Environmental influence on life-history traits: growth, survival, and fecundity in Black Brant (Branta bernicla).
Environmental (James 1983, Cooch et al. 1991a, b, Larsson and Forslund 1991, Sedinger and Flint 1991, Rhymer 1992, Saether and Heim 1993) and maternal factors (Ricklefs and Peters 1981, Sinervo 1990, Kojola 1993) strongly influence growth in numerous vertebrates. Difficulty in estimating survivorship and future fecundity of individual juveniles in most vertebrate populations has generally precluded measuring fitness consequences of such variation in growth (but see Clutton-Brock et al. 1988).
Understanding the relationship between growth and future fitness is essential to understanding the fitness consequences of reproductive "decisions" in many populations. For example, timing of reproduction influences survival of offspring (Cooke et al. 1984, Daan et al. 1990). Arctic geese are especially good subjects for examining fitness consequences of early growth because females return to their natal areas to breed (Rohwer and Anderson 1988), thereby increasing the feasibility of monitoring fitness in the same individuals whose growth histories are known. Furthermore, mechanisms by which early growth might influence fitness by affecting adult body size have been proposed and studied (Ankney and MacInnes 1978).
Female geese rely heavily on stored protein and lipid reserves to produce eggs (Ankney and MacIness 1978, Raveling 1979a, Ankney 1984) because insufficient nutrients are available in the nesting environment in late spring, when geese begin breeding, for females to meet their maintenance requirements and produce eggs (Ryder 1972, Raveling 1979b). Large body size should enable females to store larger reserves, because a given mass of reserve comprises a smaller proportion of mass of larger compared to smaller females. Furthermore, lower mass-specific metabolic rate and greater fasting endurance of large individuals (Calder 1984) may enable them to store absolutely larger lipid reserves. Ankney and MacInnes (1978) detected a significant positive relationship between culmen length and nutrient reserve size in Lesser Snow Geese (Anser caerulescens caerulescens), (hereafter snow geese) on a breeding area, and Alisauskas (1988) observed a correlation between size of nutrient reserves and body size in snow geese late in spring migration. These findings imply that body size should be positively correlated with clutch size in snow geese. Studies at La Perouse Bay (58 [degrees] N, 94 [degrees] W) on the west coast of Hudson Bay, have not detected a relationship between body size and clutch size in a more southerly population of snow geese (Cooch et al. 1992). Therefore, the relationship between body size and fecundity in snow geese remains somewhat controversial (Alisauskas and Ankney 1990, Cooch et al. 1992.).
Geese are nearly strictly herbivorous during the growth period (Owen 1980, Sedinger and Raveling 1984), in contrast to herbivorous mammals that rely on nutrient-rich milk for growth. Plant foods are relatively low in protein and metabolizable energy (Cargill and Jefferies 1984, Sedinger and Raveling 1984, 1986, Sedinger et al. 1989) and processing time in the gut restricts food intake (Sedinger and Raveling 1988), limiting the ability of goslings to compensate for low dietary nutrient concentrations by increasing food intake. Growth of goslings, therefore, responds to nutritional content of their diet (Lieff 1973, Wurdinger 1975). Because diet quality varies both spatially (Larsson and Forslund 1991, Cooch et al. 1993) and temporally (Sedinger and Raveling 1986, Manseau and Gauthier 1993), gosling size late in their first summer is strongly influenced by environmental factors (Cooch et al. 1991a, b, Larsson and Forslund 1991, Sedinger and Flint 1991). Genetic, environmental, and maternal effects shared by brood mates explained an insignificant (5%) proportion of total variation in gosling size in Black Brant (Branta bernicla nigricans) (hereafter brant) after hatch date and egg size were accounted for (Sedinger and Flint 1991). Adult size in snow geese (Cooch et al. 1991a) and Barnacle Geese (Branta leucopsis) (Larsson and Forslund 1991) is correlated with body size of the same individuals as goslings. Therefore, a substantial portion of within-population variation in adult body size of Arctic nesting geese is likely of environmental origin.
