Mate fidelity improves survival and breeding propensity of a long-lived bird.

Evolutionary and behavioural ecologists have long been interested in factors shaping the variation in mating behaviour observed in nature. Although much of the research on this topic has focused on the consequences of mate choice and mate change on annual reproductive success, studies of a potential positive link between mate fidelity and adult demographic rates have been comparatively rare. This is particularly true for long-lived birds with multi-year, socially monogamous pair bonds. We used a 26-year capture-mark-recapture dataset of 3,330 black brent Branta bernicla nigricans to test whether breeding with a familiar mate improved future breeding propensity and survival. We predicted that experienced breeders nesting with a new partner would have rates of survival similar to familiar pairs because long-lived species avoid jeopardizing survival since their lifetime fitness is sensitive to this vital rate. In contrast, we expected that any costs of breeding with a new partner would be paid through skipping the subsequent breeding attempt. We found that unfamiliar pairs had lower subsequent breeding propensity than faithful partners. However, contrary to our expectations, individuals breeding with a new mate also suffered reduced survival. These results add to a small number of studies indicating that a positive relationship between mate retention and adult demographic rates may exist in a diverse array of avian species. Given these results, researchers should consider costs of mate change that extend beyond within-season reproductive success to fully understand the potential adaptive basis for perennial social monogamy. We caution that if mate retention enhances survival prospects, improvements in annual reproductive success with pair-bond length could be a secondary factor favouring perennial social monogamy, particularly in species with slower life-history strategies. Furthermore, some cases where annual reproductive success does not improve with pair-bond duration, yet multi-year pair bonds are common, could be explained by benefits afforded by mate fidelity to adult vital rates.

Long-term research and hierarchical models reveal consistent fitness costs of being the last egg in a clutch.

Maintenance of phenotypic heterogeneity in the face of strong selection is an important component of evolutionary ecology, as are the consequences of such heterogeneity. Organisms may experience diminishing returns of increased reproductive allocation as clutch or litter size increases, affecting current and residual reproductive success. Given existing uncertainty regarding trade-offs between the quantity and quality of offspring, we sought to examine the potential for diminishing returns on increased reproductive allocation in a long-lived species of goose, with a particular emphasis on the effect of position in the laying sequence on offspring quality. To better understand the effects of maternal allocation on offspring survival and growth, we estimated the effects of egg size, timing of breeding, inter- and intra-annual variation, and position in the laying sequence on gosling survival and growth rates of black brant Branta bernicla nigricans breeding in western Alaska from 1987 to 2007. We found that gosling growth rates and survival decreased with position in the laying sequence, regardless of clutch size. Mean egg volume of the clutch a gosling originated from had a positive effect on gosling survival (ß = 0.095, 95% CRI: 0.024, 0.165) and gosling growth rates (ß = 0.626, 95% CRI: 0.469, 0.738). Gosling survival (ß = -0.146, 95% CRI: -0.214, -0.079) and growth rates (ß = -1.286, 95% CRI: -1.435, -1.132) were negatively related to hatching date. These findings indicate substantial heterogeneity in offspring quality associated with their position in the laying sequence. They also potentially suggest a trade-off mechanism for females whose total reproductive investment is governed by pre-breeding state.

Estimating correlations among demographic parameters in population models.

Estimating correlations among demographic parameters is critical to understanding population dynamics and life-history evolution, where correlations among parameters can inform our understanding of life-history trade-offs, result in effective applied conservation actions, and shed light on evolutionary ecology. The most common approaches rely on the multivariate normal distribution, and its conjugate inverse Wishart prior distribution. However, the inverse Wishart prior for the covariance matrix of multivariate normal distributions has a strong influence on posterior distributions. As an alternative to the inverse Wishart distribution, we individually parameterize the covariance matrix of a multivariate normal distribution to accurately estimate variances (σ 2) of, and process correlations (ρ) between, demographic parameters. We evaluate this approach using simulated capture-mark-recapture data. We then use this method to examine process correlations between adult and juvenile survival of black brent geese marked on the Yukon-Kuskokwim River Delta, Alaska (1988-2014). Our parameterization consistently outperformed the conjugate inverse Wishart prior for simulated data, where the means of posterior distributions estimated using an inverse Wishart prior were substantially different from the values used to simulate the data. Brent adult and juvenile annual apparent survival rates were strongly positively correlated (ρ = 0.563, 95% CRI 0.181-0.823), suggesting that habitat conditions have significant effects on both adult and juvenile survival. We provide robust simulation tools, and our methods can readily be expanded for use in other capture-recapture or capture-recovery frameworks. Further, our work reveals limits on the utility of these approaches when study duration or sample sizes are small.

Brood Size Affects Future Reproduction in a Long-Lived Bird with Precocial Young.

Estimation of trade-offs between current reproduction and future survival and fecundity of long-lived vertebrates is essential to understanding factors that shape optimal reproductive investment. Black brant geese (Branta bernicla nigricans) fledge more goslings, on average, when their broods are experimentally enlarged to be greater than the most common clutch size of four eggs. Thus, we hypothesized that the lesser frequency of brant clutches exceeding four eggs results, at least partially, from a future reduction in survival, breeding probability, or clutch size for females tending larger broods. We used an 8-year mark-recapture data set (Barker robust design) with 5 years of clutch and brood manipulations to estimate long-term consequences of reproductive decisions in brant. We did not find evidence of a trade-off between reproductive effort and true survival or future initiation date and clutch size. Rather, future breeding probability was maximized ( 0.92±0.03 [SE]) for manipulated females tending broods of four goslings and was lower for females tending smaller (one gosling; 0.63±0.09 [SE]) or larger (seven goslings; 0.52±0.15 [SE]) broods. Our results suggest that demographic trade-offs for female brant tending large broods may reduce the fitness value of clutches larger than four and, therefore, contribute to the paucity of larger clutches. The lack of a trade-off between reproductive effort and survival provides evidence that survival, to which fitness is most sensitive in long-lived animals, is buffered against temporal variation in brant.

Ultimate regulation of fecundity in species with precocial young: declining marginal value of offspring with increasing brood size does not explain maximal clutch size in Black Brent geese.

Lack 18:125-128 (1967) proposed that clutch size in precocial species was regulated by nutrients available to females during breeding. Drent and Daan 68:225-252 (1980) proposed the individual optimization hypothesis, whereby individual state determines the optimal combination of breeding date and clutch size. Neither hypothesis accounts for variation in nutrients among females at the end of egg laying, strong right truncations in clutch size distributions, or the fact that many species with precocial young are determinate layers. One solution is that there is a maximum clutch size, above which the number of fledged young declines. We manipulated brood size in Black Brent geese to decouple brood size from maternal quality and produce broods larger than the natural maximum. We recaptured marked goslings to assess variation in prefledging survival as a function of brood size and we estimated relative prefledging survival of goslings using a Bayesian hierarchical approach. We considered effects of natural clutch size, brood size and their interaction on probability that we captured goslings at about 4 weeks of age. Prefledging survival declined with increasing brood size ([Formula: see text] = -0.53; 95% CI -0.91 to -0.16), while laid clutch size had little influence on prefledging survival ([Formula: see text] = -0.04; 95% CI -0.42 to 0.32). Despite declining per capita survival with increasing brood size, the most productive brood size was six goslings, which is greater than the typical maximum clutch size of five. Thus, reduced survival in large broods, by itself, is not the sole mechanism that limits maximum clutch size. We suggest elsewhere that incubation limitation and lower residual reproductive value for females tending larger broods may be other mechanisms limiting maximal clutch size in brent.