Christopher A. Nicolai

Black Brant Harvest, Density Dependence, and Survival: A Record of Population Dynamics

Abstract We analyzed 53 years of banding and band recovery data along with estimates of harvest and population size to assess the role of harvest and density dependence in survival patterns and population dynamics of black brant (Branta bernicla nigricans) over the period 1950–2003. The black brant population has declined steadily since complete annual surveys began in 1960, so the role of harvest in the dynamics of this population is of considerable interest. We used Brownie models implemented in Program MARK to analyze banding data. In some models, we incorporated estimated sport harvest to test hypotheses about the role of harvest in survival. We also examined the hypothesis of density-dependent regulation of mortality by incorporating estimates of population size as a covariate into models of survival. For a shorter period (1985–2003), we also assessed hypotheses about the role of subsistence harvest and predation as sources of mortality. The best supported model of variation in survival and band recovery allowed survival rates to vary among 2 age classes (juv, second-yr plus ad brant) and the 2 sexes. We constrained survival probabilities to be constant within decades but allowed them to vary among decades. We also constrained band recovery rates to be constant within decades and to vary in parallel among age and sex classes. We were limited to decade-specific estimates of survival and band recovery rates because some years before 1984 lacked any banding, and banding in some other years was sparse. A competitive model constrained survival estimates to be the same for males and females. No model containing harvest or population size was competitive with models lacking these covariates (relative quasi-Akaikes Information Criterion adjusted for small sample size [ΔQAICc] > 13). In the best supported model, band recovery rates declined from 0.038 ± 0.0028 (F) and 0.040 ± 0.0031 (M) to 0.007 ± 0.0007 (F) and 0.007 ± 0.0007 (M) between the 1950s and 2000s, a clear indication that harvest rates declined over this period. Survival rates increased from 0.70 ± 0.02 and 0.71 ± 0.02 for adult males and females, respectively, in the 1950s to 0.88 ± 0.009 and 0.88 ± 0.01 for males and females, respectively, in the 1990s. Survival rates in the 1990s were among the highest estimated for brant and did not increase in the 2000s with additional reductions in sport harvest. For the shorter data set from 1985 to 2003, models containing covariates for either sport or subsistence harvest were less competitive than models lacking these terms (ΔQAICc > 3). For the best model containing subsistence harvest, the estimate of β linking subsistence harvest to survival, although imprecisely estimated, was near zero (β = −0.04 ± 0.30), consistent with the hypothesis that subsistence harvest had little impact on survival during this period. We conclude that while harvest likely influenced survival and population dynamics in earlier decades, it is most likely that continued population decline at least since 1990 is a result of low recruitment.

Carryover effects associated with winter location affect fitness, social status, and population dynamics in a long-distance migrant.

We used observations of individually marked female black brant geese (Branta bernicla nigricans; brant) at three wintering lagoons on the Pacific coast of Baja California—Laguna San Ignacio (LSI), Laguna Ojo de Liebre (LOL), and Bahía San Quintín (BSQ)—and the Tutakoke River breeding colony in Alaska to assess hypotheses about carryover effects on breeding and distribution of individuals among wintering areas. We estimated transition probabilities from wintering locations to breeding and nonbreeding by using multistratum robust-design capture-mark-recapture models. We also examined the effect of breeding on migration to wintering areas to assess the hypothesis that individuals in family groups occupied higher-quality wintering locations. We used 4,538 unique female brant in our analysis of the relationship between winter location and breeding probability. All competitive models of breeding probability contained additive effects of wintering location and the 1997–1998 El Niño–Southern Oscillation (ENSO) event on probability of breeding. Probability of breeding in non-ENSO years was , , and for females wintering at BSQ, LOL, and LSI, respectively. After the 1997–1998 ENSO event, breeding probability was between 2% (BSQ) and 38% (LOL) lower than in other years. Individuals that bred had the highest probability of migrating the next fall to the wintering area producing the highest probability of breeding.

Demographic response to perturbations: the role of compensatory density dependence in a North American duck under variable harvest regulations and changing habitat

1. Most wild animal populations are subjected to many perturbations, including environmental forcing and anthropogenic mortality. How population size varies in response to these perturbations largely depends on life-history strategy and density regulation. 2. Using the mid-continent population of redhead Aythya americana (a North American diving duck), we investigated the population response to two major perturbations, changes in breeding habitat availability (number of ponds in the study landscape) and changes in harvest regulations directed at managing mortality patterns (bag limit). We used three types of data collected at the continental scale (capture-recovery, population surveys and age- and sex ratios in the harvest) and combined them into integrated population models to assess the interaction between density dependence and the effect of perturbations. 3. We observed a two-way interaction between the effects on fecundity of pond number and population density. Hatch-year female survival was also density dependent. Matrix modelling showed that population booms could occur after especially wet years. However, the effect of moderate variation in pond number was generally offset by density dependence the following year. 4. Mortality patterns were insensitive to changes in harvest regulations and, in males at least, insensitive to density dependence as well. We discuss potential mechanisms for compensation of hunting mortality as well as possible confounding factors. 5. Our results illustrate the interplay of density dependence and environmental variation both shaping population dynamics in a harvested species, which could be generalized to help guide the dual management of habitat and harvest regulations.

