Joël Bêty

Body Condition, Migration, and Timing of Reproduction in Snow Geese: A Test of the Condition-Dependent Model of Optimal Clutch Size

The seasonal decline of avian clutch size may result from the conflict between the advantage of early breeding (greater offspring value) and the advantage of a delay in lay date (improved body condition and hence clutch size). We tested predictions of a condition‐dependent individual optimization model based on this trade‐off (Rowe et al. 1994) in a long‐distance migrant, the greater snow goose (Chen caerulescens atlantica), using data on condition, migration, and reproductive decisions of individuals. We closely tracked radio‐marked females at their main spring staging area and on their breeding grounds. Our results were consistent with predictions of the model. Early‐arriving females had a longer prelaying period and initiated their nests earlier than late arrivals. After controlling statistically for arrival date, we determined that females with high premigration condition had an earlier lay date than those in low condition. After controlling for the seasonal decline (i.e., lay date), we observed that clutch size was not related to premigration condition. Moreover, we took advantage of an unplanned manipulation of the prebreeding condition that occurred during our long‐term study. We found that a reduction in condition caused a delay in lay date. However, after controlling for the seasonal decline, it did not affect clutch size. Our study indicates that geese simultaneously adjust their lay date and clutch size according to their premigration condition and migratory behavior as predicted by the condition‐dependent optimization model.

ARE GREATER SNOW GEESE CAPITAL BREEDERS? NEW EVIDENCE FROM A STABLE-ISOTOPE MODEL

The strategy of relying extensively on stored nutrient reserves for repro- duction (capital breeding) was thought to be common in large-bodied birds breeding in harsh environments, such as arctic-nesting geese, but this view has been challenged recently. Our objective was to model inputs to the eggs from stored reserves and from local food plants in Greater Snow Geese (Chen caerulescens atlantica) breeding in the high Arctic, using a new approach based on stable-isotope signatures. Snow Geese and their eggs were collected during laying from 1999 to 2001 (N = 66 females and 110 eggs). We analyzed the isotopic signature (813C and 815N) of egg constituents (lipid-free yolk, yolk lipid, and albumen), goose tissues (lipid-free breast muscles, abdominal fat, and whole liver) and of the food plants eaten by laying geese in the Arctic (graminoids and forbs). We applied a two-isotope mixing model approach to delineate nutrient input to eggs quantitatively. Dif- ferences in the isotopic signature of endogenous reserves and arctic food plants were relatively large (5.3-8.0%0 for A813C and 7.5%c for A815N) because reserves were accu- mulated in southern staging areas where geese feed in farmlands and estuarine habitats. The percentage of egg nutrients derived from exogenous sources (food consumed in the Arctic) was higher than from endogenous (body) reserves and varied little among the three years. Isotopic signatures indicated that endogenous reserves contributed 33% of lipid-free yolk nutrients, 27% of albumen, and 20% of yolk lipid, on average. Isotopic signatures of egg constituents of individual females were more strongly related to those of liver than endogenous sources (breast muscles or abdominal fat), indicating that the endogenous isotopic signature was diluted by a dietary input in the liver. We also found evidence of seasonal variation in the use of endogenous reserves. Late-laying females apparently in- vested proportionally more endogenous reserves in their eggs than did early layers, but not those laying larger clutches. We conclude that Greater Snow Geese use a mixed capital/ income breeding strategy. Our study shows that isotopic composition of tissues can be used to infer the contribution of exogenous vs. endogenous sources of nutrients for egg formation where inputs differ isotopically.

Lower Predation Risk for Migratory Birds at High Latitudes

Predator Avoidance Strategy Selective pressures influencing bird migration can include availability of food, pressure from parasites and pathogens, and predation risk. The importance of the last of these is revealed by McKinnon et al. (p. 326; see the Perspective by Gilg and Yoccoz), who present an experimental analysis of the benefits of long-distance migration for reproduction in arctic-nesting birds. Measurements of a controlled effect of predation risk along a 3350-kilometer north-south gradient across arctic Canada provides evidence that the risk of nest predation decreases with latitude. Thus, birds migrating further north may acquire reproductive benefits in the form of reduced predation risk. Egg predation rates measured at artificial nests along a 3000-kilometer transect decrease northwards. Quantifying the costs and benefits of migration distance is critical to understanding the evolution of long-distance migration. In migratory birds, life history theory predicts that the potential survival costs of migrating longer distances should be balanced by benefits to lifetime reproductive success, yet quantification of these reproductive benefits in a controlled manner along a large geographical gradient is challenging. We measured a controlled effect of predation risk along a 3350-kilometer south-north gradient in the Arctic and found that nest predation risk declined more than twofold along the latitudinal gradient. These results provide evidence that birds migrating farther north may acquire reproductive benefits in the form of lower nest predation risk.

