No evidence for sex bias in winter inter-site movements in an Arcticnesting

Ibis (2015), 157, 401 405 Short communication No evidence for sex bias in winter inter-site movements in an Arcticnesting goose population MITCH D. WEEGMAN, 1,2 * ANTHONY D. FOX, 3 STUART BEARHOP, 1 GEOFF M. HILTON, 2 ALYN J. WALSH, 4 IAN R. CLEASBY 1 & DAVID J. HODGSON 1 1 Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Penryn, UK 2 Wildfowl & Wetlands Trust, Slimbridge, UK 3 Department of Bioscience, Aarhus University, Rønde, Denmark 4 Wexford Wildfowl Reserve, National Parks and Wildlife Service, Wexford, Ireland Understanding movement of individuals between sites is necessary to quantify emigration and immigration, yet previous analyses exploring sex biases in site fidelity among birds have not evaluated remigration (the return of marked birds that moved to alternative areas from the site at which they were marked). Using novel Bayesian multistate models, we tested whether between-winter emigration, remigration and survival rates were sexbiased among 851 Greenland White-fronted Geese Anser albifrons flavirostris marked at Wexford, Ireland. We found no evidence for sex biases in emigration, remigration or survival. Thus, sex biases in winter site fidelity do not occur in any form in this population; these techniques for modelling sex-biased movement will be useful for a better understanding of site fidelity and connectivity in other marked animal populations. Keywords: Anser albifrons flavirostris, emigration, Greenland White-fronted Goose, movement, remigration, sex bias, site fidelity, survival. Understanding the degree of avian inter-site movements is important for the conservation and management of migratory populations, particularly for interpretation of local changes in abundance, management of wintering sites and the assessment of population-level genetic diversity (Hanski & Gilpin 1991, Hestbeck et al. 1991, *Corresponding author. Email: mw384@exeter.ac.uk Merriam et al. 1991). Many populations of the Anatidae exhibit sex-biased site fidelity, whereby breeding site fidelity is a predominantly female trait and wintering site fidelity a mainly male attribute (Rees 1987, Robertson & Cooke 1999), a pattern different from most other birds (Greenwood 1980, Greenwood & Harvey 1982). Importantly, long-lived waterfowl (i.e. geese and swans) maintain pair bonds throughout life, unlike ducks (Black 1996). Among these, adult emigration and immigration between winter sites is relatively uncommon; for example, Canada Geese Branta canadensis (Raveling 1979, Hestbeck et al. 1991), Dark-bellied Brent Geese Branta bernicla bernicla (Ebbinge & St. Joseph 1992) and Pinkfooted Geese Anser brachyrhynchus (Fox et al. 1994) all exhibit a high degree of adult winter site fidelity. Hence, the great majority of geese of both sexes are site-faithful to breeding and wintering sites. Nonetheless, movement of unpaired birds between sites is important to maintain gene flow between subpopulations; highly philopatric wintering populations experience reduced gene flow and greater potential for local extinction (Rockwell & Barrowclough 1987). Among Greenland White-fronted Geese Anser albifrons flavirostris, most individuals return to the wintering site where they were marked as juveniles, but there is no evidence of sex bias in their winter site fidelity, with males as likely as females to switch winter sites between years (Wilson et al. 1991, Warren et al. 1992). In this population, pair bond formation is believed to occur away from wintering areas, as marked birds resighted with parents and siblings during winter were resightedpaired (i.e. not with family members) in subsequent years (A.D. Fox unpubl. data). As reported from other Arctic-nesting goose populations, younger birds (i.e. those aged 1 and 2) are more likely to emigrate than older birds (potentially associated with pairing of birds from different wintering provenance; Marchi et al. 2010). Birds are also more likely to emigrate when they are hatched in years of high production of young (Marchi et al. 2010), which suggests that other populationlevel processes, such as individual position in dominance hierarchies, may influence inter-site winter movement, irrespective of sex (Raveling 1970, Owen 1980). However, to date, it has not been possible to rule out sexrelated differential remigration rates (i.e. the return of birds to the wintering site at which they were marked after moving to alternative areas), which may have influenced previous estimates of sex-biased movement in this population. With the advent of more complex Bayesian multistate capture recapture models (Kery & Schaub 2012), we are better able to estimate emigration and remigration, which has not been quantified in this population. Here, we assess whether inter-site movement and survival between winters is sex-biased in Greenland White-fronted Geese caught at Wexford, Ireland, the main wintering site of this population. This study system

