Changes in the Sex Ratio of the Common Pochard Aythya Ferina in Europe and North Africa

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Changes in the sex ratio of the Common Pochard Aythya ferina in Europe and North Africa

KANE BRIDES 1*, KEVIN A. WOOD 1, RICHARD D. HEARN 1 & THIJS P.M. FIJEN 2

1Wildfowl & Wetlands Trust, Slimbridge, Gloucestershire GL2 7BT, UK. 2Plant Ecology and Nature Conservation Group, Wageningen University & Research, Droevendaalsesteeg 3a, 6708PB Wageningen, the Netherlands. *Correspondence author. E-mail: [email protected]

Abstract Assessments of the sex ratio among Common Pochard Aythya ferina flocks were undertaken in countries across Europe and into North Africa in January 2016, for comparison with results from surveys carried out over the same area in January 1989 and January 1990. The mean (± 95% CI) proportions of males in the population were estimated as 0.617 (0.614–0.620) in 1989–1990 and 0.707 (0.705–0.710) in 2016; this difference between surveys was found to be highly significant. Whilst male bias increased with latitude in both surveys, this relationship was weaker in 2016 as the increases in male bias between 1989–1990 and 2016 were greater in countries further south. Given that the sex ratio of Pochard broods is approximately 1:1 at hatching, the strong male bias observed among adult birds is indicative of lower survival of females compared with males. The results of this study suggest that factors adversely affecting female survival rate (relative to that of males) may partly explain the decline in overall Common Pochard abundance. Given the widespread and ongoing decline of this species throughout most of Europe and North Africa, further information on possible demographic drivers of change is urgently required.

Key words: Aythya ferina, Common Pochard, demography, population ecology, sex ratio, species decline.

With an estimated c. 600,000 birds in 2017). Following steady declines in winter the Central Europe, Black Sea and abundance indices since the 1990s (Nagy Mediterranean population and c. 250,000 et al . 2014), however, and also a 30–49% birds in the Northeast/Northwest Europe decline in breeding abundance over three population, the Common Pochard Aythya generations (BirdLife International 2015), ferina (hereafter Pochard) is a common its conservation status was up-listed from and widespread species across Europe Least Concern (LC) to Vulnerable (VU) on and North Africa (Wetlands International the European and global IUCN Red Lists in

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2015. Given the widespread and ongoing towards males among adults, which may be decline of Pochard throughout most of particularly evident among breeding birds. It Europe and North Africa, information therefore seems that changes in the variables highlighting possible demographic drivers that influence sex ratios can translate into of the changes in population size therefore population-level effects, such as declining is urgently required. numbers, and that information on any Sex ratio data can potentially provide change in the sex ratio for a species can useful information on the differential provide a valuable insight into its population survival rates of the sexes (Donald 2007). processes (Donald 2007). As such, sex ratio data can potentially be Any assessment of changes in sex ratio used to infer the demographic causes of must, however, account for potential declines in population size amongst avian differential spatial patterns in the species. In a review of sex ratios of adult distributions of males and females. Many birds, Donald (2007) found that increasing duck populations wintering in the northern male bias is a common feature of threatened hemisphere exhibit geographical gradients populations, and noted that such changes in in sex ratio, with greater proportions of the sex ratio may reflect differing mortality males wintering further north (Bellrose et al . risks posed to the different sexes. For 1961; Perdeck & Clason 1983; Owen & Dix instance, studies in New Zealand of the 1986). In the case of the Pochard, Owen & Kaka Nestor meridionalis septentrionalis , which Dix (1986) found that the sex ratio among is included as Endangered (EN) on the flocks of the species at sites in the United IUCN Red List, found that mainland Kingdom during winter 1983/84 was highly populations exposed to introduced correlated with latitude, with a greater male predators have sex ratios that are highly bias in more northerly areas. At a larger male-biased, whereas populations on spatial scale, analysis of sex ratio data predator-free islands have balanced sex recorded for Pochard across Europe and ratios (Greene & Fraser 1998). Although into North Africa during surveys made in there are relatively few cases where the January 1989 and January 1990 similarly mechanism for higher female mortality has found a latitudinal effect, with a higher been confirmed, in some migratory species proportion of males recorded at higher the longer migrations undertaken by the latitudes (Carbone & Owen 1995). smaller sex (usually female) may put In the study presented here we assess the them at greater risk (review in Donald sex ratios among Pochard wintering across 2007). Moreover, for waterfowl (including Europe and into North Africa in January Pochard) where the female undertakes most 2016, for comparison with those from the or all of the incubation, nesting females are 1989–1990 survey over the same area known to be sensitive to predation during (reported by Carbone & Owen 1995), and the breeding season (Sargeant et al . 1984; consider the results in light of the Baldassarre & Bolen 1994; Blums et al . population decline recorded between the 1996). These factors can lead to a skew two surveys. Given the observed decline in

