohe 2003). Similar trends have been described for the timing of breeding and generally birds have tended to lay earlier (Crick & Sparks 1999, Walther et al. 2002, Parmesan & Yohe 2003). It is clear that there should be a strong positive correlation between migration and breeding dates as suggested in some papers (Møller 1994, Brown & Brown 2000). However, papers linking both migration and breeding timing in the same population (or individuals) are very rare. Moreover, Both & Visser (2001) reveal that local weather conditions at migration time may have changed differently to those at reproduction time. In The Netherlands Pied Flycatchers Ficedula hypoleuca laid eggs earlier but the timing of migration from Africa did not alter, putting them under pressure to breed and feed quickly. Therefore, the life cycles of migratory animals must adapt to all the environments in which they spend time. This makes them vulnerable to climate change, as it is unlikely that change will be the same in all areas, or that all aspects of an animal’s behaviour will be governed by the same environmental cues. © 2004 British Trust for Ornithology, Bird Study,

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We provide evidence of a link between the timing of return to breeding grounds and the timing of egglaying. Moreover, both these timings strongly influence the number of chicks produced. The earliest returning birds start breeding earlier and consequently produce more offspring than later migrants. Natural selection can favour both earlier migrants and earlier breeders simultaneously, but cost–profit ratio for individuals could differ according to their condition (Forstmeier 2002). ACKNOWLEDGEMENTS We thank all farmers who participated in this study and M. Antczak and G. Siriwardena for comments on an earlier draft of the manuscript. We also thank the organizers and participants of the ESF Bird Migration and Climate Workshop in Konstanz for discussion.

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(MS received 24 April 2003; revised MS accepted 6 October 2003)

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