402 M. D. Weegman et al. is well suited for questions related to inter-site movement and survival because these birds are relatively long-lived (maximum age 22 years; A.D. Fox unpubl. data), allowing us to follow known individuals for extended periods of time. Greenland White-fronted Geese are of particular interest because the global population declined from 35 700 birds in 1999 to 22 100 in 2012 (Fox et al. 2013), highlighting the need for more precise estimates of inter-site winter movement to better inform models of population dynamics and improve management efforts on wintering sites. METHODS From 1983 to 2009, 851 juvenile Greenland Whitefronted Geese were caught throughout winter on baited sites at Wexford Slobs, Ireland (52 22 0 N, 6 24 0 W), using standard cannon-netting techniques. Wexford Slobs constitute the single most important wintering area for Greenland White-fronted Geese, supporting over onethird of the global population (Fox et al. 1998) comprising intensively managed grassland and cropland. All caught birds were individually marked with a metal leg ring, a white plastic leg ring with a unique alpha-numeric code, and an orange neck collar labelled with the same code (Warren et al. 1992). Collar code combinations were legible with a 20 609 spotting scope at distances of up to 800 m. Individual geese were aged by plumage characteristics (presence/absence of white frons on face and black belly bars) and sexed by cloacal examination (Cramp & Simmons 1977). The sex ratio among these birds was balanced (50.2% females). A.J.W. resighted geese weekly at Wexford throughout winter, beginning when birds arrived in autumn. A network of volunteers resighted birds annually at over 70 other known wintering sites across Great Britain and Ireland. Among all wintering sites, over 21 400 resightings of known-age marked birds were recorded during the study period. Resightings were combined over the entire winter period to estimate annual survival and movement probabilities. In cases where birds were resighted at multiple sites during the same winter, the most frequented site (i.e. Wexford or elsewhere) was assigned for that year. To determine age- and sex-specific survival (/), recapture (p) and movement (w) probabilities between Wexford and elsewhere, we developed Bayesian multistate capture recapture models in the state space framework (i.e. containing state and observation matrices, see specification in Supporting Information Appendix S1) using WINBUGS, version 1.4.3 (Spiegelhalter et al. 2007), adapting examples outlined in Kery and Schaub (2012). All individuals resighted elsewhere were pooled into one category for estimation of survival, movement and resighting probabilities elsewhere. All parameters were assumed to be constant over time and all models were run using the R2WinBUGS package in Program R, version 2.14.2 (R Development Core Team 2012). We included parameters for both sexes and two age classes (i.e. juveniles and adults, the latter being individuals 2 years old) at Wexford and elsewhere. For example, the probability of an age 1 female at Wexford in year t being elsewhere in year t + 1 could be estimated as the product of female survival at Wexford multiplied by female movement from Wexford to elsewhere. All juveniles in this study were marked at Wexford; accordingly, we could not estimate survival elsewhere at age 1 or movement from elsewhere to Wexford at age 1. Thus, our estimates of immigration to Wexford are based on remigrants (i.e. birds that previously emigrated from Wexford) that survived and returned aged 2 or older. Models with a greater number of age classes (e.g. three and four age classes) converged but were less informative (i.e. 95% credible intervals (CRI) 0.05 1.00), probably due to small sample sizes for birds observed at older ages elsewhere. Posterior means