Because of the potential for adult body size to determine size of nutrient reserves and, consequently, clutch size in Arctic geese, early environment during the growth period may play an important role in expression of life history traits. In this paper, we examine the relationship between size of brant goslings late in their first summer and adult body size, breeding propensity, and fecundity. In snow geese (Cooch et al. 1993), Barnacle Geese (Owen and Black 1989), and Emperor Geese (Anser canagicus) (Schmutz 1993), first-year survival was correlated with gosling size in late summer. Therefore, we also examine the relationship between gosling size and first-year survival in brant.
Brant nest colonially near coastal salt marshes in Alaska, arctic Canada, and Siberia (Bellrose 1980), but [greater than]70% of the breeding population nests on the Yukon-Kuskokwim (Y-K) Delta in southwestern Alaska (Sedinger et al. 1993). Brant nest in four major colonies (3000-7000 nests) and numerous smaller aggregations in tidally influenced graminoid communities of the Y-K Delta. Data reported here are based on a long-term study of brant at the Tutakoke River colony (61 [degrees] N, 165 [degrees] W) at the mouth of the Kashunuk River near Old Chevak (Mickelson 1975).
Life history data
Brant arrive between 5 and 16 May and first nests are initiated between 14 and 28 May. Seventy to 80% of nests are initiated over a 5-7 d period in nearly all years. We began searching 38 randomly located circular plots (50 m radius) after [approximately equal to]10% of nests had been initiated. These plots were searched every 4 d until egg laying was complete. All new eggs encountered on a nest visit were individually marked and recorded, and we recorded any marked eggs that were missing or obviously preyed on since the previous visit. We also measured the longest and shortest axis of each egg in the nest to the nearest 0.1 mm using dial calipers.
During incubation and hatching, we attempted to visit all nests in the colony to locate nests of color-banded adults. We marked and measured all eggs in nests of marked adults in the same manner as for nests on randomly located plots. Because many of these nests were not located until near their hatch date, we could have underestimated clutch size for these nests by an average of 0.2 eggs because of undetected predation of eggs that occurred before the nests were found (Flint et al. 1995). We estimated volume of individual eggs as a surrogate for mass using Hoyt's (1979) equation fit to a sample of brant eggs for which both volume and axis length were measured (Flint and Sedinger 1992). Total clutch volume was the sum of volumes of individual eggs. If it was not possible to measure all eggs in a clutch (e.g., if a portion of the clutch had already hatched), we assumed that unmeasured eggs had a volume equal to the average volume of measured eggs in that clutch. This introduced little error because, on average, egg volumes varied by [less than]5% within clutches (Flint and Sedinger 1992). Linear measures explained 92% of variation in egg volume (Flint and Sedinger 1992) and were more precise than weighing eggs in windy, wet field conditions on the Y-K Delta. Use of volume to index mass assumes that density of fresh eggs was constant across clutches. Egg constituents vary isometrically with egg size in geese (Ankney 1980). Furthermore, egg mass varies with developmental state of the egg (Drent 1970, Rahn and Ar 1974), and it was not feasible to measure most eggs when newly laid. Therefore, we believe linear measures of eggs provided the most precise index of egg volume (and mass) available to us under our field conditions.
We visited all nests of marked brant, including those on plots, on alternate days during hatching to determine hatch date. During and following hatching of nests, we observed brant broods from blinds on top of 3-5 m tall observation towers placed throughout the colony and brood-rearing areas. All adults observed were examined for presence of plastic tarsus bands. Band codes and number of goslings accompanying marked adults were recorded for all marked adults each time they were observed.
