Robert F. Rockwell

The snow geese of La Pérouse Bay : natural selection in the wild

Introduction The Lesser Snow Geese of La Perouse Bay Population structure and gene flow Fitness components model of Snow Goose life cycle Pairing, mating, and parental care Annual variation in fitness components Age effects Heritability of quantitative traits Plumage colour Clutch size The timing of reproduction Egg size Body size Synthesis and conclusions References Author index Subject index

BODY SIZE VARIATION IN LESSER SNOW GEESE: ENVIRONMENTAL PLASTICITY IN GOSLING GROWTH RATES'

We examined the influence of timing of reproduction and brood size on growth rates of goslings of nidifugous Lesser Snow Geese (Anser caerulescens caerulescens) using data collected at La Perouse Bay, Manitoba from 1978 to the present. Gosling growth rates declined significantly during the season, and the declines were independent of the parental genotype. Goslings from larger broods grew significantly faster, but there was no significant annual variation in the seasonal growth rate patterns with either hatch date or brood size. The hatch date and brood size effects could not be accounted for by systematic differences in either egg size or body size of the female parent. Goslings that grew more slowly due to the effects of hatch date or brood size were significantly smaller as adults. Because gosling growth rates influence final adult size, they may have a significant effect on various life history traits in this species.

Long-Term Decline in Body Size in a Snow Goose Population: Evidence of Environmental Degradation?

(1) The effect of environmental factors on annual body size variation was investigated in a breeding population of lesser snow geese (Anser caerulescens caerulescens L.) La Perouse Bay, Manitoba, using data from 1969 to the present. The population size has increased from approximately 2000 to 10 000 pairs during this period (Cooch et al. 1989). (2) Annual mean gosling body mass, tarsus and culmen length (measured at fledging) declined significantly by approximately 16% (240 g), 4% (3.4 mm), and 2% (0.93 mm) respectively in cohorts hatching between 1976 and 1988. The decline in gosling size has led to a significant decline in size of locally bred adults. Annual mean adult female body mass, tarsus and culmen lengths declined by approximately 15% (270 g), 6% (5.4 mm), and 4% (2.3 mm), respectively, in cohorts hatching between 1969 and 1986. (3) Gosling growth and subsequent adult size was also reduced in years with cold, wet weather after hatch. (4) The decline in gosling size was not dependent on the pattern of annual variation in egg or hatch mass, nor systematic changes in annual mean hatch date or post-hatch weather. Goslings reared by individual adult females showed a decline in size over time, suggesting that the general decline reflects a non-genetic change in gosling growth rates during the fledging period. The most probable non-genetic factor is a long-term reduction in food available to the geese at La Perouse Bay. This reduction in food availability may reflect over-exploitation of primary food plants by the geese.

RETROSPECTIVE ANALYSIS OF DEMOGRAPHIC RESPONSES TO ENVIRONMENTAL CHANGE: A LESSER SNOW GOOSE EXAMPLE

In general, analysis of population dynamics can proceed either prospectively or retrospectively. In the case of the former, asymptotic expectations are generally derived (analytically or numerically) from analysis of the potential effects of perturbation of the elements of the life table. However, the vital rates that are indicated by prospective analysis to contribute the most to projected growth rate are not necessarily those that have contributed to observed variation in growth rate over time. We used a retrospective analysis to analyze the life table responses of a population of long-lived herbivorous geese to a systematic reduction in food abundance within the traditional breeding colony. Typical of long-lived species, adult survival rate has been shown previously in a prospective perturbation analysis to have the largest potential impact on projected growth of the population. However, despite a significant long-term increase in adult survival over the course of the study, there has been a long-term decline in growth rates of the population inhabiting the traditional sampling areas, although absolute numbers of individuals in both populations increased over time. Retrospective assessment of the relative contributions of variation in underlying vital rates (adult and juvenile survival, in situ recruitment, emigration and immigration rates into the population) to projected growth showed that the long-term dynamics of the nesting population primarily reflected the combined effects of changes in postfledging juvenile survival, while changes in the postnesting population were most influenced by variation in juvenile survival and adult fidelity rate to the traditional brood-rearing areas. Decreases in both juvenile survival and fidelity reflect systematic reductions in food abundance over the course of the study. Our results confirm previous suggestions that philopatry to the brood-rearing areas may be a significantly more plastic trait than fidelity to nesting areas.

LONG-TERM CHANGES IN SURVIVAL RATES OF LESSER SNOW GEESE'

Survival rates of adult and juvenile Lesser Snow Geese (Anser caerulescens caerulescens) were estimated based on hunter recoveries from over 80 000 geese banded between 1970 and 1988 at La Perouse Bay, near Churchill, Manitoba. Adult survival rate increased significantly from -78% in 1970 to nearly 88% in 1987. Similar increases in adult survival, although not significant, occurred between 1969 and 1979 at a second colony of Snow Geese at Cape Henrietta Maria in Northern Ontario. These increases coincided with a decline in the proportion of Snow Geese being shot each year on the flyways, suggesting that reduced mortality from hunting may be responsible for the increased sur- vival. In contrast, survival rates of fledglings over their first year decreased significantly from a mean of 60% in 1970 to -30% in 1987, despite the reduction in hunting pressure. This indicates that young geese have been suffering increasing rates of nonhunting mortality, most likely prior to leaving the breeding grounds or on their first autumn migration. Their increased mortality appears to be related to slower growth rates and reduced body size induced by deteriorating feeding conditions on the breeding grounds. This study shows that different processes can influence mortality rates at different stages of the life cycle. The long-term changes indicate that mean values of age-specific survival rates are not adequate to describe the dynamics of this population.

