J Ornithol (2013) 154:769–774 DOI 10.1007/s10336-013-0941-8
ORIGINAL ARTICLE
Seasonal variation in Eurasian Wigeon Anas penelope sex and age ratios from hunter-based surveys Kevin Kuhlmann Clausen • Lars Dalby • Peter Sunde • Thomas Kjær Christensen Bjarke Egelund • Anthony David Fox
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Received: 11 December 2012 / Revised: 28 January 2013 / Accepted: 22 February 2013 / Published online: 13 March 2013 Ó Dt. Ornithologen-Gesellschaft e.V. 2013
Abstract Demographic monitoring is vital for tracking and modelling the population dynamics of highly mobile bird populations. However, different types of monitoring can sometimes lead to different outcomes, and understanding the causes of equivocal results is an important step to advance future monitoring schemes. This study found consistent seasonal variation in Eurasian Wigeon Anas penelope sex and age ratios among Danish hunter-based wing surveys, and describes how accounting for this variation might explain reported discrepancies between this and other monitoring methods. Early season flocks were dominated by adult males, and juvenile proportions were highest in November and significantly lower before and after this peak. Nationwide field assessments undertaken in January 2012 showed no significant differences from sex and age ratios in the wing survey data from that particular hunting season (2011/2012), indicating that this survey is a good predictor of Wigeon demography. These results highlight the need to account for consistent temporal variation in such demographic time series when using the results to model population parameters.
Communicated by F. Bairlein. K. K. Clausen (&) L. Dalby P. Sunde T. K. Christensen B. Egelund A. D. Fox Wildlife Ecology Group, Department of Bioscience, Aarhus University, Grena˚vej 14, 8410 Rønde, Denmark e-mail:
[email protected] L. Dalby Ecoinformatics and Biodiversity Group, Department of Bioscience, Aarhus University, Ny Munkegade 114, 8000 Aarhus C, Denmark
Keywords Wing survey Field counts Reproduction Demography Hunting bag Management Zusammenfassung Jahreszeitliche Schwankungen in Geschlechterverha¨ltnis und Altersstruktur bei der Eurasischen Pfeifente (Anas penelope) anhand von Erhebungen durch Ja¨ger Fu¨r die Nachverfolgung und Modellierung populationsdynamischer Prozesse ist bei hochmobilen Vogelpopulationen ein demographisches Monitoring von entscheidender Bedeutung. Unterschiedliche Arten des Monitorings ko¨nnen jedoch gelegentlich zu unterschiedlichen Resultaten fu¨hren, und das Versta¨ndnis der Ursachen von mehrdeutigen Ergebnissen ist ein wichtiger Schritt in der Entwicklung zuku¨nftiger Monitoring-Systeme. In den Erhebungen da¨nischer Ja¨ger fanden sich durchga¨ngig jahreszeitliche Schwankungen im Geschlechterverha¨ltnis und in der Altersstruktur der Eurasischen Pfeifente (Anas penelope); diese Studie beschreibt, wie die Beru¨cksichtigung solcher Unterschiede eventuell die Diskrepanzen zwischen den Ergebnissen aus unterschiedlichen Monitoring-Methoden erkla¨ren ko¨nnte. Schwa¨rme zu Beginn der Saison wurden von adulten Ma¨nnchen dominiert, wa¨hrend der Anteil junger Vo¨gel im November am ho¨chsten und vor und nach diesem Spitzenwert signifikant niedriger war. Im Januar 2012 landesweit durchgefu¨hrte Freiland-Bewertungen zeigten keine signifikanten Unterschiede im Geschlechterund Altersverha¨ltnis zu den Erhebungen dieser speziellen Jagdsaison (2011/2012), was nahelegt, dass eine solche Erhebung ein guter Pra¨diktor fu¨r demographische Aussagen bei Pfeifenten ist. Diese Ergebnisse unterstreichen die Notwendigkeit, konsistente zeitliche Schwankungen in derartigen demographischen Zeitreihen zu beru¨cksichtigen,
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wenn die Ergebnisse dazu benutzt werden, Populationsmerkmale zu modellieren.
