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Appendix S1. Study area and data collection
We used data from a long-term study of breeding snow geese at La Pérouse Bay ‘LPB’ and
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the larger Cape Churchill Peninsula ‘CCP’ region in northern Manitoba (58º44’ N, 94º28’ W).
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The study area is located in the southern part of the species’ breeding range, and is also used as a
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temporary staging area for birds migrating to more northern colonies in spring. The local
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population grew from 2,000 pairs in the 1960s to > 50,000 pairs in 2011 (Rockwell et al. 2011).
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As the population grew, the nesting area expanded along the eastern coast of the CCP as well as
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several kilometers inland from the coastal LPB saltwater marsh.
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Left panel: Progressive expansion of the lesser snow goose nesting colony from 1973 to 2008
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from the historic colony at La Pérouse Bay to the Cape Churchill Peninsula. Right panel: zone 1
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encompasses the coastal saltmarsh that the original nesting colony used for brood rearing (i.e.,
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the coastal saltmarsh of La Pérouse Bay); zone 2 encompasses the inland zone of freshwater
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marsh adjacent to zone 1; zone 3 is located in the north-east corner of the CCP; and zone 4
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extends south of zone 3 where most of the goslings are now raised. Note: the gray area between
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Hudson Bay and the landmass is the extent of the fluctuating saltmarsh.
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Goose families dispersing away from traditional brood-rearing grounds initially stayed within
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the confines of the coastal salt marshes (RFR unpublished data). Since the 1990s, family groups
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have been using freshwater marshes (RFR unpublished data), but several lines of evidence
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suggest that even under changing conditions, snow goose broods still show strong seasonal
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fidelity to a particular area and habitat type. Goslings encountered during our annual banding
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efforts were 5-6 weeks old. Most of the within-season habitat dispersal is likely to occur in the
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first 7-10 days after hatching, after which broods move relatively little (Mainguy et al. 2006).
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Mellor and Rockwell (2006) found that goslings collected at inland freshwater habitat have
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substantially lower loads of caecal nematodes than do goslings collected in coastal habitats. This
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is consistent with the hypothesis that the population is structured in coastal and inland segments.
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In addition, Winiarski et al. (2012) used stable isotopes in forage plants and gosling tissue
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samples taken at regular intervals during brood rearing to determine how gosling resource use
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differed between habitats, changed during growth, and affected gosling growth rate and body
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size. They showed that goslings throughout brood rearing consistently used freshwater marsh or
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tidal marsh, with no evidence of large-scale movement or diet changes during gosling growth
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(Winiarski et al. in press). Based on this evidence, we are comfortable assigning each gosling to
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a habitat zone that reflects where it foraged during development.
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Every year in late July or early August, when the adults are molting and before the young can
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fly, adults and goslings are rounded up, aged, sexed, and banded. Birds are marked with uniquely
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numbered USFWS aluminum leg bands and previously marked birds are recorded as recaptures.
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Body mass (to nearest gram) and tarsus length (to nearest mm) were measured during banding
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drives for a sample of goslings intermittently between 1978 and 2005 (measurements not taken
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in 1992, 1993, 2002, 2003, and 2004; banding did not take place in 1996 and 1997). Sample
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sizes for female goslings across habitat zones and years are presented here:
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Here, we used data on gosling captures from 1978 to 2005 for available years and
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information on subsequent recaptures and hunter recoveries from 1978 to 2008, since we wanted
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to estimate survival for known-age individuals (i.e., individuals marked as goslings). During this
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time period we banded and measured morphological variables for 8086 goslings (4125 females
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and 3961 males).
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Sex differences in gosling development are significant (Cooch et al. 1996, Cooch et al.
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1997); we decided to restrict all analyses to females because they are the philopatric sex for
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which the effect of variability in gosling body condition on juvenile survival later in life can be
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assessed. Because males are not philopatric to their natal breeding grounds, we only had 11
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recaptures for males marked as goslings. Relying on dead recoveries provided too little
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information to estimate male juvenile survival since only 5.2% of the males banded, measured,
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and released as goslings were recovered dead.
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Thorough descriptions of the study site, field methods, and data collection protocols are
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provided by Davies et al. (1988) and Cooke et al. (1995).
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Literature cited
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Cooch EG, Lank DB, Cooke F (1996) Intraseasonal variation in the development of sexual size
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dimorphism in a precocial bird: evidence from the lesser snow goose. Journal of Animal
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Ecology, 65, 439-450
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Cooch EG, Lank DB, Robertson RJ, Cooke F (1997) Effects of parental age and environmental
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change on offspring sex-ratio in a precocial bird. Journal of Animal Ecology, 66, 189-202.
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Cooke F, Rockwell RF, Lank DB (1995) the Snow Geese of La Pérouse Bay. In: Natural
selection in the wild. Oxford University Press, Oxford, UK
Davies JC, Rockwell RF, Cooke F (1988) Body-size variation and fitness components in Lesser
Snow Geese (Chen caerulescens caerulescens). The Auk, 105, 639-648
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Mainguy J, Gauthier G, Giroux J-F, Bêty J (2006) Gosling growth and survival in relation to
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brood movements in Greater Snow Geese (Chen caerulescens atlantica). The Auk, 123(4),
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1077-1089
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69
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Mellor AA, Rockwell RF (2006) Habitat Shifts and Parasite Loads of Lesser Snow Geese (Chen
caerulescens caerulescens). Écoscience, 13, 497-502
Rockwell RF, Gormezano LJ, Koons DN (2011) Trophic matches and mismatches: can polar
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bears reduce the abundance of nesting snow geese in western Hudson Bay? Oikos, 120(5), 696-
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709
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Winiarski KJ, McWilliams SR, Rockwell RF (In Press) Rapid environmental degradation in a
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subarctic ecosystem influences resource use of a keystone avian herbivore. Journal of Animal
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Ecology, 81(5):1132-42
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