Resource-dependent weather effect in the reproduction of the White Stork Ciconia ciconia Damijan Denac1 Denac D. 2006. Resource-dependent weather effect in the reproduction of the White Stork Ciconia ciconia. Ardea 94(2): 233–240. Weather affects the breeding success of White Stork Ciconia ciconia, but the effect has not been studied in the context of different food resources or habitat quality. The aim of this study was to determine whether the impact of weather conditions on breeding success was dependent on habitat quality. The effect of weather on reproduction was analysed in two populations that differed significantly in the availability of suitable feeding habitats. Multiple regression analyses revealed that of the weather variables analysed (average temperature and rainfall in April, May and June), rainfall in May and temperature in June explained a sig- nificant part of the variation in numbers of fledged chicks per pair, but only in the population with poorer food resources. The lack of weather influence in the population with richer food resources was tentatively explained by the larger brood sizes. More effective heat conservation in larger broods, and thus lower chick mortality during cold weather, could be the underlying mechanism for the different response to weather in the two populations. Key words: breeding success, Ciconia ciconia, habitat quality, resource dependent weather-effect, White Stork 1National Institute of Biology, Vecv na pot 111, SI–1000 Ljubljana, Slovenia ([email protected]) INTRODUCTION optimal feeding habitats, whereas fields, especially cornfields, are suboptimal (Sackl 1987, Pinowski et The influences of food as a resource and weather al. 1991). Consequently, breeding pairs surrounded as an agent affecting the birds’ distribution, migra- by optimal feeding habitats are more productive tion and reproduction are particularly well known than those breeding in suboptimal sites (Barbraud (Gill 1995, Newton 1998, Elkins 2004). Reproduc- et al. 1999, Dziewiaty 2002, Nowakowski 2003). tion success of the White Stork Ciconia ciconia is Similarly, adults and nestlings are affected by directly affected by food availability, being higher weather. Adults both during migration and winter- in peak vole years (Tryjanowski & Ku´zniak 2002), ing (Sæther et al. 2006) and after arrival on the in pairs breeding closer to rich feeding sites breeding grounds (Tryjanowski et al. 2004). Until (Tortosa et al. 2002, 2003) or in the presence of young storks develop thermoregulatory ability livestock, where storks have better access to the (Tortosa & Castro 2003), rainy and cold weather food resources (Tryjanowski et al. 2005). White conditions can significantly reduce breeding suc- Storks select foraging habitats according to prey cess by causing high chick mortality (Jovani & availability and accessibility (Alonso et al. 1991). Tella 2004). The effect of weather on reproduction In Europe, meadows, pastures and wetlands are of White Stork has been studied in univariate 234 ARDEA 94(2), 2006 Figure 1. The representative habitat of the White Stork population in the north-eastern part of Slovenia is composed of large, intensively used fields (left). Storks of the south-eastern population have larger areas of food-rich meadows avai- lable (right) (photos D. Denac). models only, assuming stable food resources ond, south-eastern population has been recently (Carrascal et al. 1993, Bert & Lorenzi 1999, Moritzi established and is situated in two mesoregions – et al. 2001, Jovani & Tella 2004). However, the Bela krajina and the Krsvka lowland (together effect of one environmental parameter (e.g. ambi- 658 km2), colonized by White Storks from 1975 ent temperature) may become evident only when onwards. Since then, their numbers have increas- other environmental parameters (e.g. food avail- ed, and the south-eastern population accounts for ability) affecting reproduction are also taken into 7–12% of the 1999–2004 national breeding popu- account. Recently, attempts were made to study a lation (Denac 2001, D. Denac unpubl. data). number of environmental parameters jointly in Number of chicks fledged was recorded for all multi-way models, e.g. for the Buzzard Buteo buteo nests in each year from 1999 to 2004 (see Results (Krüger 2004). So far, the influence of weather on for sample sizes in each year). Breeding success reproduction success of White Stork has not been was measured as the number of chicks fledged per studied under different food conditions. The aim breeding pair, and breeding pairs were determined of this study was to explore the impact of weather following standardised census methodology conditions on breeding success in conjunction with (Schulz 1999). Habitat quality between both popu- habitat