Individual variation in timing of migration: causes and reproductive consequences in greater snow geese (Anser caerulescens atlanticus)

Decisions made by birds during migration to breeding grounds can strongly affect the fitness of individuals. We investigated possible causes and reproductive consequences of inter-individual variation in the migratory behavior of an arctic-nesting species, the greater snow goose (Anser caerulescens atlanticus), by radio-tracking females at their staging area and on their breeding grounds. Females showed relatively high repeatability in the duration of migration (ri=0.37) and arrival date on the breeding grounds (ri=0.42) suggesting that these traits are characteristics of individuals. Conversely, no individual consistency in departure date from the staging area was detected (ri=−0.02) indicating that environmental factors may have a large influence. Females paired with dominant males departed slightly earlier from the staging area than females accompanied by subordinate males. However, neither social status on the staging area (i.e. paired vs unpaired) nor dominance scores were associated with arrival time of individuals. Finally, the probability of breeding was positively related to arrival date indicating a reproductive cost of arriving too early on the breeding grounds. The combination of breeding probability and seasonal decline in breeding success nonetheless suggests that females arriving a few days earlier than the median arrival date attained highest reproductive success. Our results show that assessing the fitness consequences of early arrival by focusing solely on breeding females would lead to an overestimation of the genuine benefits. This study also indicates possible genetically based differences among individuals in migration duration and arrival time on the breeding grounds.

Trophic Interactions in a High Arctic Snow Goose Colony

Abstract We examined the role of trophic interactions in structuring a high arctic tundra community characterized by a large breeding colony of greater snow geese (Chen caerulescens atlantica). According to the exploitation ecosystem hypothesis of Oksanen et al. (1981), food chains are controlled by top-down interactions. However, because the arctic primary productivity is low, herbivore populations are too small to support functional predator populations and these communities should thus be dominated by the plant/ herbivore trophic-level interaction. Since 1990, we have been monitoring annual abundance and productivity of geese, the impact of goose grazing, predator abundance (mostly arctic foxes, Alopex lagopus) and the abundance of lemmings, the other significant herbivore in this community, on Bylot Island, Nunavut, Canada. Goose grazing consistently removed a significant proportion of the standing crop (∼40%) in tundra wetlands every year. Grazing changed plant community composition and reduced the production of grasses and sedges to a low-level equilibrium compared to the situation where the presence of geese had been removed. Lemming cyclic fluctuations were strong and affected fox reproduction. Fox predation on goose eggs was severe and generated marked annual variation in goose productivity. Predation intensity on geese was closely related to the lemming cycle, a consequence of an indirect interaction between lemming and geese via shared predators. We conclude that, contrary to the exploitation ecosystem hypothesis, both the plant/herbivore and predator/prey interactions are significant in this arctic community.

Long-term monitoring at multiple trophic levels suggests heterogeneity in responses to climate change in the Canadian Arctic tundra

Arctic wildlife is often presented as being highly at risk in the face of current climate warming. We use the long-term (up to 24 years) monitoring records available on Bylot Island in the Canadian Arctic to examine temporal trends in population attributes of several terrestrial vertebrates and in primary production. Despite a warming trend (e.g. cumulative annual thawing degree-days increased by 37% and snow-melt date advanced by 4–7 days over a 23-year period), we found little evidence for changes in the phenology, abundance or productivity of several vertebrate species (snow goose, foxes, lemmings, avian predators and one passerine). Only primary production showed a response to warming (annual above-ground biomass of wetland graminoids increased by 123% during this period). We nonetheless found evidence for potential mismatches between herbivores and their food plants in response to warming as snow geese adjusted their laying date by only 3.8 days on average for a change in snow-melt of 10 days, half of the corresponding adjustment shown by the timing of plant growth (7.1 days). We discuss several reasons (duration of time series, large annual variability, amplitude of observed climate change, nonlinear dynamic or constraints imposed by various rate of warming with latitude in migrants) to explain the lack of response by herbivores and predators to climate warming at our study site. We also show how length and intensity of monitoring could affect our ability to detect temporal trends and provide recommendations for future monitoring.

ARE BODY CONDITION AND REPRODUCTIVE EFFORT OF LAYING GREATER SNOW GEESE AFFECTED BY THE SPRING HUNT

Abstract A spring hunt was implemented on the staging areas of Greater Snow Geese (Chen caerulescens atlantica) in Quebec in 1999 and 2000. We evaluated whether this activity, which occurred during the period of spring nutrient storage, may have affected the body condition and reproductive effort of laying geese. We collected laying females in years with a spring hunt (1999–2000, n = 34) and compared them with birds collected in years without a hunt (1989–1990, n = 10). All indices of body condition and clutch size were significantly lower in years with a hunt than in years without, and laying dates were delayed. Tracking of radio-marked females on the staging and breeding areas showed that a lower proportion of females reached the nesting areas in years with a hunt (28% in 1999–2000, n = 80) than in years without (85% in 1997–1998, n = 80) and that fewer females nested (9% vs. 56%, respectively). Our results suggest that the spring hunt negatively affected nesting geese. ¿Son Afectadas por la Cacería de Primavera la Condición Física y el Esfuerzo Reproductivo de Chen caerulescens atlantica? Resumen. La cacería de primavera fue implementada en áreas de escala de Chen caerulescens atlantica en Quebec en 1999 y 2000. Evaluamos si la cacería, que se llevó a cabo durante el período primaveral de acumulación de nutrientes, pudo haber afectado la condición fisica y el esfuerzo reproductivo de los gansos. Colectamos hembras que estuvieran poniendo huevos en años con cacería de primavera (1999–2000, n = 34) y las comparamos con aves colectadas en años sin cacería (1989–1990, n = 10). Todos los índices de la condición física y del tamaño de la nidada fueron significativamente menores en años con cacería que en años sin cacería, y se retrasaron las fechas de puesta. El seguimiento de hembras marcadas con radio en áreas de escala y cría mostró que una menor proporción llegó a las áreas de nidificación en años con cacería (28% en 1999–2000, n = 80) que en años sin cacería (85% en 1997–1998, n = 80), y que menos hembras nidificaron (9% vs. 56%, respectivamente). Nuestros resultados sugieren que la cacería de primavera afecta negativamente a los gansos nidificantes.

