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Keywords:

  • breeding biology;
  • climate change;
  • nest fate;
  • prairies;
  • timing of breeding;
  • waterfowl

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix
  • 1
    Increases in average global temperature during the twentieth century have prompted calls for research on the effect of temperature variation on avian population dynamics. Particular attention has been paid to the hypothesis that increased temperatures may affect a species’ ability to shift their breeding efforts to match the phenology of their prey, and thus result in reduced reproductive success (the ‘mismatch hypothesis’).
  • 2
    We used data from a long-term study of breeding ducks to investigate how duck nest success varied with clutch initiation date, and to test whether spring temperature affected this relationship in a manner consistent with the mismatch hypothesis. We modelled five possible functional forms of how nest success might vary with clutch initiation date and spring temperature, and used an information-theoretic approach to determine which model best described the nesting outcomes of five dabbling duck species nesting in Saskatchewan, Canada.
  • 3
    Probability of nest success for the five species did not vary strongly with clutch initiation date, and we found evidence consistent with the mismatch hypothesis for one species, northern pintail Anas acuta, although weight of evidence was weak.
  • 4
    Overall nest success of all five species was positively associated with spring temperature. These results suggest that increasing spring temperature alone (within the range observed in this study) may not affect nest success in a manner that would result in lower populations of breeding ducks.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix

Average global temperature has increased between 0·3 and 0·6 °C over the last 100 years, primarily due to an increase in average spring and winter temperatures (Houghton et al. 2001). This global warming has been cited as a conservation concern for bird species throughout the world (McCarty 2001), and prompted calls for research on the effect of temperature variation on avian population dynamics (Visser et al. 2003). Reviews of effects of climate change on bird populations have identified three basic results: (1) several northern-latitude birds have expanded their range northward (Thomas & Lennon 1999); and (2) breeding dates have advanced (McCarty 2001); and (3) the fitness consequences of increased temperatures may depend on species’ ability to shift their breeding efforts to match the phenology of their prey (the ‘mismatch hypothesis’; Visser et al. 1998). Populations that cannot adjust their breeding efforts to warmer temperatures during the breeding season may experience reduced reproductive output, either due to physiological constraints of timing of egg production (Visser et al. 1998; Thomas et al. 2001), or due to a decoupling of the cues that trigger migration in the wintering grounds with temperature in the breeding grounds (Both & Visser 2001). The current trend for higher temperatures is predicted to continue (Houghton et al. 2001), creating an emerging need to better understand the interrelationships among spring temperature, timing of breeding, and reproductive output in bird populations.

According to the mismatch hypothesis, increased spring temperatures result in faster development rates of prey than the development rates of eggs and nestlings (McCarty 2001). This difference creates a temporal mismatch between the emergence of prey and laying date of breeding birds. Evidence for the mismatch hypothesis comes primarily from studies that demonstrate increased selection differentials or reduced reproductive output for later-laid nests than for earlier-laid nests during warm springs relative to cooler springs (e.g. Thomas et al. 2001; Gjerdrum et al. 2003). From a statistical perspective, the effect of temperature on reproductive output proposed by the mismatch hypothesis can be thought of an interaction between lay date and spring temperature. This relationship is one of five possible functional forms (Fig. 1). The first and simplest form would be an effect of spring temperature on overall reproductive output, with no relationship with lay date within each season (Fig. 1A). Reproductive output could also increase (or decrease) linearly over the breeding season (Fig. 1B), as would occur with changes in available cover, predator densities, or prey for nesting birds over the season. This linear relationship between lay date and reproductive output may also vary among seasons with different spring temperatures, as would be expected under the mismatch hypothesis (Fig. 1C). Alternatively, reproductive output may also vary within a season such that nests laid at the midpoint of the nesting season would differ from nests laid relatively earlier or later (Fig. 1D), which would occur if predation rates depend on abundance of nests, e.g. the ‘predator swamping’ hypothesis (Darling 1938; Ims 1990). Last, the effect of synchronous laying might vary among seasons with different spring temperatures (Fig. 1E) due to complex interactions among cover for nesting birds, available prey and predator densities. This pattern would also be consistent with the mismatch hypothesis.

image

Figure 1. Schematic of five possible models depicting the relationship between probability of hatch of duck nests (measured as daily survival rate), spring temperature, and relative clutch initiation date (CID). Separate lines represent years with different mean spring temperature.