©Wildfowl & Wetlands Trust Wildfowl (2017) 67: 100–112 102 Pochard sex ratios the number of Pochard recorded over the January 2016) were used in the analysis, past 20 years, and the increasing male bias with 45.5% ( n = 834 flocks; 106,288 found in declining populations of other individual birds) of the sex ratio samples species, we hypothesised that the male bias collected on the 2016 IWC focal dates (16– in the Pochard population would be greater 17 January). in the more recent survey. For some surveys, sex could not be determined for all individuals within the Methods flock, in which case the sex ratio in the sample was assigned to the full flock to yield Sex ratio survey weighted estimates of the total numbers of National coordinators of the annual males and females, after Carbone & Owen International Waterbird Census (IWC), (1995). Following the approach of earlier which has been organised by Wetlands studies ( e.g . Sheldon 1998; Hardy 2002; International (previously the International Donald 2007), sex ratio was expressed as the Waterfowl and Wetlands Research Bureau; proportion of males within the sample, IWRB) each year since 1967, were asked calculated as: to organise sex ratio determinations of Sex ratio = n /(n + n ), Pochard to be undertaken by their network m m f of volunteer counters during the mid- where nm and nf refer to the total numbers January IWC in 2016. Bird-watchers were of males and females, respectively. This also invited to submit data collected outside formula allowed the sex ratio to be of the IWC counts and the project was calculated for any sample of birds, including heavily promoted on social media, using for an individual country or the total the Twitter hashtag #Pochard to encourage population. Whilst data for individual flocks interest among the bird-watching community. were not reported in Carbone & Owen Observers were asked to record for each (1995), data on the numbers of males and site visited the total flock size, the number females were presented for each country, of birds for which sex was determined, the which allowed comparison with our 2016 number of males and the number of data. Countries for which data were not females, location name, latitude and habitat. available for either the 1989–1990 or 2016 Data were submitted online via the Duck surveys were not included in the analysis. Specialist Group website (http://www. Because of the strong relationship between ducksg.org/projects/compoch/), or via male bias and latitude, standardising various online recording portals used the surveyed area to a consistent set by waterbird counters and birdwatchers, of countries was necessary to permit including BirdTrack and Observation.org. comparison of the 1989–1990 and 2016 In order to ensure that the results were survey results. In total, data from 13 comparable with those of the 1989–1990 countries were available from both the survey ( i.e . Carbone & Owen 1995), only 1989–1990 and 2016 surveys: Algeria, data collected during a 16-day period (9–24 Britain, Denmark, France, Germany,

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Greece, Hungary, Italy, the Netherlands, proportion of males in the 1989–1990 Republic of Ireland, Romania, Spain and survey differed significantly from the Switzerland. For the 1989–1990 and 2016 proportion of males recorded in 2016 surveys, we calculated the sex ratio for each (Crawley 2005). Second, for each survey we country based on the total numbers of used a two-tailed binomial test to assess the males and females in that country, whilst the significance of the deviation of the total population sex ratio was based on the total numbers of males and females for: (i) each numbers of males and females in all 13 country, and (ii) the total population, from a countries. 1:1 ratio. In both cases, the binomial tests To verify that the coverage achieved by allowed 95% confidence intervals to be the 1989–1990 and 2016 surveys were estimated for the proportion of males in the comparable, we estimated the proportion of sample, based on the approach of Clopper the total population of Europe and North & Pearson (1934). Africa that were within the countries For the country-level weighted sex ratio included in the surveys, based on the mean data for both surveys, we used linear models IWC counts over the four years leading up with Gaussian error structures to assess the to the survey years ( i.e . in January 1985–1988 relationships between the sex ratio and January 2012–2015). The IWC mean (expressed as the proportion of males) and: count data showed that these 13 countries (i) survey year, (ii) central latitude for each included in both surveys accounted for country, and (iii) the interaction between 0.724 and 0.729 of the total numbers of year and latitude. The inclusion of survey Pochard in 1985–1988 and 2012–2015, year, and its interaction with latitude, respectively, suggesting that the 1989– allowed us to test whether the proportion of 1990 and 2016 surveys were based on males differed between years, and whether almost identical proportions of the total the magnitude of any difference was populations. consistent over the range of latitudes surveyed. Following inspections of the Statistical analyses model residuals, to meet the assumptions of All statistical analyses were carried out using the linear modelling approach we square- R version 3.3.0 (R Development Core Team root transformed the response variable (sex 2016), with statistically significant results ratio) (Zuur et al . 2010). attributed where P < 0.05; all P values were adjusted using Holm-Bonferroni Results corrections to account for multiple The mean (± 95% CI) proportion of males comparisons (Holm 1979). First, to assess in the population was estimated as 0.617 whether the sex ratio of: (i) each country, (0.614–0.620) in 1989–1990, and 0.707 and (ii) the total population, differed (0.705–0.710) in 2016 (Table 1); a 2-sample significantly between the two surveys, we binomial test for equality of proportions used a 2-sample binomial test for equality indicated that this 0.09 difference in the sex χ2 of proportions to assess whether the ratio was highly significant ( 1 = 1893.58,