ANNUAL SURVIVAL AND SITE FIDELITY OF NORTHERN PINTAILS BANDED ON THE YUKON-KUSKOKWIM DELTA, ALASKA

Abstract We banded northern pintails (Anas acuta; n = 13,645) at a single site on the Yukon-Kuskokwim Delta (YKD), Alaska, USA, from 1990 to 2001. We used recaptures from our site in combination with hunter recoveries to model annual survival, recovery rates, and fidelity to our capture location. Most recoveries (>90%) occurred in the Pacific Flyway with 64% reported from Californias Central Valley. Our top candidate models allowed survival to vary by sex but not by age or year. Estimated annual survival was 77.6% (95% CI: 73.9–81.0%) for males and 60.2% (95% CI: 53.2–67.0%) for females. Reporting rates varied by age, sex, and year; estimates for adult males exceeded those for adult females by 3.5 times. Within sexes, reporting rates of hatch-year pintails exceeded those of adults. Estimated recovery rates were considerably lower than those estimated during the 1950s–1970s for winter banded pintails (Hestbeck 1993b), but there were no differences in survival rates. This suggests that changes in harvest regulations may not have influenced annual survival in this population. The propensity of banded pintails to return to our capture site (fidelity rate) varied between sexes and was positively correlated with water conditions in prairie Canada. Our estimates of fidelity rates varied from 77.4% to 87.2% for males and 89.8% to 94.3% for females. Our fidelity estimates suggest that some level of subpopulation structuring may exist for northern pintails. Additionally, our estimates of fidelity support previous observations of northern pintails overflying poor wetland habitat conditions on the Canadian prairies.

Life-history strategies of North American elk: trade-offs associated with reproduction and survival

Abstract The principle of energy allocation states that individuals should attempt to maximize fitness by allocating resources optimally among growth, maintenance, and reproduction. Such allocation may result in trade-offs between survival and reproduction, or between current and future reproduction. We used a marked population of North American elk (Cervus elaphus) to determine how energetic costs of reproduction in the current year affect survival and reproduction in the subsequent year. Using a multistate mark–recapture model we examined the influence of individual and environmental variation on trade-offs between these 2 life-history traits. We observed no difference in survival probabilities between pregnant and nonpregnant individuals or as a function of recruiting an offspring. Nonetheless, there was a negative effect of recruiting an offspring in the current year on becoming pregnant the following year. Increased body condition, and higher precipitation, contributed to greater probabilities of becoming pregnant in a particular year regardless of reproductive state and previous recruitment. Costs associated with reproduction led to a reduced probability of future reproduction rather than a reduction in survival. These findings are consistent with risk-sensitive reproductive allocation, where adult survival is maintained through variation in reproductive effort resulting in high and stable adult survival and more-variable reproduction.

Are there trade‐offs between pre‐ and post‐fledging survival in black brent geese?

1. The growth period is an important determinant of fitness later in life through its effects on first-year survival and future reproduction. Choices by adult females about where to rear their offspring strongly affect growth rates and offspring fitness in geese. 2. Individual female black brent (Branta bernicla nigricans) tend to raise their broods in the same areas each year, and these areas are consistently ranked with respect to growth rates of goslings. Therefore, some females consistently rear their broods on areas resulting in lower post-fledging fitness. 3. We explore the potential that growth rates of offspring (and associated fitness consequences) are traded off against other vital rates influencing fitness of either adult females or goslings. Growth of goslings primarily influences fitness after fledging, so one hypothesis is that survival before fledging, which is influenced by predation, is traded off against growth rates and post-fledging survival. 4. We estimated pre-fledging and post-fledging survival for goslings reared on areas used by broods from the Tutakoke River black brent colony. We examined recaptures, recoveries by hunters and resightings of brent marked as goslings with webtags and standard leg rings. These data were analyzed using capture-mark-recapture models in program mark to derive separate estimates of pre- and post-fledging survival for 18 cohorts (1987-2004) of black brent goslings across seven brood rearing areas (BRAs). 5. Estimates of pre-fledging survival probability varied from 0·00 ± 0·00 (mean ± 95% confidence interval) to 0·92 ± 0·1; and estimates of post-fledging survival probability varied from 0·00 ± 0·00 to 1·00 ± 0·08. Substantial variation existed both among BRAs and years but post-fledging survival declined substantially during the study. 6. Pre- and post-fledging survival were positively correlated, exhibiting a quadratic relationship (ß(post-fledging survival)  = 1·00 (±0·47)x-0·83 (±0·480)x(2) , where x = pre-fledging survival). Therefore, we did not find a trade-off between pre- and post-fledging survival in black brent goslings across BRAs, suggesting that factors other than foraging conditions and predation on goslings must influence selection of BRAs.