Each year since 1986, during adult remigial molt, we captured brant in corral traps. Banding "drives" varied in size from [less than]100 to [greater than]2000 individuals. We banded in major brood-rearing areas (determined by radio-telemetry, P. L. Flint, personal observation) accessible to us by boat. Therefore, we recaptured brant in the same areas where they were originally marked. No banding was conducted during rain to avoid hypothermia in goslings. Captured brant were fitted with plastic bands containing unique alphanumeric codes and standard U.S. Fish and Wildlife Service metal bands. Previously banded adults were recorded. We weighed ([+ or -]5 g for goslings, [+ or -]10 g for adults) and measured ([+ or -]0.1 mm) culmen and tarsus of all previously marked brant. A sample of newly marked adults and goslings was also weighed and measured. To assess mass dynamics of females following hatch, we trapped a sample of females on their nests when their clutches were hatching. These females were weighed and measured using our standard procedure. Females from this sample were recaptured during banding in late summer, and thus provided a sample for which we knew mass change over a known period following hatch. We recorded presence of a brood patch on all females, indicating that they nested in the current year. Recaptured females, as well as those observed during nesting or brood rearing, were scored as present on the colony in a given year.
Goslings were still growing during the banding period. We therefore adjusted measures of mass, culmen, and tarsus for variation in capture date by regressing these measures against number of days between peak of hatch and capture date in the year when the individual was captured. Residuals from these regressions provided estimates of the three measures for each individual relative to other individuals captured on the same date. Because hatch dates of goslings included in these analyses were unknown, a substantial proportion of the variation in these residuals is associated with variation in gosling age.
We used principal components analysis (BMDP 4M, Dixon 1985) to calculate the first principal component (PC1) score for each individual based on the correlation matrix of measurements. We calculated two sets of PC1 scores, the first based only on culmen and tarsus measurements, while the second also included mass. Because PC1 scores contained mass in some analyses, we refer to PC1 scores as body size rather than structural size. For goslings, we used residuals from the regressions of measurements against days following peak of hatch for principal components analysis. We used PC1 scores to index body size in all analyses.
We included body mass to calculate one set of gosling and adult PC1 scores despite the fact that adult mass varies by as much as 40% annually in Arctic geese (Ankney 1982, Alisauskas and Ankney 1990). Even so, of variables that we measured, body mass should be most closely correlated with overall body size and capacity to store nutrient reserves (Moser and Rusch 1988). Furthermore, adult female geese are at annual low body mass at hatching (Raveling 1979a, Ankney 1984), and average female mass increased relatively little between hatching and banding (see Results). Also, the small increment in mass above the annual minimum occurred after nesting and 10 mo before the next breeding season. Therefore, our PC1 scores did not contain nutrient reserves that were devoted to reproduction in the year of measurement. For goslings, mass is substantially more sensitive to nutrient intake than are either culmen or tarsus length. For example, in two studies, mass of goslings (controlled for age) varied by 15% among areas or years, whereas tarsus varied by 4 to 9% and culmen varied [less than]2% (Cooch et al 1991b, Aubin et al. 1993). Thus, mass and, consequently, gosling PC1 scores containing mass provide a greater range of variation against which to compare life history traits. Furthermore, it is unlikely that mass of goslings contained nutrient reserves because growing birds turnover 20 to 80% of muscle protein daily (MacDonald and Swick 1981, Lauterio et al. 1986, Tomas et al. 1991). Rapid turnover of muscle protein results in the complete replacement of body protein by newly synthesized protein every few days during growth. Lipid did not likely contribute substantially to variation in gosling mass, because lipid represents a small and relatively constant proportion of gosling mass after the first few days of life (Sedinger 1986). Finally, the substantial mass to be gained by the average gosling, between our measurements and fledging ([approximately equal to]400 g), further reduces the likelihood that mass of goslings contained nutrient reserves.
We restrict analyses to females because they are strongly philopatric to their natal colony (Rohwer and Anderson 1988; M. S. Lindberg and J. S. Sedinger, personal observation), whereas dispersal by males substantially reduces number of males for which we have known histories. Sex was determined by cloacal examination (Owen 1980). To assess relative survival of goslings, we scored individuals as having survived their 1st yr if they were observed on the colony at least 1 yr after they were initially banded. Thus, survival analyses reported here are intended only to assess survival of goslings relative to other goslings and do not represent unbiased estimates of annual survival. The relationship between gosling size at banding in late summer (PC1 score) and survival until at least 1 yr was assessed using logistic regression with body size as the independent variable and hatch year as a categorical variable. Individuals who were resighted were scored as 1 and unresighted individuals were scored as 0 in the analysis.