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P < 0.001). The numbers of males were birds (811 and 1,361, respectively) to the significantly higher than expected for a 1:1 total sample, and hence did not counteract ratio for every country in the 1989–1990 the overall pattern of greater male bias in survey except Algeria and Greece (which 2016 that was observed for the total had significant female biases), and for every population. country in the 2016 survey except Algeria We found significant positive effects of (which had no significant bias) (Table 1). Of both latitude and year on the proportions the 13 countries compared between 1989– of males in each country ( F3,22 = 18.65, 2 1990 and 2016, eight showed significantly P <0.001, R adj = 67.9%; Table 2; Fig. 2). greater male bias in 2016, whilst two showed Furthermore, we found evidence of a significantly reduced male bias (Table 1; Fig. significant, negative interaction between 1). However, these two countries, Hungary latitude and year, such that the increases in and Romania, contributed relatively few male bias in 2016 were greater for countries

Figure 1. The size and direction of the difference in male bias in the sex ratio (expressed as the proportion of males) between the 2016 and 1989–1990 surveys. Positive values indicate a greater male bias in 2016 compared with 1989–1990. The statistical significance of the difference is indicated for each country (see Table 1): *** = P <0.001, ** = P <0.01, n.s. = P > 0.05.

Table 2. The mean (± s.e.) estimates and significance of the effect sizes of the parameters in our linear model on square-root-transformed sex ratio (proportion of males) in the 13 countries surveyed in both 1989–1990 and 2016. All P values were adjusted using Holm-Bonferroni corrections for multiple comparisons and are statistically significant.

Parameter Estimate s.e. t value P value

Intercept –38.3006 11.6570 –3.29 0.029 Latitude 0.7781 0.2483 3.13 0.034 Year 0.0193 0.0058 3.31 0.029 Latitude * Year –0.0004 0.0001 –3.09 0.034

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Figure 2. The mean (± 95% CI) effect of latitude on Pochard sex ratio, based on the 1989–1990 (solid circles and lines) and 2016 (open circles and dashed lines) surveys.

©Wildfowl & Wetlands Trust Wildfowl (2017) 67: 100–112 Pochard sex ratios 107 at lower latitudes, albeit sample sizes for in 2016 (0.71 males in all birds surveyed) Algeria in 2016 were low (Table 2; Fig. 2). than in 1989–1990 (0.62 males). The two surveys covered the same 13 countries and Discussion an almost identical proportion of the The 2016 survey found, as expected, that population, suggesting that the observed Pochard sex ratios continue to show a change in sex ratio represents a real change strong male bias, as previously described by in population structure, rather than being an Owen & Dix (1986) and Carbone & Owen artefact of the sampling process. Given that (1995), with a relatively greater proportion we had data only for two survey periods, it of females wintering in countries in the was not possible to determine whether the south of the range. We also found evidence difference in sex ratios formed part of a that this latitudinal effect on sex ratio was trend of increased male bias over time. The weaker in 2016 than in 1989–1990, however, change in the proportion of males recorded with lower latitude countries showing the in each of the different countries, and the greatest increases in male bias. Males are continued relationship between sex ratio typically dominant over females in this and latitude, does however reinforce the species, and therefore are able to occupy view that there was a greater proportion of more favourable wintering areas that are males in the population in 2016. Countries closer to the breeding grounds, resulting in at lower latitudes were more likely to show the sub-dominant females moving further an increased male bias in the sex ratio in afield (Choudhury & Black 1991). The 2016 compared with 1989–1990, whereas greater capacity of males to withstand cold more northerly countries typically showed temperatures has also been proposed as an little or no change. For instance, we found explanation for the tendency observed in no difference in the proportion of males small-bodied birds, including ducks, for a reported in Denmark between study higher proportion of males to winter at periods, which concurs with Christensen & higher latitudes (Ketterson & Nolan 1976; Fox (2014) who similarly found no Nichols & Haramis 1980; Owen & Dix significant trend in the sex ratio of Pochard 1986; Carbone & Owen 1995; Evans & Day wintering in Denmark between 1982 and 2001). For example, Owen & Dix (1986) 2010 on analysing hunter-shot wing calculated that the lower critical temperature samples. Nonetheless, that age checks for for Pochard (representing the minimum air Pochard in low-latitude countries such as temperature at which an animal can Algeria, Greece, Italy and Spain, and also maintain its basal metabolic rate whilst across the whole study area, all showed resting without incurring additional stronger male bias in 2016 may have thermoregulatory costs) was 7.1°C for consequences for Pochard population males but 8.4°C for females. dynamics and for the success of any The analyses also indicated that the male conservation measures. For example, bias for all Pochard surveyed across Europe Donald (2007) noted that increasingly and North Africa was significantly greater male-biased sex ratios can lead to lower