Introduction Demographic monitoring is a vital pre-requisite for understanding and modelling population change, especially with regard to providing evidence-based options for population management (Beissinger and Westphal 1998; de Kroon et al. 2000; Blackwell et al. 2004). Despite the widely acknowledged importance of deriving basic estimates of survival and fecundity, accurately quantifying these parameters remains difficult and time-consuming, and often relies on proxy measures to derive estimates (Lakhani and Newton 1983; McClintock and White 2009). In Denmark, the national wing survey has been an effective way of estimating the annual sex and age ratio composition of waterfowl populations among shot birds (Clausager 2004). The voluntary submission of wings by hunters has contributed to one of the few long-term ([30 years) European monitoring programmes that support the management and conservation of game species. However, data compiled from the wing survey are sampled from a small subset of the overall population, which is spatially and temporally restricted to hunted areas within the hunting season (Bunnefeld et al. 2009; Imperio et al. 2010). Moreover, the composition of this sample may be susceptible to changes in hunting effort (Christensen 2005). Previous analysis suggested that wing survey data might show biased sex and age ratios compared to field counts. Mitchell et al. (2008) reported Eurasian Wigeon Anas penelope juvenile proportions obtained from wing surveys consistently higher than those from catches and field counts, and Madsen (2010) showed a similar pattern among Pink-footed Geese Anser brachyrhynchus. Similarly, biases in hunting bag sex ratios have been described, e.g. for Greater Snow Geese Chen caerulescens (Giroux and Be´dard 1986) and Canvasbacks Aythya valisineria (Olson 1965). These potential biases need to be taken into account with the interpretation of wing survey demographic data. Guillemain et al. (2012) showed substantial seasonal variation in the sex and age ratio among wing survey data, and the apparent inconsistencies between this and other methods may result from neglecting to account for the temporal components in measures of population demography. Whereas most alternative methods to estimate sex and age ratios (e.g. from field counts, catches, etc.) are spatially and temporally highly restricted, the Danish wing survey data are derived from material gathered from throughout Denmark during the course of entire hunting seasons. How temporal variation may affect our sex and age ratio
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estimates for the population as a whole is currently unknown, but acknowledging the limitations of different monitoring data is an important step to improve our ability to support the adaptive management of harvested wild populations (Williams and Nichols 2001; Elmberg et al. 2006). Quantifying the potential impacts of such temporal variation is essential if we are to justify recommending continued management decisions based on these datasets. In this study, we describe seasonal variation in sex and age ratios among samples of hunter-supplied Eurasian Wigeon (hereafter ‘‘Wigeon’’) wings from birds staging in Denmark, using data from the Danish wing survey compiled over 10 years. In addition, we compare sex and age ratios from the 2011/2012 hunting season with an independent estimate obtained from nationwide assessments made in the field. We conclude by discussing possible pitfalls of surveying the demographic composition of a flyway population without adjusting for sampling date, and whether the apparent differences reported between estimates obtained from hunting bags and field assessments might relate to differences in the temporal window of these surveys.
Methods We used data on sex and age distribution of Wigeon submitted by hunters from 10 hunting seasons (September 1– January 15 and, since 2011, January 31), 2002/2003–2011/ 2012 via the Danish wing survey (organised from Aarhus University). The sex and age of submitted wings were determined using plumage characteristics and standard
Fig. 1 The 41 sites in Denmark visited during field counts to determine sex and male age ratios of Eurasian Wigeon Anas penelope in January 2012
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survey protocols (Boyd et al. 1975; Clausager 2004). Field counts of Wigeon were carried out with telescopes at 41 different sites across Denmark (Fig. 1) during a 10-day period (January 13–22), ensuring the complete body moult of first-winter birds necessary to confidently distinguish sex and male age ratios in the field. Male Wigeon are unique among dabbling ducks in that the age of individuals can easily be determined from a distance based on the colour of their forewing (white in adults, grey in first-winter males). This enabled a quick and precise determination of age distribution among males within flocks. To account for potential spatial differences, both habitat and region were assigned to each flock, and compiled data therefore included total counts, sex ratios, age ratios in males, habitat type (freshwater lake, freshwater wetland, shallow brackish), and region (Jutland, Funen, Sealand). Wing survey data were analysed from a 10-year period (2002/2003–2011/2012) as well as separately from the 2011/2012 hunting season. In both wing samples, as well as in the count data, the proportion of males among all birds and the proportion of juveniles among all males were estimated with generalised linear models (GLIMMIX procedure in SAS 9.3; SAS Institute) as a binomially distributed response variable with a logit-link function and binomially distributed error terms. Date effects were modelled as the cubic, quadratic or linear function that provided the best fit. In the wing sample analyses, we tested for and accounted for annual variation in baseline values by incorporating hunting season as a random effect. When it appeared that both sex and age ratios varied significantly as a function of date, we also conducted post hoc tests for possible temporal variation in the response variables among years by adding year 9 date to the fixed effects. Field count response parameters were tested for (fixed) effects of habitat type, region and date assessed by means of AICc weights. As no overdispersion appeared to be present within the binomial response data (Chi square/df ratios: 0.53–1.1), no dispersion adjustments were applied in any of the models. We assessed the contribution of fixed effects on the basis of type I and type III F statistics. The statistical significance of random effects was tested on the basis of the z score of the covariate parameter values and their standard errors.
