Two closely-related, different-sized species of geese nest sympatrically south of the Queen Maud Gulf (QMG) in Canadas central arctic. Following a period of high population growth rate in both species within the QMG, the population growth rate of larger-bodied lesser snow geese (Chen caerulescens caerulescens; hereafter snow geese) has slowed most recently to roughly half that observed in smaller-bodied Rosss geese (Chen rossii). I focused on factors that influence productivity and recruitment in these two species, to improve our understanding of life history variation associated with interspecific differences in body size, and to test for density-dependent population responses. I used long-term data (1991 to 2008) to compare spring nutrient reserves, breeding strategies, clutch sizes, nest success, and juvenile survival in Rosss and snow geese breeding at Karrak Lake, Nunavut; a large breeding colony located within the QMG.<p> Long-term patterns of spring body condition (i.e., fat and protein reserves) diverged in prospective breeding female Rosss and snow geese implying that differences in food acquisition ability had become more acute. Snow geese displayed larger reductions in protein and fat reserves through time compared to Rosss geese thereby suggesting a differential density-dependent response in the ability to store nutrient reserves, a prerequisite for breeding in both species. Decreased per capita food availability influenced the timing of reproduction in both species. Nesting phenologies of Rosss and snow geese, adjusted for variation in phenology of local spring climate, have become later by 6.5 and 5.0 days, respectively, since 1991. Nutritional strategies (i.e., reliance on reserves versus local food) used for clutch formation differed between species. Rosss geese displayed greater reliance on stored reserves (i.e., capital breeding) than did snow geese, though both used endogenous reserves (> 62% of yolk protein, > 48% of albumen, and > 73% of yolk lipid) for clutch formation. Rosss and snow geese experienced declines of 28% and 23% in body masses from arrival to post-laying and also until hatch demonstrating that endogenous reserves are the main nutrient sources for incubation. Still, constraints of small size forced Rosss geese to use a mixture of local food plants and reserves for incubation metabolism.<p> I then examined differences in clutch size, nest success, and juvenile survival to understand of the role of recruitment in the interspecific divergence of population trajectories. I did not find strong interspecific differences in clutch size and nest success. Overall, snow geese had a larger mean clutch size, which was expected based on benefits of a larger-body size. Clutch sizes decreased with delays in breeding and decreasing protein reserves of arriving females, although Rosss geese displayed larger declines with decreasing protein reserves. Mean apparent nest success for Rosss geese was 4.5% higher compared to snow geese. Nest success showed large declines (11%) in both species with increasing population size at the breeding colony. However, nest success of snow geese decreased twice as fast with delays in breeding compared to Rosss geese. Last, I found no evidence of negative density dependence in juvenile survival over time. Juvenile survival was higher in snow geese (48%) compared to Rosss geese (38%), consistent with a life history prediction based on body size differences. Despite lower juvenile survival, recruitment by Rosss geese is likely greater than that of snow geese because of earlier sexual maturity, higher breeding probability and/or greater nest success.<p> Ultimately, small body size of Rosss geese may produce an ideal life history schedule under resource limitation at this colony i.e., one that maximizes fitness compared to larger snow geese. Life history characteristics of Rosss geese (e.g., absolutely lower energy requirement, have a flexible breeding strategy, higher reproductive effort, an earlier age of sexual maturity, a shorter breeding cycle allowing delayed arrival and nest initiation on arctic breeding areas, and shorter time required by goslings to attain adult size), in addition to their smaller bill morphology may allow exploitation of a wider niche space (i.e., one that includes marginal quality and low quantity vegetation) relative to snow geese. Because there were no large differences in components of recruitment considered here, other components of recruitment (age of sexual maturity, breeding probability) may be affected more strongly by diminished spring nutrition in snow geese and thus have a larger influence on local population dynamics.

Two closely-related, different-sized species of geese nest sympatrically south of the Queen Maud Gulf (QMG) in Canadas central arctic. Following a period of high population growth rate in both species within the QMG, the population growth rate of larger-bodied lesser snow geese (Chen caerulescens caerulescens; hereafter snow geese) has slowed most recently to roughly half that observed in smaller-bodied Rosss geese (Chen rossii). I focused on factors that influence productivity and recruitment in these two species, to improve our understanding of life history variation associated with interspecific differences in body size, and to test for density-dependent population responses. I used long-term data (1991 to 2008) to compare spring nutrient reserves, breeding strategies, clutch sizes, nest success, and juvenile survival in Rosss and snow geese breeding at Karrak Lake, Nunavut; a large breeding colony located within the QMG.<p> Long-term patterns of spring body condition (i.e., fat and protein reserves) diverged in prospective breeding female Rosss and snow geese implying that differences in food acquisition ability had become more acute. Snow geese displayed larger reductions in protein and fat reserves through time compared to Rosss geese thereby suggesting a differential density-dependent response in the ability to store nutrient reserves, a prerequisite for breeding in both species. Decreased per capita food availability influenced the timing of reproduction in both species. Nesting phenologies of Rosss and snow geese, adjusted for variation in phenology of local spring climate, have become later by 6.5 and 5.0 days, respectively, since 1991. Nutritional strategies (i.e., reliance on reserves versus local food) used for clutch formation differed between species. Rosss geese displayed greater reliance on stored reserves (i.e., capital breeding) than did snow geese, though both used endogenous reserves (> 62% of yolk protein, > 48% of albumen, and > 73% of yolk lipid) for clutch formation. Rosss and snow geese experienced declines of 28% and 23% in body masses from arrival to post-laying and also until hatch demonstrating that endogenous reserves are the main nutrient sources for incubation. Still, constraints of small size forced Rosss geese to use a mixture of local food plants and reserves for incubation metabolism.<p> I then examined differences in clutch size, nest success, and juvenile survival to understand of the role of recruitment in the interspecific divergence of population trajectories. I did not find strong interspecific differences in clutch size and nest success. Overall, snow geese had a larger mean clutch size, which was expected based on benefits of a larger-body size. Clutch sizes decreased with delays in breeding and decreasing protein reserves of arriving females, although Rosss geese displayed larger declines with decreasing protein reserves. Mean apparent nest success for Rosss geese was 4.5% higher compared to snow geese. Nest success showed large declines (11%) in both species with increasing population size at the breeding colony. However, nest success of snow geese decreased twice as fast with delays in breeding compared to Rosss geese. Last, I found no evidence of negative density dependence in juvenile survival over time. Juvenile survival was higher in snow geese (48%) compared to Rosss geese (38%), consistent with a life history prediction based on body size differences. Despite lower juvenile survival, recruitment by Rosss geese is likely greater than that of snow geese because of earlier sexual maturity, higher breeding probability and/or greater nest success.<p> Ultimately, small body size of Rosss geese may produce an ideal life history schedule under resource limitation at this colony i.e., one that maximizes fitness compared to larger snow geese. Life history characteristics of Rosss geese (e.g., absolutely lower energy requirement, have a flexible breeding strategy, higher reproductive effort, an earlier age of sexual maturity, a shorter breeding cycle allowing delayed arrival and nest initiation on arctic breeding areas, and shorter time required by goslings to attain adult size), in addition to their smaller bill morphology may allow exploitation of a wider niche space (i.e., one that includes marginal quality and low quantity vegetation) relative to snow geese. Because there were no large differences in components of recruitment considered here, other components of recruitment (age of sexual maturity, breeding probability) may be affected more strongly by diminished spring nutrition in snow geese and thus have a larger influence on local population dynamics.