For individual females that were known to have survived (i.e., observed at least 1 yr after hatching), we examined the relationship between their size as goslings and their likelihood of breeding as follows. Individual females were scored as having nested if they were flushed from a nest, observed with a brood during brood rearing, or captured during banding with a brood patch. Our assessment of breeding, therefore, included nesting, whether or not females were successful. We restricted analysis to females from the 1986 and 1987 cohorts, because only females from these two cohorts had at least two opportunities to nest within the years (1986-1990) considered in our analyses; most female geese do not begin nesting until 2 or 3 yr of age (Finney and Cooke 1978, Rockwell et al. 1983, J. S. Sedinger, personal observation). We restricted breeding records in this analysis to those in the 2nd or 3rd yr so that females from both cohorts had an opportunity to be detected as breeders in exactly 2 yr. We divided the sample of 108 females who had been measured as goslings and were resighted at least 1 yr later into three groups of approximately equal size based on their size as goslings. We then used [[Chi].sup.2] contingency table analysis to test the hypothesis that females of different size varied in their likelihood of breeding, given they had survived. We also performed an alternative analysis to examine the relationship between gosling size and later breeding propensity in which we used logistic regression to examine the relationship between breeding (breed = 1, not breed = 0) and PC1 scores.
We examined the relationship between gosling size and adult size by regressing body sizes of individuals recaptured at 1 yr of age or older against body sizes of the same individuals as goslings. To determine whether it was necessary to control for adult age in our analysis of the relationship between gosling and adult body size we used ANOVA to examine the relationship between mean size measures (mass, culmen, and tarsus) and age (1, 2, 3+ yr). Because mass is potentially seasonally dynamic, we used analysis of covariance (ANCOVA), with age in years as a fixed factor and days following peak of hatch as the covariate in analysis of mass and adult age. We also examined age-related variation in the PC1 scores of adults using one-way ANOVA with age (1, 2, 3+ yr) as a fixed factor.
We examined the relationship between body size of adults and their investment in eggs using ANCOVA. We used clutch volume as the dependent variable for this analysis, because nutrient reserves of female geese actually regulate mass (as indexed by volume) of eggs produced. It is important to distinguish between a physiological hypothesis of regulation of egg production by nutrient reserves, in which body size plays a role, and variation in the life history trait, clutch size. Clutch mass reflects nutrient investment in the clutch, which may be regulated by nutrient reserves. Clutch mass is clearly correlated with clutch size, but female geese vary substantially in egg size and failure to account for egg size in analyses of regulation of clutch size will fail to account for substantial variation in investment in the clutch. In both snow geese (Ankney and Bisset 1976) and brant (Flint and Sedinger 1992), females with the same total investment in eggs can produce different clutch sizes because of variation in egg size. Therefore, using clutch volume in our analysis controls for variation in egg size, which varies by 78% among female brant (Flint and Sedinger 1992). We used adult body size as the covariate and age as a fixed factor in these analyses. We controlled for female age in our analysis of the relationship between clutch size (and volume) and female size, because clutch size may increase with female age in geese (Rockwell et al. 1983, Forslund and Larsson 1992). Most females used in analyses of the relationship between adult female size and clutch size were not initially color-banded as goslings. They were not, therefore, of known age. For these females, we estimated their minimum age by assuming they were 2-yr-old when first captured with adult plumage. Uncertainty about female age likely reduced our power to detect a relationship between female age and clutch size (or volume). Our principal goal in this and other analyses described below, however, was to assess the relationship between female size and investment in a clutch. To the extent that estimated female age reduced unexplained variation in these analyses, inclusion of female age increased our power to detect a relationship between body size and clutch size or clutch volume. To directly assess the relationship between fecundity and adult size, we performed an alternative analysis (ANCOVA) using clutch size as the dependent variable and PC1 score and mean egg volume as covariares. Age was again a fixed factor in this analysis. We also examined the relationships between clutch volume (and clutch size) produced by adult females and size of the same individuals when they were goslings, by using ANCOVA, with clutch volume (or clutch size) as the dependent variable, gosling PC1 score as the covariate, and age as a fixed factor. This analysis provided a direct assessment of the relationship between early growth and subsequent fecundity.