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ratio varied as a highly significant cubic function of date (F3,9987 = 22.83, p \ 0.0001) from a maximum of c. 70 % males in early September to a low of c. 59 % by midOctober, after which it increased again to c. 65 % by late December (Fig. 2a). The sex distribution did not appear to vary much between years (z = 1.83, p = 0.068). The proportion of juveniles among males varied as a highly significant quadratic function of date (F2,5613 = 148.04, p \ 0.0001) from a low of c. 21 % in early September to a maximum of c. 58 % in November, after which it fell again to below 30 % by early January (Fig. 3a). The proportion of juveniles varied significantly between years (z = 2.04, p = 0.042), but there was no interaction between years and date (F18,5595 = 0.72, p = 0.79). Field count and wing sample from the 2011/2012 hunting season A total of 3,967 Wigeon were assigned to sex and male age classes at 41 sites throughout Denmark (Fig. 1). Of these, 2,310 were males and 559 juvenile males. Within the count
Results Wing survey analyses From a total 17,298 wings received from the 2002/2003 to 2011/2012 hunting seasons, 10,297 (60 %) were males, of which 5,729 (56 %) were juveniles. The wing survey sex
Fig. 2 Variation in the proportion of males in the Danish wintering population of Wigeon (±95 % confidence limits) as a function of date based on a wing survey data in the period 2002/2003–2011/2012 and b field counts (dashed line) and wing survey data (solid line) from the 2011/2012 hunting season
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The 2011/2012 wing sample consisted of 1,432 wings of which 887 (61.9 %) were males and 447 juvenile males (50.4 % of all males). If comparing unadjusted overall mean estimates from the two methods, both proportion of males (z = 2.21, p = 0.027) and proportion of juveniles among males (z = 6.32, p \ 0.00001) were overestimated in the wing survey. While male proportion differed by only 4 %, the proportion of juveniles among males estimated from the wing survey was twice that estimated from field counts (50.4 vs. 24.2 %). However, when the temporal variation of sampling date was taken into account, the estimates obtained by the two methods were nearly identical (Figs. 2b, 3b).
Discussion
Fig. 3 Variation in the proportion of juveniles among males in the Danish wintering population of Wigeon (±95 % confidence limits) as a function of date based on a wing survey data in the period 2002/2003–2011/2012 and b field counts (dashed line) and wing survey data (solid line) from the 2011/2012 hunting season
data, some support was found for a date effect on sex ratio (Table 1; F1,138 = 6.66, p = 0.011), as the proportion of males increased from c. 55 to 62 % from January 13 to 22, with an overall estimated mean of 57.8 % (95 % CL 56.3–59.3). We found no support for a date effect on the proportion of juveniles among males, and the estimated mean from field counts was 24.2 % (95 % CL 22.5–26.0). Neither habitat type nor region seemed to influence sex and age ratios (Table 1). Table 1 Model selection summary of candidate models to explain variation in sex and age ratios of Wigeon assessed from field counts in Denmark, January 2012 ER evidence ratio of models with covariates compared to model with no effects (wimodeli/ wimodelwith no effec). See text for definition of explanatory factors. Most supported results in bold
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The decade of Wigeon wing survey data included in this study showed a consistent seasonal trend in sex and age ratios across all 10 years. Results indicate that the sex ratio was relatively constant with male percentages around 60 %, although early autumn seemed to show consistently higher numbers. This pattern is in agreement with adult males generally leaving the breeding areas well in advance of female and juvenile birds on moult migration (Mitchell et al. 2008; Guillemain et al. 2012), and this early influx of males to the autumn staging areas is well reflected in the wing survey. Age ratios among males showed a consistent unimodal pattern with highest juvenile percentages in late October–early November, and significantly lower proportions before and after this peak. The low percentage representative of the early season is probably an effect of the adult male phenology described above, whereas the lateseason drop could be explained by high mortality rates among juveniles (Guillemain et al. 2012). The unimodal pattern in age ratio seemed to persist irrespective of fluctuations in breeding success and number of juveniles, and percentage of first winter males was consistently higher in mid-autumn. The relatively large confidence intervals associated with the age ratio is merely an expression of this annual variation in reproductive output from the
Sex ratio
Age ratio among males
AICC
DAICC
ER
AICC
DAICC
wi (%)
ER
No effects
479.34
4.8
3
Region (R)
476.27
1.7
15
–
434.56
0.0
35
–
4.6
440.05
5.5
2
0.1
Date (D) Habitat (H)
474.58 481.60
0.0 7.0
34 1
10.8 0.3
436.51 434.73
1.9 0.2
13 32
0.4 0.9
wi (%)
R?D
474.70
0.1
32
10.2
440.05
5.5
2
0.1
D?H
478.46
3.9
5
1.6
436.66
2.1
12
0.3
R?D?H
477.19
2.6
9
2.9
439.72
5.2
3
0.1
J Ornithol (2013) 154:769–774