©Wildfowl & Wetlands Trust Wildfowl (2017) 67: 100–112 108 Pochard sex ratios per capita productivity where intense male Pochard, declines in wintering numbers in competition hinders female reproduction. the west of the range have tended to be Given the current skew, productivity in greater than the overall population trend Pochard is likely to be limited not by males (e.g. 65% decline between winters 1988/89 but by the number of adult breeding and 2013/14 in the UK, Hayhow et al . females, as found in other duck populations 2017; c. 60% during 2004–2013 in the (e.g . Hoekman et al . 2002). Netherlands, Hornman et al . 2015) and Given that the sex ratio of Pochard increases have been observed further east broods is approximately 1:1 at hatching (e.g. wintering numbers have increased in (Blums & Mednis 1996), the male bias Sweden, Nilsson 2008). However, our observed among full-grown birds suggests a finding that the 13 countries included in lower survival of females compared with both sex ratio surveys held a consistent males. Moreover, the declining population proportion of 0.72–0.73 of the total size together with the greater male bias number of Pochard counted by the IWC recorded in 2016 indicates that female suggests that any redistribution of birds survival rates may have decreased more from the westernmost countries was largely sharply than those of males. Theoretically within the area covered by this survey and male survival rates could have increased thus did not affect the overall sex ratio more than those for females, but given the assessment. overall population trend this is unlikely. The There are several potential direct and observed change in the sex ratio could indirect factors which may explain the alternatively have resulted from a shift in apparent relative decrease in female survival. winter distribution, resulting in a lower The two most likely are changes in levels of: proportion of all females being counted (i) direct and indirect hunting mortality, and during the IWC, but this seems also unlikely (ii) predation. Pochard is a widely huntable for two key reasons: (1) the proportion of species, being legal quarry in at least 26 the total numbers that were included in the countries throughout Europe (Powolny & two sex ratio surveys were almost identical, Czajkowski, in press). Waterbird hunting and (2) where shifts in duck distribution pressure within Europe is widely recognised have been found in Europe they have as being greatest in southern Europe typically been to the north and east, which (Mateo 2009) where the largest proportion given the positive relationship between the of female Pochard overwinter, a view proportion of males and latitude should supported by a recent compilation of have resulted in more females and fewer hunting bag estimates for 17 European males being counted in 2016, which was not countries by Powolny & Czajkowski (in the case. Shifts in winter distribution, press). In addition, Pochard are huntable generally northeast towards breeding areas, from 1 August in several east European have been demonstrated for a wide range of countries, which could cause additional species, including some diving ducks, in selective hunting mortality on breeding recent decades (Lehikoinen et al. 2013). For females and naïve juveniles prior to their