Environmental change and the cost of philopatry: an example in the lesser snow goose

The consequences of philopatric and dispersal behaviours under changing environmental conditions were examined using data from the colony of Lesser Snow Geese (Anser caerulescens caerulescens) breeding at La Pérouse Bay, Manitoba, Canada. In response to increased population size and decreased food abundance over time, increasing numbers of family groups have been dispersing from the traditional feeding areas. Goslings from dispersed broods were significantly heavier (7.3%), and had longer culmens (3.1%), head lengths (2.6%) and marginally longer tarsi (1.9%) on average than goslings that remained within La Pérouse Bay itself. These differences were consistent in each of 5 years. There was no evidence that the larger size of dispersed goslings was due to either a tendency for larger adults to disperse to alternative sites, or increased mortality of smaller goslings among dispersed broods. The most likely cause for the larger size of goslings from dispersed broods was the significantly greater per capita availability of the preferred salt-marsh forage species at non-traditional brood-rearing areas. The larger goslings in non-traditional feeding areas showed significantly higher firstyear survival, suggesting that the use of deteriorating traditional feeding areas may currently be maladaptive in this population.

Foraging geese, vegetation loss and soil degradation in an Arctic salt marsh

Abstract The North American mid‐continent population of Lesser snow geese (Chen caerulescens caerulescens L.) has increased by ca. 7% per year, largely as a result of geese feeding on agricultural crops in winter and on migration. We describe the long‐term effects of increasing numbers of geese at an arctic breeding ground (La Pérouse Bay, Manitoba) on intertidal salt‐marsh vegetation. Between 1985 and 1999 goose grubbing caused considerable loss of graminoid vegetation along transects in intertidal marshes. Loss of vegetation led to bare sediment with a plant cover of less than 2%. Changes in vegetation could not be described by simple linear, geometric or exponential functions; most losses occurred between 1988 and 1990 and losses were staggered in time between individual transects, some of which had all vegetation removed. Between 1979 and 1999 the standing crop in July in remaining intact heavily‐grazed swards of Puccinellia phryganodes and Carex subspathacea fell from 40–60 g m−2 to 20–30 g m−2. Intense grazing on remaining patches of sward has restricted growth of these clonal forage plants and hypersalinity of bare sediments has precluded re‐establishment of vegetation. Between 1989 and 1993 numbers of faecal droppings in grazed plots reached maximum values of 15–22 droppings m−2 wk −1. Since then peak values have remained at less than 13 droppings m −2 wk −1. The loss of vegetation and changes in soil conditions have resulted in the establishment of an alternative stable state (hypersaline bare sediment). Nomenclature: Porsild & Cody (1980).

NATAL AND BREEDING PHILOPATRY IN A BLACK BRANT, BRANTA BERNICLA NIGRICANS, METAPOPULATION

We estimated natal and breeding philopatry and dispersal probabilities for a metapopulation of Black Brant (Branta bernicla nigricans) based on observations of marked birds at six breeding colonies in Alaska, 1986–1994. Both adult females and males exhibited high (>0.90) probability of philopatry to breeding colonies. Probability of natal philopatry was significantly higher for females than males. Natal dispersal of males was recorded between every pair of colonies, whereas natal dispersal of females was observed between only half of the colony pairs. We suggest that female-biased philopatry was the result of timing of pair formation and characteristics of the mating system of brant, rather than factors related to inbreeding avoidance or optimal discrepancy. Probability of natal philopatry of females increased with age but declined with year of banding. Age-related increase in natal philopatry was positively related to higher breeding probability of older females. Declines in natal philopatry with year of banding corresponded negatively to a period of increasing population density; therefore, local population density may influence the probability of nonbreeding and gene flow among colonies.

Relative effects of survival and reproduction on the population dynamics of emperor geese

Populations of emperor geese (Chen canagica) in Alaska declined sometime between the mid1960s and the mid-1980s and have increased little since. To promote recovery of this species to former levels, managers need to know how much their perturbations of survival and/or reproduction would affect population growth rate (X). We constructed an individual-based population model to evaluate the relative effect of altering mean values of various survival and reproductive parameters on X and fall age structure (AS, defined as the proportion of juv), assuming additive rather than compensatory relations among parameters. Altering survival of adults had markedly greater relative effects on A than did equally proportionate changes in either juvenile survival or reproductive parameters. We found the opposite pattern for relative effects on AS. Due to concerns about bias in the initial parameter estimates used in our model, we used 5 additional sets of parameter estimates with this model structure. We found that estimates of survival based on aerial survey data gathered each fall resulted in models that corresponded more closely to independent estimates of A than did models that used mark-recapture estimates of survival. This disparity suggests that mark-recapture estimates of survival are biased low. To further explore how parameter estimates affected estimates of X, we used values of survival and reproduction found in other goose species, and we examined the effect of an hypothesized correlation between an individuals clutch size and the subsequent survival of her young. The rank order of parameters in their relative effects on A was consistent for all 6 parameter sets we examined. The observed variation in relative effects on A among the 6 parameter sets is indicative of how relative effects on A may vary among goose populations. With this knowledge of the relative effects of survival and reproductive parameters on A, managers can make more informed decisions about which parameters to influence through management or to target for future study. J. WILDL. MANAGE. 61(1):191-201

Age and reproductive success in female lesser snow geese: experience, senescence and the cost of philopatry

We studied age-related reproductive success in female lesser snow geese Chen caerulescens caerulescens, using data from the breeding colony at La Perouse Bay, Manitoba, 1973-88. We assessed age-specific variation in both individual components of reproductive success and their cumulative product using individuals aged 2-19 years. The composite measure of reproductive success increased among birds 2-6 years of age. Increased reproductive success over these ages was due to significant increases in clutch size and gosling survival and significant reductions in probabilities of total nest and total brood failure. After age 6, the composite measure of reproductive success declined significantly