TABLE 1. Factor score coefficients for the first principal component (based on culmen, tarsus, and mass) for goslings and adult Black Brant.
Adults[dagger] Measure Goslings(*) Adult size Clutch size
Culmen 0.393 0.484 0.438 Tarsus 0.398 0.480 0.497 Mass 0.415 0.436 0.482
* Gosling measures were adjusted for expected age (days) before principal components analysis.
[dagger] Principal components analysis was performed on two samples of adults, one used for comparison of gosling and adult size, and one used to examine the relationship between adult body size and clutch size.
We used Levene's test (Dixon 1985) to examine homogeneity of variance among treatment levels in ANCOVAs. We also examined residuals from relationships between dependent variables and covariates to check for homogeneity of variance with respect to covariates, because heterogeneity of variance can bias hypothesis tests (Neter et al. 1990). Hypothesis tests are robust to deviations from normality when sample sizes are relatively large (Neter et al. 1990).
Tarsus and culmen lengths, and body mass of goslings increased significantly as a function of time between peak of hatch and capture date ([ILLUSTRATION FOR FIGURE 1 OMITTED]; P [less than] 0.02 for all analyses). Culmen, tarsus and mass, adjusted for time of capture of goslings, all loaded positively on the first principal component (Table 1), which explained 77% of the variation in the original measurements. When only culmen and tarsus were included in the analysis, PC1 explained 80% of the variation in the original data. We performed principal components analyses on two different samples of adults, (1) those used to compare adult and gosling size, and (2) those used to examine the relationship between adult body size and clutch size. In both cases, measures of tarsus, culmen and mass loaded positively onto PC1 (Table 1), which explained 50 and 51% of the variation in the two samples, respectively. PC1 scores based only on culmen and tarsus explained 81% and 61% of the variation in the original samples of adults, respectively.
Logistic regression models relating probability of re-sighting goslings after 1 yr to PC1 score and year fit data for both sets of PC1 scores ([[Chi].sup.2] Goodness of Fit Test, P = 0.91 for PC1 scores excluding mass and P = 0.77 for PC1 scores containing mass). PC1 score was significantly related to resighting probability for PC1 scores excluding mass (P = 0.0054) and for PC1 scores including mass (P = 0.0013; [ILLUSTRATION FOR FIGURE 2 OMITTED]). Year was also significantly related to resighting rate (P [less than] 0.0001), which resulted from higher resighting rates for goslings from the 1986 and 1987 cohorts.
Larger goslings became larger adults ([ILLUSTRATION FOR FIGURE 3 OMITTED]; F = 15.27; df = 1, 37, P = 0.0004). Culmen, tarsus, and mass measurements did not vary with adult age (F [less than] 0.6; df: 2, 291, P [greater than] 0.5, all 3 measures), nor did PC1 scores (F = 1.11; df = 2, 127, P = 0.33), so we pooled all adults for PC calculations. Neither Levene's test nor examination of residuals indicated heterogeneity of variance in any of our analyses.