The tundra food web of Bylot Island in a changing climate and the role of exchanges between ecosystems

Abstract: It is increasingly recognized that ecosystems are not closed systems and that exchanges of resources across ecosystem boundaries can have repercussions on food webs, especially in low productivity systems such as the terrestrial Arctic. However, because these exchanges can take multiple forms, assessing their significance in the functioning of the tundra food web is difficult. In this paper, we first review some important concepts related to resource exchanges between ecosystems and examine their relevance to the study of trophic interactions in the arctic tundra. An analysis of the Bylot Island food web in the Canadian Arctic using a mass-balance trophic model suggests that predators are the dominant force controlling this food web. However, an important feature of this ecosystem is that several top predators benefit from allochthonous inputs, either through the presence of migratory birds during the summer or the use of the marine environment as a foraging ground in winter. We also show that migratory birds may act as autochthonous resource exporters for lower trophic levels, for instance by removing nitrogen from the nutrient pool when young produced locally migrate south and die away from the system. Although these resource exchanges may be a general feature of several arctic terrestrial ecosystems, their importance in the functioning of the tundra food web remains to be determined. Through long-term monitoring, we found that primary production in wetlands of Bylot Island increased by 85% over a 20-y period, likely a consequence of the warming trend observed in the area. However, we have not detected any changes at higher trophic levels, which is consistent with a top-down control of this food web. Given the importance of resource exchanges between ecosystems in the dynamics of the tundra food web, a full investigation of the effects of climate change will require a broader cross-ecosystem perspective.

Disentangling trophic relationships in a High Arctic tundra ecosystem through food web modeling

Determining the manner in which food webs will respond to environmental changes is difficult because the relative importance of top-down vs. bottom-up forces in controlling ecosystems is still debated. This is especially true in the Arctic tundra where, despite relatively simple food webs, it is still unclear which forces dominate in this ecosystem. Our primary goal was to assess the extent to which a tundra food web was dominated by plant-herbivore or predator-prey interactions. Based on a 17-year (1993-2009) study of terrestrial wildlife on Bylot Island, Nunavut, Canada, we developed trophic mass balance models to address this question. Snow Geese were the dominant herbivores in this ecosystem, followed by two sympatric lemming species (brown and collared lemmings). Arctic foxes, weasels, and several species of birds of prey were the dominant predators. Results of our trophic models encompassing 19 functional groups showed that <10% of the annual primary production was consumed by herbivores in most years despite the presence of a large Snow Goose colony, but that 20-100% of the annual herbivore production was consumed by predators. The impact of herbivores on vegetation has also weakened over time, probably due to an increase in primary production. The impact of predators was highest on lemmings, intermediate on passerines, and lowest on geese and shorebirds, but it varied with lemming abundance. Predation of collared lemmings exceeded production in most years and may explain why this species remained at low density. In contrast, the predation rate on brown lemmings varied with prey density and may have contributed to the high-amplitude, periodic fluctuations in the abundance of this species. Our analysis provided little evidence that herbivores are limited by primary production on Bylot Island. In contrast, we measured strong predator-prey interactions, which supports the hypothesis that this food web is primarily controlled by top-down forces. The presence of allochthonous resources subsidizing top predators and the absence of large herbivores may partly explain the predominant role of predation in this low-productivity ecosystem.

Manipulating individual state during migration provides evidence for carry-over effects modulated by environmental conditions.

Despite observational evidence of carry-over effects (COEs, events occurring in one season that produce residual effects on individuals the following seasons), to our knowledge no experimental studies have been carried out to explore how COEs might affect reproductive output. We simulated an environmental perturbation affecting spring-staging migrants to investigate COEs in greater snow geese (Anser caerulescens atlanticus). During three consecutive years, 2037 females captured during spring staging (approx. 3000 km south of their Arctic breeding grounds) were maintained in captivity (with or without access to food) for 0–4 days. Duration of captivity (but not food treatment) negatively affected reproductive success, probably through stress response. Reproductive success was reduced by 45–71% in 2 years, but not in a third year with unusually favourable breeding conditions. This unprecedented manipulation indicates that COEs can have a strong effect on individual reproductive success in long-distance migrants, but that this effect can be partly compensated for by good environmental conditions on the breeding ground.