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Although the mismatch hypothesis has most often been considered for birds during the fledging period, it may also apply to the egg-laying and incubation period. In particular, breeding ducks may provide a good test case for these ideas as a strong connection exists between spring temperature and clutch initiation date of many duck species, with warmer springs eliciting earlier laying than during colder springs (Langford & Driver 1979; Hammond & Johnson 1984; Krapu & Reinecke 1992; Greenwood et al. 1995; Blums et al. 1996; Blums, Clark & Mednis 2002). Temperature may affect the ultimate fate of duck nests through a variety of mechanisms, including effects on ducks themselves, their invertebrate prey, their predators, and on their wetland and upland habitats. Ducks leave the nest more frequently and for longer intervals during warm periods than during cooler periods (Afton & Paulus 1992), and may thus attract attention of predators and expose nesting females to greater predation risk to themselves or their clutches during warmer springs than during cooler springs. The development and emergence of invertebrate prey (e.g. Chironomidae) that form an important dietary component of nesting ducks is also temperature dependent (Ward 1992). In addition, ephemeral and seasonal wetlands that provide prey for nesting ducks disappear at faster rates in warm springs than in cool springs (Larson 1995). Growth rates of plants also depend on temperature (Myneni et al. 1997), and thus the availability of nest-concealing plant cover for nesting ducks will vary among seasons, which may affect predation rates. As well as spring temperatures, winter temperatures on breeding areas may also affect the subsequent nesting success. Severe winter temperatures may result in increased mortality of nest predators, and higher nest success might be expected following cold winters than following milder winters. Alternatively, if severe winter temperatures also result in high mortality of rodents and other prey that provide alternatives to duck eggs, then lower nest success can be expected following colder winters than milder winters. Thus, several mechanisms exist by which spring and winter temperatures can affect overall nest success and its relationship with clutch initiation date, and the outcome will likely represent an integration of these mechanisms over the nesting period of each breeding season.

We use data from a long-term study of breeding ducks at St Denis National Wildlife Area (NWA), Saskatchewan, Canada, to examine the relationship between lay date, temperature and nest success. Data on nesting ducks, including clutch initiation date and subsequent nest fate, were collected at St Denis NWA between 1980 and 2000. Over this time period, the site experienced diverse weather conditions (Fig. 2), allowing us to examine this question in detail. In addition, previous studies at this site have demonstrated that timing of breeding affects brood survival and recruitment probability, such that earlier hatched broods experience greater probability of survival and recruitment than do later hatched broods (Dzus & Clark 1998; Dawson & Clark 2000). Therefore, an analysis of the effect of spring temperature on the relationship between clutch initiation date and nest success may also allow inference about its possible role in determining covariation between brood survival and nest success, and ultimately on recruitment probability.

image

Figure 2. Variable weather conditions at St Denis National Wildlife Area, Saskatchewan, Canada, 1980–2000. Plots A–C depict mean spring temperature (°C), previous mean winter temperature (°C), and May pond density (number per ha).

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Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix

study site: st denis national wildlife area

The 361-ha St Denis National Wildlife Area (NWA; 52°12′ N, 106°5′ W) is located approximately 40 km east of Saskatoon, Saskatchewan, Canada, and has over 100 temporary and permanent wetlands, most of which are fringed by grasses, shrubs or trees. Approximately 50% of the previously cultivated land has been seeded to a mixture of bromegrass Bromus spp. and alfalfa Medicago sativa for nesting cover. Full details of the study site can be found in Clark & Shutler (1999).

nest fate and clutch initiation date

Nest searches at St Denis NWA were conducted from early May to mid-July of each year between 1980–1981 and 1983–2000, by flushing females off the nest as heavy ropes or cable-chain devices were pulled between two all-terrain vehicles (Clark & Shutler 1999), or on foot and beating cover. Three or four nest searches were performed each year. When a nest was found, species was identified by watching the female, or by species-typing the eggs and/or feather lining of the nest (Klett et al. 1986). Clutch initiation date was determined by counting eggs (assuming one egg was laid per day, and that no eggs were lost) and candling to assess the stage of embryo development (Weller 1956). Nest fate (hatched, depredated, abandoned) was determined in subsequent visits following criteria in Klett et al. (1986). Nests that were abandoned or destroyed due to human interference (researcher disturbance, farm machinery), or were located inside predator fences or on islands, or whose fate was unknown were excluded from the analyses. The species considered in this study were the five most common dabbling duck species found nesting in the area: mallard Anas platyrhynchos, northern pintail A. acuta, blue-winged teal A. discors, northern shoveler A. clypeata and gadwall A. strepera.