are presented with 95% CRI. To examine sex-specific differences in survival and movement probabilities, we set males as the intercept in all models and tested whether the difference between female and male rates was identifiably different from zero (i.e. whether the 95% CRI overlapped zero). We report approximate P-values for these tests, citing the proportion of each posterior distribution lying below zero. We claim significance between categories when this proportion is < 2.5% or > 97.5%. Additional model specification and code can be found in Appendix S1. RESULTS Of 851 Greenland White-fronted Geese marked as juveniles between 1983 and 2009, 187 (22%) emigrated from Wexford in their lifetime. Of these, 54 (29%) birds remigrated back to Wexford in following years. The proportion of female posteriors below the intercept for survival at Wexford (P = 0.89) and elsewhere (P = 0.61) was non-significant (Supporting Information Fig. S1), implying no differences in survival between sexes at Wexford for ages 1 (male posterior mean 0.72, 95% CRI 0.68 0.76, female mean 0.69, 0.65 0.73) and 2+ (male mean 0.80, 0.77 0.82, female mean 0.78, 0.75 0.82) or elsewhere for ages 2+ (male mean 0.82, 0.78 0.85, female mean 0.81, 0.76 0.85; Fig. 1). The proportions of female posteriors below the intercept for emigration (P = 0.64) and remigration (P = 0.27) were also non-significant (Fig. S1). Thus, there were also no differences between sexes in emigration probabilities at ages 1 (male mean 0.18, 0.14 0.22, female mean 0.17, 0.13 0.22; Fig. 1) and 2+ (male mean 0.11, 0.09 0.14, female mean 0.11, 0.08 0.13) or remigration

No sex bias in winter movement by geese 403 probabilities at ages 2+ (male mean 0.11, 0.08 0.15, female mean 0.13, 0.09 0.18). Generally, survival at Wexford among males and females increased from age 1 (mean 0.71, 0.67 0.74) to age 2+ (mean 0.79, 0.76 0.81), while emigration probability declined (mean 0.18, 0.14 0.22 to mean 0.11, 0.09 0.13). Resighting probability at Wexford (mean 0.89, 0.87 0.91) was greater than elsewhere (mean 0.58, 0.54 0.63). DISCUSSION Based on capture histories of Greenland White-fronted Geese marked at Wexford, Ireland, we found no evidence of sex bias in winter emigration or survival rates in this population using novel Bayesian approaches to such analyses. These findings support those of previous studies of the same population of Greenland Whitefronted Geese (Wilson et al. 1991, Warren et al. 1992), and of studies of Light-bellied Brent Geese Branta bernicla hrota (Harrison et al. 2010), Black Brant Branta bernicla nigricans (Lindberg et al. 2007) and Lesser Snow Geese Chen caerulescens caerulescens (Williams et al. 2008). However, the data presented here are the first to show that there is also no sex-biased remigration back to Wexford by emigrants in subsequent years and therefore that there is no evidence to suggest that winter site fidelity in this population is sex-biased in any form. These findings strengthen the argument against male-biased winter site fidelity among long-lived waterfowl, whereby only two previous studies have suggested its occurrence; in the case of Canada Geese, yearling and adult birds showed no sex bias in winter homing rate (Raveling 1979), whereas in Bewick s Swans Cygnus columbianus bewickii, there were no significant differences between the proportions of males and females returning in their second winter (Rees 1987). Male-biased winter site fidelity might be predicted to occur in ducks (not geese or swans) because they pair during winter, are annually monogamous and exhibit shorter-term pair bonds which break up after breeding, as summarized by Robertson and Cooke (1999). (a) (b) (c) Figure 1. (a) Posterior means of age-specific survival (age 1 at Wexford, age 2+ at Wexford, age 2+ elsewhere) between male (dark grey bars; 95% credible intervals) and female (light grey bars; 95% credible intervals) Greenland White-fronted Geese marked at Wexford, Ireland, 1983 2009. (b) Posterior means of age-specific movement (age 1 emigration, age 2+ emigration, age 2+ remigration) between male (dark grey bars; 95% credible intervals) and female (light grey bars; 95% credible intervals) Greenland White-fronted Geese marked at Wexford, Ireland, 1983 2009. (c) Posterior means of resighting probability of Greenland Whitefronted Geese at Wexford and elsewhere, 1983 2009. All birds were marked at Wexford, Ireland.