quality. lations was compared as the size of potential feed- ing habitat within the defined survey area around each nest. The survey area was a circle with a METHODS radius 1.5 km from the nest, which is the distance from the nest covered by the majority of foraging Two White Stork populations in Slovenia were flights (Sackl 1985, Alonso et al. 1991, Dziewiaty chosen to test how the weather influences breed- 1992, Nowakowski 2003). Using GIS and the ing success under different food conditions. The Actual land use map (Ministry of Agriculture, first, north-eastern population was situated in two Forestry and Food 2002), the surface of potential geographical mesoregions – the Murska and Dravs- White Stork feeding habitats (intensive meadows, ka lowlands (together 1026 km2), both of which extensive meadows, fields) within the survey areas have been traditional White Stork breeding sites was calculated. The fields category included all for at least 100 years, with 53–62% of the land sown with annual crops or temporarily uncul- 1999–2004 national breeding population. The sec- tivated due to crop rotation, as well as vegetable Denac: RESOURCE-DEPENDENT WEATHER EFFECT 235 and flower gardens. Intensive meadows included intensive meadows) within the defined survey improved meadows that are frequently mown or areas and significantly more (P < 0.001) subopti- grazed, and the extensive meadows category mal foraging habitat (fields) than pairs in the included all land covered with natural vegetation south-eastern population (Fig. 2). Pooled for all that is grazed or mown, but not intensively. Exten- years, the south-eastern population had signifi- sive and intensive meadows were defined as opti- cantly better breeding success than the north-east- mal and fields as suboptimal foraging habitats ern population (t-test: t = –5.18, P < 0.001; (Sackl 1987, Pinowski et al. 1991; Fig. 1). The null Fig. 3). hypothesis of no difference in habitat quality between the populations was tested using one-way MANOVA, followed by the post hoc Tu key honest 6 North-eastern population significant difference (HSD) test for unequal n. South-eastern population 5 ) Daily measurements of rainfall and average 2 temperature were gained from all climatic stations 4 in each mesoregion (Murska lowland – seven sta- tions, Dravska lowland – four, Krsvka lowland – 3 three, Bela krajina – three) for the period between 2 1 April and 30 June for 1999–2004. Data from the surface area (km stations were averaged to obtain daily values char- 1 acteristic for a single mesoregion. Rainfall 0 amounts were summed and the temperatures aver- fields intensive extensive meadows meadows aged to produce characteristic values for rainfall and temperature for each month (April, May, Figure 2. White Stork breeding pairs in the south-eastern June), within each year and mesoregion. population had more optimal feeding habitats (extensive Two-way ANOVAs were used to test whether and intensive meadows) and fewer suboptimal habitats mesoregions differed in rainfall and temperature, (fields) within the survey areas than pairs from the north- introducing month and mesoregion as factors. eastern population. Box plots indicate median, 25th and Forward multiple regression analysis was used to 75th percentiles, and outliers. explain the specific contribution of independent weather variables (rainfall in April, May and June and temperature in April, May and June) to the 123 15 113 16 119 17 111 18 101 18 126 28 variability in breeding success. Analyses were car- 5 poorer population ried out separately for each population. Both pop- richer population ulations occurred in two mesoregions, and data 4 were gained for six years, thus correlations were calculated for twelve data for the population. 3 2 RESULTS 1 mean breeding success The surface areas of potential feeding habitats dif- 0 fered significantly between the populations (one- 1999 2000 2001 2002 2003 2004 way ANOVA: F = 0.533, P < 0.001). The 3,171 Figure 3. Mean (± SD) White Stork breeding success Tu key HSD test further revealed that pairs in the (number of fledged chicks/breeding pairs) in the popula- north-eastern population had significantly less tion with poorer (north-eastern) and richer food resources (P < 0.001) optimal foraging habitat (extensive, (south-eastern). Numbers of breeding pairs are indicated. 236 ARDEA 94(2), 2006 Table 1. Partial regression coefficients (± standard error of the estimate and P-values) between weather variables and breeding success in the poorer (n = 12) and richer population (n = 12). Coefficients were only calculated for variables included in the final model. Multiple R2 (adjusted) = percentage of variance explained including all factors; R2 (adjus- ted) = percentage of variance explained by final model.