weather data

Environment Canada has operated a weather monitoring station at St Denis NWA since 1989, and we compared data collected by that station with weather data from the Saskatoon Airport (52°10′ N, 106°43′ W), the next nearest weather station. Weather data from the Saskatoon Airport covered the entire time period of the breeding duck data. Mean daily temperature at St Denis NWA was highly correlated with mean daily temperature at the Saskatoon airport (R2 = 0·987, n = 3048). Regression of mean daily temperature (MDT) between the two sites indicated a near 1 : 1 conversion (MDT(St Denis NWA) = −0·50 + 0·99 × MDT(SK Airport)), and so temperature data from the Saskatoon Airport were used in the analyses. Mean spring temperature was calculated as the mean daily temperature between 1 April and 30 June of each year, and mean previous winter temperature was calculated as mean daily temperature between 1 January and 28/29 February of each year.

Precipitation at St Denis NWA was not strongly correlated with precipitation at Saskatoon Airport [R2 = 0·432 for daily accumulation (n = 2920), and R2 = 0·517 for monthly accumulation (n = 109)]. We therefore used May wetland (pond) density as a measure of environmental conditions as estimated for Stratum 30 from the aerial waterfowl surveys (Smith 1995). This estimate of pond density correlated strongly with pond counts done locally for years 1982–2000 (R2 = 0·825, n = 19; RGC, unpublished data), provided data for more years than the local pond counts, and allowed us to look at effects of pond density from previous years (Drever et al. 2004). Each year of breeding duck data was then assigned an estimate of pond density in the year of study, and an estimate of the previous year's pond density.

data analyses

Nesting occurs earlier in the year during warmer springs than during cooler springs (Hammond & Johnson 1984; Greenwood et al. 1995). To allow comparisons among years, clutch initiation date was converted to relative clutch initiation date. For the five duck species nesting at St Denis NWA, relative clutch initiation date was calculated as deviations from the yearly median clutch initiation date for each species (x − x50%), a measure that made our analyses comparable with other studies published from the same site (Dzus & Clark 1998; Dawson & Clark 2000). To test whether timing of egg-laying was associated with spring temperature, we regressed the median clutch initiation date in each breeding season for each species against mean spring temperature.

We used ‘Mayfield logistic regression’ to model daily survival rate of duck nests as a function of weather variables and relative clutch initiation date (Aebischer 1999; Hazler 2004). Each nest represents multiple trials of a binomial process (failure/success) where the number of trials is the number of days the nest was under observation (‘exposure days’). Because nests are found in different stages of the nesting cycle, nests that fail early are least likely be found, which can upwardly bias estimates of nest success (Mayfield 1961, 1975). The Mayfield logistic regression reduces this bias by including exposure days, and also allows the incorporation of individual covariates into the analysis (Hazler 2004). The fate of each nest was categorized as ‘success’ if 1 or more eggs hatched, or ‘failure’ if no eggs hatched, either due to nest predation or abandonment. Note that because of programming considerations, Mayfield logistic regression models nest failure rates, and thus signs of coefficients and intercepts must be reversed to interpret their effects on survival rate (Hazler 2004). We thus converted these mortality rates into daily survival rates (DSR) such that DSR = (1 − daily mortality rate).