Trade-offs between offspring fitness and future reproduction of adult female black brent.

1. Successful reproduction requires numerous decisions, and some of which may require trade-offs between current and future reproduction. We studied effects of choice of foraging patches on gosling growth and future breeding by mothers in black brent (Branta bernicla nigricans) geese. 2. Specific foraging areas consistently produced high-quality goslings over 21 years. We found a consistent ranking of gosling mass, corrected for age, across brood rearing areas (BRAs) and years [Akaike model weights, Σw(i)  = 1·00 for models including additive effects of BRA and year]. Growth of goslings largely determines their future fitness, so areas where goslings grew most rapidly also produced goslings with the highest mean fitness. 3. We used a multistate robust design capture-mark-recapture approach to estimate the probability of transitioning from a breeding state to a non-breeding (unobservable) state as a function of quality of BRA. 4. In the best supported model, transition from a breeding state to a non-breeding state was positively related to gosling growth rates across BRAs. Thus, future reproduction was lower for females using BRAs that produced higher-quality goslings. Our results are consistent with trade-offs by individual brent between fitness of their current offspring and their own reproductive value.

Spatial variation in life‐history trade‐offs results in an ideal free distribution in Black Brant Geese

Ideal free distribution theory predicts that individuals distribute themselves so fitness is equal among patches. In this paper we evaluate all components of adult fitness to assess the hypothesis that individuals distribute themselves among seven brood-rearing areas so that trade-offs among different life history traits result in equal mean fitness among individuals using different areas. We used estimates of vital rates (clutch size, nest survival, pre-fledging survival, post-fledging survival, juvenile survival, and breeding probability) to estimate brood-rearing-area-specific per capita recruitment rates and survival for adult females. We summed brood-rearing-area-specific per capita recruitment and adult survival to calculate brood-rearing-area-specific estimates of lambda. We found little variation in lambda among brood-rearing areas and lifetime fitness implications of changing brood-rearing area were negligible (< 1% brood-rearing area mean fitness). We conclude that adult female Black Brant distribute themselves in an ideal free manner, resulting in equal fitness among females using these areas.

Intercolony Variation in Growth of Black Brant Goslings on the Yukon-Kuskokwim Delta, Alaska

ABSTRACT Recent declines in black brant (Branta bernicla nigricans) are likely the result of low recruitment. In geese, recruitment is strongly affected by habitat conditions experienced by broods because gosling growth rates are indicative of forage conditions during brood rearing and strongly influence future survival and productivity. In 2006–2008, we studied gosling growth at 3 of the 4 major colonies on the YukonKuskokwim Delta, Alaska. Estimates of age-adjusted gosling mass at the 2 southern colonies (approx. 30% of the world population of breeding black brack) was low (gosling mass at 30.5 days ranged 346.7 ± 42.5 g to 627.1 ± 15.9 g) in comparison to a third colony (gosling mass at 30.5 days ranged 640.0 ± 8.3 g to 821.6 ± 13.6 g) and to most previous estimates of age-adjusted mass of brant goslings. Thus, our results are consistent with the hypothesis that poor gosling growth is negatively influencing the brant population. There are 2 non-mutually exclusive explanations for the apparent growth rates we observed. First, the population decline may have been caused by density-independent factors and habitat capacity has declined along with the population as a consequence of the unique foraging feedback between brant and their grazing habitats. Alternatively, a reduction in habitat capacity, as a result of changes to the grazing system, may have negatively influenced gosling growth, which is contributing to the overall long-term population decline. We found support for both explanations. For colonies over habitat capacity we recommend management to enhance foraging habitat, whereas for colonies below habitat capacity we recommend management to increase nesting productivity.

Differences in growth of Black Brant goslings between a major breeding colony and outlying breeding aggregations

Abstract We compared growth rates of Black Brant (Branta bernicla nigricans) goslings from two dispersed nesting aggregations to those from the large Tutakoke River Colony on the Yukon-Kuskokwim Delta, Alaska during summers, 1999 and 2000. Approximately 20% of the Black Brant population on the Yukon-Kuskokwim Delta, Alaska nests outside of the four major colonies. Dispersal to these outlying breeding locations is hypothesized as a mechanism by which individuals reduce the negative effects of density dependence associated with major colonies. Growth rates of goslings varied among brood rearing areas both associated with dispersed nesting aggregations (n = 4) and associated with the Tutakoke River Colony (n = 7). Mean mass of goslings, adjusted for age, from brood rearing areas associated with dispersed nesting aggregations ranked sixth, eighth, and ninth of nine brood rearing areas sampled in 1999, and sixth and ninth of nine brood rearing areas sampled in 2000. Mean age-adjusted mass of goslings with the largest mass were 198 and 139 g lighter, respectively, from brood rearing areas associated with a dispersed nesting aggregation than those from brood rearing areas associated with the Tutakoke River colony in 1999 and 2000. Our findings suggest that goslings from dispersed nesting aggregations we sampled are unlikely to have an advantage over goslings from a major colony with respect to survival, adult body size, and recruitment.