population, which translates into a vertical movement of the entire curve. The high level of agreement with the independently derived estimates of sex and age ratios from field counts indicate that the wing survey is an appropriate tool to estimate demography in this population, suggesting little indication of potential bias. However, temporal variation in the demographic measures highlighted in this study underlines the need to take account of such variation when comparing snap-shot field assessments of sex and age ratios with whole-season averages from the wing survey. As shown above, simple comparisons between age ratios from these two methods would indicate a 2011/2012 wing survey estimate twice as high as the equivalent field count assessment without adjustment for temporal variation in the former. This is potentially the main reason for the apparent discrepancies reported by Mitchell et al. (2008) between Wigeon wing surveys and other measures of reproduction. The broad temporal window that (of necessity) characterises the wing surveys is potentially an important strength to this proxy measure of fecundity in the population as a whole. Field counts and trapping methods are often restricted in time and space, and the scope for incorporating seasonal variation can be very limited. As a consequence, estimated sex and age ratios from use of these methods might be misleading if not adjusted for seasonal variation. Another important outcome of these findings relates to using juvenile proportions as a measure of reproduction (Robertson 2008). At least for Wigeon, such assessments should preferably incorporate juvenile percentages from the entire autumn season, as temporally restricted surveys might significantly over- or underestimate actual reproduction dependent on the degree of overlap with the ‘‘wave’’ of juveniles passing along the flyway, when viewed from a static geographical perspective (Fig. 3). The same care should be taken when inferring survival rates or incorporating age class-specific life-history traits in demographic population models. Even though the geographical scale of this study covers most of the Danish staging areas, our analysis is still limited to parts of the entire Northwest European flyway. Elevated juvenile mortality (Guillemain et al. 2012) and sex-specific migration phenology might influence sex and age ratios further along the flyway. The pattern of seasonal variation (Figs. 2, 3) might therefore be different in other countries (e.g. United Kingdom, France). The insignificant interaction between year and date found in our study indicates that Wigeon phenology has been more or less constant throughout the study period. A significant interaction term, corresponding to horizontal shifts along the x-axis in Figs. 2 and 3, would have pointed to changes in either life-history traits or timing of migration
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as a response to environmental or climatic change. However, a thorough clarification of such changes would necessitate the analysis of time series data in excess of 10 years, and require that only very minor changes in hunting legislation and in the distribution and timing of the hunting effort of the desired species had occurred during this time span. This study demonstrates that the temporal component of demographic measures is very important when using them to infer population level life-history traits. This is probably even more pronounced when dealing with a migratory species like Wigeon, which show phenological movements specific to different age or sex classes. As a consequence, interpretation of demographic parameters from shot birds submitted to wing surveys should always be founded on sound knowledge of the ecology of individual species. To further strengthen the temporal congruence between methods described in this study, additional field surveys quantifying sex and age ratios throughout the entire hunting season should be conducted, because such data are not currently available. Although regular field counts are fundamental to national monitoring programs, the determination of sex and age of individual birds is rarely, if ever, carried out. Furthermore, field discrimination of sex and age classes can be very difficult, especially in early autumn during body moult. Therefore, gathering enough earlyseason demographic data to back up the hunter-based survey might prove difficult without including other methods. Our results demonstrate the value of sex- and age-specific differences in plumage that can be easily recognised in the field. These differences are especially pronounced in Wigeon, but studies of plumage appearance in other dabbling duck species might find similar but less distinct differences between sex and age classes that may prove useful for demographic studies in these species. Acknowledgments We owe an enormous debt to all the hunters who have voluntarily contributed to the Danish wing survey over many years and to Ib Clausager for initiating and running the scheme for many years. We acknowledge funding from the Forest and Nature Agency (latterly Nature Agency) to support the determination of the sex and age ratios among the wing samples of common quarry species and contributed to the analysis presented here. We would also like to thank the Danish Agency for Science, Technology and Innovation for financial support to L.D.’s PhD project.
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