We compared five logistic regression models using an information-theoretic approach that considered several alternatives of how nest success, measured as daily nest survival rate, might vary with climate variables, clutch initiation date, and their interactions (Burnham & Anderson 2002). These five models corresponded to five functional forms shown in Fig. 1. Model 1 contained only the weather variables (spring temperature, previous winter's temperature, pond density and previous year's pond density), and assumed no variation with relative clutch initiation date. The next four models included all four weather variables and allowed daily survival rate to vary with relative clutch initiation date, either as a simple linear relationship or as a quadratic relationship. Model 2 assumed a linear relationship between daily survival rate and relative clutch initiation date regardless of weather conditions. Model 3 included spring temperature, clutch initiation date, and an interaction term between spring temperature and relative clutch initiation date. By allowing the relationship between daily survival rate and relative clutch initiation date to vary with spring temperature, this model was a formulation of the mismatch hypothesis. In particular, the mismatch hypothesis predicts that nest success should decline with lay date at a faster rate during warm springs than during cold springs. Model 4 included a quadratic term of relative clutch initiation date and assumed that daily nest survival rate of nests would vary nonlinearly over the breeding season, e.g. daily nest survival could increase in the beginning of the nesting period to a midpoint and then decrease later in the nesting period. Model 5 allowed this quadratic relationship to vary with spring temperature by including terms for interactions between spring temperature and relative clutch initiation date and between spring temperature and the quadratic term for relative clutch initiation date. Because it included an interaction between spring temperature and relative clutch initiation date, this model could also be a formulation of the mismatch hypothesis. Thus, we interpreted support for either Model 3 or Model 5 as support of the mismatch hypothesis, although these two models may suggest different mechanisms.

All models were constructed using proc genmod (SAS Institute 1997). For four species (mallard, blue-winged teal, gadwall and northern shoveler), the most highly parameterized model (Model 5) showed evidence of mild overdispersion. Deviance/DF values (φ) ranged between 1·2 and 1·4, where such values should be close to 1·0 if no overdispersion exists (SAS Institute 1997). To correct for overdispersion, we adjusted the likelihood functions and standard errors of parameters for each model by setting the SCALE option to the square root of the observed value of φ from Model 5 (SAS Institute 1997). For northern pintail, the φ-value from Model 5 was 0·98, so the SCALE option was set to 1 (Burnham & Anderson 2002). For each model, we calculated Akaike's Information Criterion modified for small samples (AICc), the difference in AICc between each model and the model with the minimum AICc (ΔAICc), and the Akaike weight (w) (Burnham & Anderson 2002). Inference about the relationship between nest survival rates and weather variables was based on all five models by using model averaging, where parameter estimates and their variances are calculated as weighted averages over all models (Burnham & Anderson 2002). The Akaike weights, which provide the relative level of support for each model, were used as the weighting factors.

We could not use model averaging to incorporate model selection uncertainty into the effect of relative clutch initiation date. The use of model averaging for β parameters can be problematic as a way to formally include model uncertainty when linear and quadratic forms of the same variable are compared (Blums et al. 2005). Therefore, when considering the role of clutch initiation date on nest success, we relied more heavily on inferences available from model selection, rather than model-averaging. In addition, we used a graphical approach to aid in interpreting results. We calculated predicted values of daily nest survival rates for all species from the five models based on mean values for pond density, previous year's pond density, and winter temperature, and a range of observed values of mean spring temperature and relative clutch initiation date. These values were then averaged over the five models using the Akaike weights as weighting factors to calculate predicted values that accounted for model selection uncertainty. These model-averaged predicted values were then plotted against relative clutch initiation date.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix

A total of 2503 nests from St Denis NWA was considered (Table 1). Mallard nests were the most common, followed by blue-winged teal, gadwall, northern shoveler, and northern pintail. Most nests of all five species failed due to predation, which was the most common fate, and only a small fraction, approximately 5% overall, were abandoned. For all five species combined, median lay date occurred relatively earlier in the breeding season during warmer than during cooler springs (Fig. 3; β = −1·46, 95% CI = [−2·65 −0·27], R2 = 0·40).

Table 1.  Number of nests and their fates found at St Denis National Wildlife Area, Saskatchewan, Canada, 1980–2000, considered in this study
SpeciesnHatched (%)Depredated (%)Abandoned (%)
Mallard 86629637
Northern pintail  9836604
Gadwall 48643525
Blue-winged teal 67137594
Northern shoveler 38235605
Total250335595
image

Figure 3. Clutch initiation date, spring temperature, daily nest survival of ducks nesting in St Denis National Wildlife Area, Saskatchewan, Canada, 1980–2000. Plots A–E depict the median lay date as a function of mean spring temperature for each breeding season. Plots F–J depict the relationship of daily nest survival of duck nests with relative clutch initiation date (CID) along a gradient of spring temperature. The three lines in each graph depict predicted values at different values of mean spring temperature. Species names refer to: mallard Anas platyrhynchos; northern pintail A. acuta (‘Pintail’); blue-winged teal A. discors (‘Teal’); northern shoveler A. clypeata (‘Shoveler’); gadwall A. strepera.