404 M. D. Weegman et al. If females were site-faithful to natal areas and males to their winter quarters, it would be conceivable that females would shift their winter quarters when pairing to accompany their male to its wintering area. However, we now know this not to be the case; therefore, it seems more likely that the peak in between-winter shifts in sites among birds is due to individuals of differing winter provenance pairing and moving between wintering areas. If inter-site movement between wintering areas occurs after pair bond formation, then the relatively high emigration estimates we found could suggest that competition at Wexford may be too strong for lone pairs, which are most likely repeatedly to lose aggressive encounters for foraging opportunities to larger social units (Boyd 1953, Raveling 1970, Black & Owen 1989). Furthermore, Greenland White-fronted Geese are unique among waterfowl in maintaining parental bonds for 6 years and longer (Warren et al. 1993, M.D. Weegman unpubl. data), which may contribute to the lack of sex bias in winter inter-site movement. Parents and offspring remaining together for such extended periods would take part in any inter-site movement as a family unit, which because of the balanced sex ratio would not result in any sex bias. The annual emigration probability from Wexford (nearly 20% for 1-year-olds and 11% for 2+-year-olds) indicates that there is large inter-site movement (albeit not sex-biased) and connectivity among 70 known wintering flocks of Greenland White-fronted Geese. Our emigration probabilities are similar to those of Marchi et al. (2010), using different approaches to model ageand sex-specific emigration in this population. In this study, we build on Marchi et al. (2010) by calculating remigration probabilities; we found that over 10% of emigrants aged 2+ years return (i.e. remigrate) to Wexford during their lifetime. This supports the hypothesis that some departing Wexford birds return after sampling conditions at wintering sites elsewhere. A small proportion (< 5%) of remigrants moved sites at least twice between years, perhaps evidence of extreme exploratory behaviour or low social rank (Stahl et al. 2001). Although we know a great deal about between-year changes in wintering sites of known-age birds captured at Wexford, we know relatively little about the nature and extent of immigration to Wexford from other wintering sites, which must explain the stability in numbers there (as the number of juveniles has remained low; Fox et al. 2013) during the period of recent global population decline. However, our estimates of remigration may not be representative of birds immigrating to Wexford because remigrants have prior knowledge of the site; such prior knowledge could be incorporated into more complex multistate memory models (e.g. 2-order Markov models; Hestbeck et al. 1991, Brownie et al. 1993) in the future. Development of marking programmes at other known wintering sites would also allow for more robust estimates of immigration to Wexford as well as global inter-site movement at the metapopulation level for identification of wintering flocks of particular conservation concern. Together, these estimates will inform a metapopulation model for increased conservation efforts at all known wintering sites. We thank the National Parks and Wildlife Service of Ireland, particularly the offices of J. Wilson, D. Norriss, O. Merne and D. Tierney, for their support. 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Winter fidelity and apparent survival of lesser snow Goose populations in the Pacific flyway. J. Wildl. Manage. 72: 159 167. Wilson, H.J., Norriss, D.W., Walsh, A., Fox, A.D. & Stroud, D.A. 1991. Winter site fidelity in Greenland White-fronted Geese Anser albifrons flavirostris, implications for conservation and management. Ardea 79: 287 294. Received 20 May 2014; revision accepted 1 February 2015. Associate Editor: Richard Phillips. SUPPORTING INFORMATION Additional Supporting Information may be found in the online version of this article: Figure S1. (a) Posteriors from the relationship between male and female survival in Greenland Whitefronted Geese at Wexford, 1983 2009. In multistate models, males were set as the intercept (zero) and female posteriors were compared with the intercept; thus, the proportion of posteriors below zero (i.e. female survival; indicated by red bars) shows non-significant sex differences (approximate P = 0.89). (b) Posteriors from the relationship between male and female survival in Greenland White-fronted Geese elsewhere, 1983 2009. In multistate models, males were set as the intercept (zero) and female posteriors were compared with the intercept; thus, the proportion of posteriors below zero (i.e. female survival; indicated by red bars) shows nonsignificant sex differences (approximate P = 0.61). (c) Posteriors from the relationship between male and female emigration from Wexford in Greenland Whitefronted Geese, 1983 2009. In multistate models, males were set as the intercept (zero) and female posteriors were compared with the intercept; thus, the proportion of posteriors below zero (i.e. female emigration; indicated by red bars) shows non-significant sex differences (approximate P = 0.64). (d) Posteriors from the relationship between male and female remigration back to Wexford in Greenland White-fronted Geese, 1983 2009. In multistate models, males were set as the intercept (zero) and female posteriors were compared with the intercept; thus, the proportion of posteriors below zero (i.e. female remigration; indicated by red bars) shows non-significant sex differences (approximate P = 0.27). Appendix S1. Further specification of Bayesian multistate models (including formulas, definition of parameters, etc.).