Larger adults laid clutches with larger volumes ([ILLUSTRATION FOR FIGURE 4 OMITTED]; F = 8.23; df = 1, 126, P = 0.005 based on PC1 scores containing mass and F = 4.72; df = 1, 126, P = 0.03 for PC1 scores excluding mass). This was primarily a result of variation in clutch size; when egg volume and body size were used as covariates, and age as the main effect (ANCOVA), clutch size was significantly related to body size (F = 6.32; df = 1, 125, P = 0.009, PC1 scores including mass, and F = 4.80; df = 1, 125, P = 0.03, PC1 scores excluding mass) but not egg volume (F = [less than or equal to] 0.06; df = 1, 128, P [greater than] 0.8, analyses based on both sets of PC1 scores). Clutch volume varied with age in the expected direction (older individuals produced clutches with larger volume), but the effect of age in this analysis was not significant (F = 2.16; df = 2, 125, P = 0.12). When PC1 scores were based only on culmen and tarsus measurements, individuals who were larger as goslings tended to produce clutches with larger volumes when they became adults (F = 2.93; df = 1, 28, P = 0.098). The relationship between gosling size and eventual clutch volume was stronger when mass was included in PC1 scores (F = 4.96; df = 1, 32, P = 0.033; [ILLUSTRATION FOR FIGURE 5 OMITTED]). The relationship between gosling size and eventual clutch size, controlled for egg size, was not significant (F = 2.50; df = 1, 27, P = 0.126) when PC1 scores excluded mass, but the effect of egg size was nearly significant (F = 3.33; df = 1, 27, P = 0.079). When PC1 scores included mass, gosling size was significantly related to eventual clutch size, controlled for egg size (F = 4.59; df = 1, 27, P = 0.04).
Nearly identical proportions of surviving females from the 1986 and 1987 cohorts were observed breeding; 71% and 72% of the large individuals in the two cohorts, respectively, and 50% and 53% of the small plus medium individuals in the two cohorts nested. We, therefore, pooled the two cohorts to increase the power of our analysis of the relationship between gosling size and future breeding propensity. In the 1986 and 1987 cohorts combined, the largest goslings were significantly more likely to breed ([[Chi].sup.2] = 9.64; df = 2, P = 0.008, based on PC1 scores containing mass) than were smaller goslings. Logistic regression of breeding (breed or not breed) on gosling PC1 scores also indicated a significant relationship (P = 0.04 for PC1 scores containing mass and P = 0.01 for PC1 scores excluding mass).
The correlation between adult body size (PC1 scores containing mass) and clutch volume could be spurious if females who nested earlier and laid larger clutches (Cooch 1961, Ryder 1972, Cooper 1978, Ely and Raveling 1984) also gained more mass between hatching and capture in late summer, compared to later nesting females who laid smaller clutches and had less time to gain mass between hatch and capture in late summer. To examine the possibility that mass gain between hatch and recapture in late summer was related to the length of the interval between these events, we regressed mass gain against the number of days between hatch and recapture. Mean body mass of females weighed at hatching ([Mathematical Expression Omitted]) and at banding ([Mathematical Expression Omitted]) differed by only 68 g, 6.7% of mean mass at hatching. Furthermore, for a sample of 18 females weighed both at hatch and at banding in the same year, the relationship between mass gain and number of days since they hatched their clutches was not significant (t = -1.50; df = 16, P [greater than] 0.15) and the slope (-14.3) was actually negative. Females with more time between the hatching of their clutches and capture at banding actually gained less mass than those with less time available. We conclude that mass dynamics following hatch did not confound the relationship between clutch volume and adult body size in our analysis.
Variation in gosling size
Growing domestic animals compensate for periods of food deprivation by increasing nutrient intake when restored to ad libidum rations (Wilson and Osbourn 1960). Consequently, domestic animals increase growth rates when nutrients are available following food shortage and rapidly return to a "normal" growth trajectory. Such "compensatory growth" is less likely in wild species because breeding is usually timed so that growth of young coincides with the seasonal peak of nutrient availability (Lack 1954, Millar 1977). In seasonal environments, growth is likely slowed and then terminated annually when nutrients become limiting. Inability to fully compensate for poor nutrition during growth has been reported numerous times in ungulates (Klein 1964, Skogland 1983, Clutton-Brock et al. 1988).