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We found that daily survival rate of nests, calculated from the Mayfield logistic regressions, did not vary strongly with relative clutch initiation date for the five species studied at St Denis NWA, and the data provided weak weight of evidence for the mismatch hypothesis. Model 1, which included only weather variables, was ranked most parsimonious of the set for mallard, northern pintail, gadwall and northern shoveler (Table 2). For blue-winged teal, Model 4, which included the quadratic form of relative clutch initiation date, was ranked as most parsimonious of the set, but this model received almost equal ranking with Models 1 and 2 (Table 2). Akaike weights for Models 3 and 5 had low values for all five species, ranging between 0·09 and 0·19 for Model 3, and 0·02–0·08 for Model 5, indicating the mismatch hypothesis had low overall support from the data. Relative to the other species, support for Model 3 was highest for the northern pintail (w = 0·19). All five species, however, had considerable model selection uncertainty, as most models were ≤ 4 AICc values of the best one, and the Akaike weight of the best model was < 0·6 in each case (Table 2).

Table 2.  Ranking of models relating daily nest survival to weather variables (mean spring temperature, pond density, previous year's pond density, and mean winter temperature) and relative clutch initiation date (CID) for five duck species breeding in St Denis National Wildlife Area (NWA), Saskatchewan, Canada, 1980–2000
Model no.12345
ParametersWeatherWeather, linear CIDWeather, linear CID, interaction with spring temperatureWeather, Quadratic CIDWeather, quadratic CID, interaction with spring temperature
K678810
  1. Model no. refers to the model number used in Fig. 1 and in the text. n = sample size, K = number of parameters, ΔAICc = the difference in Akaike's Information Criterion, corrected for small sample sizes, between each model and the model with the minimum AICc, w = Akaike weight, %C = per cent concordance between predicted probabilities and observed responses. Values in bold designate the most parsimonious model for each species.

Mallard, Anas platyrhynchos (n = 866)
ΔAICc0·00 1·01 2·91 2·43 5·80
w 0·46 0·28 0·11 0·14 0·03
%C55·058·058·357·758·3
Northern pintail, Anas acuta (n = 98)
ΔAICc0·00 2·19 2·02 4·03 5·89
w 0·53 0·18 0·19 0·07 0·03
%C52·957·259·657·960·3
Gadwall, Anas strepera (n = 486)
ΔAICc0·00 1·89 3·59 3·05 6·74
w 0·55 0·21 0·09 0·12 0·02
%C51·253·052·553·653·5
Blue-winged teal, Anas discors (n = 382)
ΔAICc 0·54 0·45 2·410·00 2·78
w 0·25 0·26 0·10 0·32 0·08
%C51·856·155·956·356·7
Northern shoveler, Anas clypeata (n = 671)
ΔAICc0·00 1·89 2·57 1·61 4·76
w 0·45 0·18 0·13 0·20 0·04
%C52·854·753·556·456·3

Model-averaged predicted values suggested that overall daily nest survival rates at St Denis NWA were higher during warm springs than during cooler springs, and showed a weak tendency to increase with relative clutch initiation date for mallard, gadwall, blue-winged teal and northern shoveler (Fig. 3, Appendix 1). For northern pintail, the slope of the relationship between daily nest survival and relative clutch initiation date varied slightly with mean spring temperature. During cooler springs (mean spring temperature = 9 °C), daily nest survival rates were lower for earlier laid nests than for later laid nests. During warmer springs (mean spring temperature = 13 °C), earlier laid nests tended to have higher survival rates than later laid nests (Fig. 3). The effect of relative clutch initiation date was not strong, and did not counteract the overall effect of mean spring temperature, such that overall nest success of pintails was higher during warm springs than during cool springs.

Model-averaged parameter estimates for the effect of weather variables indicate that daily nest survival rates of all five species were strongly influenced by mean spring temperature, rather than other weather variables. With the exception of gadwall, parameter estimates for the effect of mean spring temperature had 95% confidence intervals that did not encompass 0 for all species (Appendix 1), indicating that overall nest success tended to be higher during warmer than cooler springs. The model-averaged parameter for the effect of spring temperature on nest success of gadwall had a 95% confidence interval that encompassed 0, although its effect was in the same direction as the other species. In contrast, pond density, previous years’ pond density, and previous winter temperature did not strongly influence daily nest mortality, as parameter estimates for the effects of these environmental variables all had 95% confidence intervals that encompassed 0 for all species (Appendix 1).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix

Duck species that nest later in the breeding season in the North American prairies, such as gadwall and blue-winged teal, often have higher nest success than species that nest earlier in the season, such as mallard and northern pintail (Klett, Shaffer & Johnson 1988; Beauchamp et al. 1996). This difference among species, however, did not appear to be mirrored at the intraspecific level. We found that daily nest survival rates showed only a weak tendency to increase with relative clutch initiation date, and not for all five species considered. These results are similar to Greenwood et al. (1995), who found a relatively slow decline in predation rates on duck nests as the season progressed, and Emery et al. (2005) who found that duck nest success in unmanaged habitats tended to be higher later in the breeding season. The weak effect of relative clutch initiation date on probability of nest success contrasts with its strong effect on clutch size and brood survival of dabbling ducks, which show consistent declines over the breeding season (Rotella & Ratti 1992; Dzus & Clark 1998; Guyn & Clark 1999; Krapu et al. 2004), indicating that breeding ducks do not likely face a trade-off between nest success and other demographic vital rates.

Daily survival rates of nests differed among breeding seasons for all five species at St Denis NWA, and we found that overall nest success tended to be higher during warmer than cooler springs. The effect of spring temperature may partly explain the large annual fluctuations in nest success observed at sites where predators are not managed over the Prairie Pothole Region (Drever et al. 2004), although the mechanism behind this positive association between nest success and spring temperature remains unclear. Given the high percentage of nests that failed due to predation (Table 1), mechanisms that deal with predation risk could be considered more likely than mechanisms that deal with probability of nest abandonment independent of predation risk (e.g. food supply for nesting ducks). For example, prey other than duck eggs may be more available to predators during warm springs, which might result in decreased predation rates on nests (Byers 1974; Crabtree & Wolfe 1988; Ackerman 2002). Similarly, faster growth rates of plants during warm seasons better provide nests with concealment from predators, although this hypothesis may be more applicable to avian predators than mammalian predators (Clark & Nudds 1991).

Overall, we found weak evidence for the mismatch hypothesis occurring during the nesting period of breeding ducks. The possible exception might be northern pintail, a species that nests early in the season, and for which we found weak to moderate evidence that the slope of the relationship between daily survival rate and lay date varies with spring temperature in a manner consistent with the mismatch hypothesis (Fig. 3). The nesting biology of the northern pintail may make it more susceptible to changes in ambient temperature than other species. Pintails tend to feed in shallow ephemeral ponds (Stewart & Kantrud 1973) whose presence on the landscape is affected by evapotranspiration rates determined by temperature (Larson 1995). Thus, the food supply for pintails might diminish earlier in the season during warm springs than during cool springs, which could result in greater abandonment rates or longer periods when nests are left unattended. In addition, pintails tend to nest under sparse nesting cover and thus receive little protection from solar radiation (Shutler, Gloutney, & Clark 1998), and so may be more directly affected by changes in temperature than other species. We stress that the evidence for the mismatch hypothesis was weak, although the sample size for northern pintails was the lowest of the five species considered (n = 98 nests), and thus the weak effect of clutch initiation date may have resulted from low power. Given that pintail populations have recently declined (Miller & Duncan 1999), more research into the effects of spring temperature on their nesting behaviour and food supplies may be warranted.

We found an overall positive effect of spring temperature on nest success for the five species in our study, which suggests that birds respond to temperature in a complex fashion. Previous studies of the possible consequences of climate change for ducks have focused on availability of wetland habitats (Poiani & Johnson 1991; Bethke & Nudds 1995; Sorenson et al. 1998). Sorenson et al.'s (1998) models, based on correlations between numbers of ducks and number of wetlands, suggested that increased temperatures will likely result in frequent drought conditions and loss of wetlands, leading to an almost 50% reduction in breeding waterfowl numbers. These population responses to climate change would be governed by processes not considered here (e.g. emigration and immigration rates due to settling patterns depending on availability of ponds; Johnson & Grier 1988). Thus, while we found no evidence that warm spring temperatures might act on nest success in a fashion that would lead to lowered populations of ducks, nesting ducks may experience deleterious population consequences during warm springs in the future through other mechanisms.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix

We thank the numerous people that have collected data at St Denis NWA. Kirsten Hazler provided helpful suggestions on the use of Mayfield logistic regression. Delta Waterfowl Foundation, the University of Guelph, and the National Science and Engineering Research Council of Canada provided funding to MCD. The St Denis Auto-Weather Station has been maintained by a number of agencies over the years, including Environment Canada, McMaster University, Canadian Wildlife Service, and the National Water Research Institute. Suggestions by Peter Blums, Glenn Desy, Dan Haydon, Tom Nudds, and Yolanda Wiersma improved an earlier version of the manuscript.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix
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Appendix

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Appendix

Appendix 1

Table 3.  Model-averaged parameter estimates of five models to explain variation in daily nest failure rate of duck nests according to weather variables, clutch initiation date (CID), and interactions between clutch initiation date and spring temperature at St Denis National Wildlife Area (NWA), Saskatchewan, Canada, 1980–2000. Species names refer to mallard Anas platyrhynchos, northern pintail A. acuta, gadwall A. strepera, blue-winged teal A. discors and northern shoveler A. clypeata
ParameterEstimateStandard errorLower 95% CLUpper 95% CLt-value
Mallard
 Intercept−0·800·860·88−2·48−0·9
 Pond density−0·010·020·02−0·04−0·6
 Spring temperature−0·230·06−0·11−0·34−3·9
 Previous winter temperature−0·020·020·03−0·07−0·8
 Previous year's pond density0·010·010·04−0·020·7
 Relative ClD−0·00010·010·02−0·020·0
 Interaction between relative CID and spring temperature−0·0010·0030·005−0·01−0·4
 Relative CID (quadratic)−0·00030·0010·001−0·001−0·6
 Interaction between relative CID (quadratic) and spring temperature0·00010·00020·0005−0·00020·6
Northern pintail
 Intercept2·6442·1176·793−1·5051·2
 Pond density−0·0680·0740·076−0·212−0·9
 Spring temperature−0·4080·172−0·071−0·745−2·4
 Previous winter temperature0·0110·0740·156−0·1350·1
 Previous year's pond density−0·0370·0420·046−0·120−0·9
 Relative ClD−0·0800·1060·128−0·287−0·8
 Interaction between relative CID and spring temperature0·0170·0110·038−0·0051·5
 Relative CID (quadratic)−0·0010·0020·004−0·005−0·4
 Interaction between relative CID (quadratic) and spring temperature0·00040·00040·0010·0000·9
Gadwall
 Intercept−1·5811·2570·884−4·045−1·3
 Pond density0·0380·0370·110−0·0341·0
 Spring temperature−0·1500·0900·027−0·327−1·7
 Previous winter temperature0·0610·0400·141−0·0181·5
 Previous year's pond density0·0460·0290·102−0·0111·6
 Relative ClD−0·0170·0330·047−0·080−0·5
 Interaction between relative CID and spring temperature0·0040·0070·019−0·0100·6
 Relative CID (quadratic)0·00050·0010·003−0·0020·5
 Interaction between relative CID (quadratic) and  spring temperature0·000020·00040·001−0·0010·04
Blue-winged teal
 Intercept0·4780·8682·179−1·2240·6
 Pond density−0·0290·0190·008−0·066−1·5
 Spring temperature−0·2600·063−0·137−0·383−4·1
 Previous winter temperature0·0300·0260·080−0·0201·2
 Previous year's pond density−0·0120·0170·022−0·045−0·7
 Relative ClD−0·0140·0190·022−0·050−0·8
 Interaction between relative CID and spring temperature0·0020·0050·012−0·0070·5
 Relative CID (quadratic)0·0010·0010·004−0·0020·8
 Interaction between relative CID (quadratic) and spring temperature−0·00030·00030·0003−0·001−1·0
Northern shoveler
 Intercept0·1001·1362·326−2·1260·1
 Pond density0·0080·0230·054−0·0370·4
 Spring temperature−0·2340·083−0·072−0·396−2·8
 Previous winter temperature0·0480·0320·110−0·0141·5
 Previous year's pond density−0·0220·0200·019−0·062−1·1
 Relative ClD0·0240·0410·105−0·0560·6
 Interaction between relative CID and spring temperature−0·0070·0060·005−0·019−1·2
 Relative CID (quadratic)0·0010·0010·002−0·0010·7
 Interaction between relative CID (quadratic) and spring temperature−0·00010·00030·001−0·001−0·3