©Wildfowl & Wetlands Trust Wildfowl (2017) 67: 100–112 Pochard sex ratios 109 departure to winter quarters, with most Anas crecca lower female survival rates males having already migrated away by the compared with males were apparent during beginning of August (Fox et al. 2016). Spatial the breeding season, likely resulting from differences in hunting pressure may also greater predation of females, but not during influence female survival indirectly through winter. Such nest predation is often by non- mortality on the birds ingesting spent lead native mammal species, such as Racoon Dog gunshot. The prevalence of lead gunshot Nyctereutes procyonoides and, particularly, ingestion by waterbirds varies considerably American Mink Mustela vison (e.g. Blums et al . among European countries but tends to be 1996). Although data on the population higher in southern Europe (Mateo 2009). trends of invasive non-native mammals are Furthermore, two recent studies have scarce, it is widely recognised that they are demonstrated a relatively high susceptibility increasing, especially Raccoon Dog which of Pochard to lead ingestion, both within now occurs in 21 countries in Europe the UK (Green & Pain 2016) and across (Genovesi et al. 2009). Europe (Andreotti et al . in press). Whilst the The causes of the changes in sex ratio in latter did not investigate sex differences in time and space remain unknown and further Pochard mortality attributable to lead research to estimate and compare male and poisoning, the authors estimated that it female survival rates ( e.g . using a capture- accounts for the death of 56,511 Pochard in mark-resight approach; White & Burnham Europe each year (Andreotti et al . in press). 1999) would provide valuable demographic Given the apparent elevated risk to female information needed to help understand Pochard from direct and indirect hunting these changes. Such an analysis of marked related mortality, greater hunting pressure in birds would allow spatial and temporal southern Europe therefore could be an patterns in survival to be assessed, and important factor in the apparent decrease in potential drivers of survival rates to be the proportion of females in the population. examined ( e.g . Wood et al . in press). Increased female mortality from predation Furthermore, such an approach can identify is another possible reason underlying the whether changes in survival are limited to shift in the sex ratios between surveys, with particular age classes, sexes, or regions, all of a recent review of the decline in Pochard which are currently unknown for Pochard. numbers suggesting that predation by a Previous research has estimated survival range of species (including non-native rates for female Pochard breeding in Latvia mammals), could be a major threat to (e.g . Blums et al. 1996, 2002), but we breeding Pochard populations (Fox et al . currently lack comparable survival estimates 2016). Female Pochard spend more time on for males. Collecting information on the breeding grounds, incubating eggs and hunting bag sizes, and other sources of raising ducklings, than do males, leading to hunting mortality, could permit assessment an increased level of predation, particularly of the relative impact on male versus female during the incubation period. Devineau et al . Pochard. It would also be advantageous to (2010) reported that for Green-winged Teal initiate routine collection of sex ratio data as

©Wildfowl & Wetlands Trust Wildfowl (2017) 67: 100–112 110 Pochard sex ratios part of standard waterbird surveys, such as go to Nick Moran and Neil Calbrade for the the IWC. Such data would help to provide a provision of counts submitted to BirdTrack better understanding of the patterns of and the Wetland Bird Survey, Johannes Wahl inter-annual variation in sex ratios, and thus and Nicolas Strebel for supplying data from differentiate between long-term trends and Ornitho, to Hisko de Vries for data from fluctuations between years. Annual data Observation.org and Timme Nyegaard for collection is ongoing in some European the provision of data from Dofbasen. countries, but currently too few to describe Jonathan Cooper kindly helped with the population-scale trends. Surveys of duck sorting of count data. Thibaut Powolny wings from hunted birds can also provide a (OMPO) and Matt Ellis (BASC) kindly valuable additional source of data for provided the European hunting bag assessing trends in the sex and age estimates. We are grateful to Eileen Rees, composition of the population (Christensen Tony Fox, Geoff Hilton and Matthieu & Fox 2014). Guillemain for their helpful comments Overall, we currently lack the knowledge on an earlier version of our manuscript. to explain the demographic causes and Finally, our grateful thanks go to Matthieu underlying drivers of the observed decrease Guillemain for his input at the project in the proportion of female Pochard planning stage and everyone at the Pochard wintering in Europe and North Africa. discussion at the 4th Pan-European Duck Nevertheless, our findings suggest that Symposium in Finland for their thoughts female survival in relation to that of males is and useful discussions on data needs for now lower than during the late 1980s, and Pochard. that aspects of the life-cycle more strongly pertinent to females are likely to be References contributing to the observed decline in Andreotti, A., Guberti, V., Nardelli, R., Pirrello, overall population size. Better monitoring S., Serra, L., Volponi, S. & Green, R.E. In and analyses of demographic information press. Economic assessment of wild bird will help to elucidate the situation and to mortality induced by the use of lead gunshot facilitate the further development of in European wetlands. Science of the Total targeted conservation and management Environment doi: 10.1016/j.scitotenv.2017. actions for this declining duck species. 06.085. Baldassarre, G.A. & Bolen, E.G. 1994. Waterfowl Acknowledgements Ecology and Management . John Wiley & Sons, Inc., New York, USA. We thank the national coordinators and Bellrose, F.C., Scott, T.G., Hawkins, A.S. & Low, the many volunteers across Europe and J.B. 1961. Sex ratios and age ratios in North North Africa who collected, submitted and American ducks. Illinois Natural History Survey collated the data upon which our analyses Bulletin 27: 391–471. are based. We also thank Szabolcs Nagy and BirdLife International. 2015. European Red List of Tom Langendoen at Wetlands International Birds . Office for Official Publications of the for the provision of IWC data. Our thanks European Communities, Luxembourg.

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