Gosling size in late summer is associated primarily with the date on which goslings hatch, which influences both age and growth rate (Sedinger and Flint 1991, Cooch et al. 1991a), and quality of habitat on which they are reared (Larsson and Forslund 1991), with additional influence of egg size (Sedinger and Flint 1991). In Black Brant [less than]10% of variation in gosling size was explained by brood membership, after controlling for hatch date and egg size (Sedinger and Flint 1991), suggesting that environmental and maternal factors are major determinants of gosling size in late summer. Declining growth rate of goslings with later hatch date in high latitude environments is related to deteriorating foraging conditions as the season progresses following hatch (Sedinger and Raveling 1986). Because growth rate is negatively correlated with hatch date, the latest hatching goslings are smallest because they are both younger and they grow more slowly than the earliest hatching goslings (Sedinger and Flint 1991). Declining growth rate precludes late hatching goslings from completely compensating for their shorter growing period. Our finding that smaller goslings become smaller adults is consistent with the hypothesis of incomplete compensation in growth rates and with observations in snow geese and Barnacle Geese (Cooch et al. 1991a, b, Larsson and Forslund 1991, 1992). When combined with the major role of environmental factors in gosling growth, the correlation between gosling and adult size is consistent with substantial regulation of adult body size by environment during the growth period (Larsson and Forslund 1991).
Early growth and life history traits
Smaller goslings survived their 1st yr less well than did larger goslings. We do not know when differential mortality occurred, but in Barnacle Geese, survival of small goslings during their first autumn migration was lower than for larger goslings (Owen and Black 1989). Brant migrate from the Y-K Delta to Izembek Lagoon at the tip of the Alaska Peninsula, and from there to Mexico (Einarsen 1965). These migrations are energetically costly (D. V. Derksen, personal communication) and smaller individuals may be at a disadvantage. We predict that a substantial portion of difference between small and larger goslings in early survival will be manifested during autumn migration in their 1st yr.
Size of goslings late in their first summer influenced the size individuals eventually achieved as adults, and in our study adult size was correlated with number and total volume of eggs laid by females. This relationship is further supported by our observation that gosling size was correlated with both number and volume of eggs eventually produced by the same individuals as adults. Our observations are consistent with the model, first proposed by Ankney and MacInnes (1978), that, in geese, stored reserves of protein and lipid regulate a female's investment in her clutch and the maximum potential size of such nutrient reserves is determined by female body size.
Our findings differ from those of Cooch et al. (1992) who did not find a relationship between female body size and either clutch size or clutch volume. Because the snow goose is the species for which the model was originally developed (Ankney and MacInnes 1978), the difference between our results and those of Cooch et al. (1992) is of interest. The difference between our two studies could result from either biological differences between brant and snow geese nesting at La Perouse Bay or from methodological differences between studies. Cooch et al. (1992) included culmen, tarsus, head length, but not body mass in development of PC1 scores because of the variable nature of body mass in snow geese (Alisauskas and Ankney 1990). Because minimum lean body mass is most likely to be highly correlated with body size (Moser and Rusch 1988), exclusion of mass from PC1 scores may have reduced the correlation between PC1 score and body size in Cooch et al.'s (1992) study relative to ours. Finally, in most analyses of the relationship between body size in Lesser Snow Geese, egg size was not controlled for (Davies et al. 1988, Cooch et al. 1992), thereby leaving a substantial proportion of variation in clutch investment unaccounted for. In recent analyses (E. G. Cooch, personal communication), controlling for egg size produced a positive correlation between body size and clutch size for snow geese in 8 of 9 yr.
The analyses of Cooch et al. (1992) are based on snow geese nesting at La Perouse Bay, a relatively low latitude colony compared to the principal historic range of this population (Bellrose 1980). As Cooch et al. (1992) point out, nutrient dynamics and body size-clutch size relationships for snow geese at La Perouse Bay may differ substantially from those for geese nesting at higher latitudes. We, therefore, believe that recent analyses of the relationship between clutch size (or volume) and body size in snow geese are inconclusive.
A positive correlation between juvenile size and survival as well as between adult body size and reproductive investment has been observed in large-bodied mammals (Clutton-Brock et al. 1988). Increased investment by large-bodied mammals may either be expressed as increased fecundity or breeding frequency (Gaillard et al. 1992, Cameron et al. 1993) or increased investment in individual offspring (Clutton-Brock et al. 1988, Kojola 1993). We expect the relationship between body size and reproductive investment to be strongest in large birds and mammals that rely at least partially on nutrient reserves for breeding.
Evolution of life history traits
Numerous studies have detected significant positive heritabilities for life history traits of birds, including body size, egg size, and clutch size (reviewed in Boag and VanNoordwijk 1987, Lessels et al. 1989). James (1983) and Rhymer (1992) showed substantial geographic variation in environmental effects related to growth and clutch size. These studies call into question the ability of fostering studies at the local scale to control for environmental effects completely. Larsson and Forslund (1991) observed substantial variation in growth rate and final adult size for goslings of Barnacle Geese reared on areas only 7 km apart.
Hatch date is an important determinant of gosling size in late summer, not only because earlier hatching goslings are older, but also because earlier hatching goslings grow more rapidly than those hatching later (Sedinger and Flint 1991, Cooch et al. 1991a). A negative correlation between fledging size and hatch date also occurred in small passerines with altricial young (Alatalo and Lundberg 1986, Price 1991), which was attributed to declining food abundance later in the breeding season. In passerines, however, adult size was not negatively correlated with their breeding date (Alatalo and Lundberg 1986, Price 1991); hatch date did not, therefore, contribute substantially to common environment effects in heritability estimates. In geese, however, smaller females tended to nest later (Cooch et al. 1991). Because body size in late summer in turn influences other traits, such as adult body size and clutch size, and mothers and daughters have similar nesting dates (Findlay and Cooke 1982), our data suggest that hatch date is an important component of common environment that must be controlled for to avoid overestimating heritability of life history traits in geese. This may be especially true for clutch size in geese, which is relatively weakly heritable in these birds (Findlay and Cooke 1987).
Larger goslings survived at a higher rate and were larger and more fecund as adults. These fitness advantages accruing to goslings that were larger at fledging, combined with seasonal declines in growth rate (Sedinger and Flint 1991, Cooch et al. 1991a) and the shorter growing season available to later-hatching goslings should favor the earliest possible nesting by Arctic breeding geese. Most populations of geese nest as early as secure nest sites become available (Barry 1962, Ryder 1972, Eisenhauer and Kirkpatrick 1977), and in some colonial species thousands of nests are initiated within a 10-d period (e.g., Findlay and Cooke 1982). Substantial variation in fitness is associated with this apparently minor variation in nesting date, and it is therefore necessary to explain the remaining variation in breeding dates of geese.
Drent and Daan (1980) and Daan et al. (1990) proposed that variation in breeding dates in birds was maintained by variation in individual quality. Individuals in poor condition, or occupying poor quality territories, may delay breeding to increase size of nutrient reserves or allow food to increase seasonally on their territories. Decline in size of breeding female geese with advancing nesting date (Cooch et al. 1991a) is consistent with this hypothesis. A delay in breeding should therefore enable these individuals to produce, or feed more young than they otherwise would have. Increased fecundity associated with delayed breeding is counterbalanced, however, by lower recruitment of young from late nests. Under this hypothesis individuals optimize their reproductive fitness by breeding at a time that maximizes recruitment of their offspring. Individuals in good condition before breeding (or who occupy good quality territories) breed earlier and produce more offspring, whereas individuals in poor condition delay breeding and produce fewer offspring, producing the ubiquitous seasonal decline in clutch size. Our data and those for other species (Cooch et al. 1991) suggest that costs of delayed breeding include lower future fecundity of offspring as well as lower survival. The importance of early growth in life histories of other species (Skogland 1983, Clutton-Brock et al. 1988, Sinervo 1990) suggests that hypotheses similar to that of Daan et al. (1990) should be considered for other species.
This research was supported by the Alaska Fish and Wildlife Research Center and Migratory Bird Management, Region 7, U.S. Fish and Wildlife Service. N. Chelgren assisted with field work and analysis of data. We thank C. D. Ankney, E.G. Cooch, J. Fox, and K. Schwaegerle for commenting on earlier drafts of the manuscript.
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|Author:||Sedinger, James S.; Flint, Paul L.; Lindberg, Mark S.|
|Date:||Dec 1, 1995|
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