Factors influencing the abundance of wintering western snowy plovers at Crown Memorial State Beach

Survival during the nonbreeding season, when mortality from food shortages and raptor predation is highest, influences shorebird population growth. These selection pressures, as well as anthropogenic influences, can shape wintering shorebird habitat use patterns. The western snowy plover (Charadrius alexandrinus nivosus) is a small shorebird that uses sand-spits, dune-backed beaches, open areas around estuaries for foraging and roosting. The Pacific Coast population of western snowy plovers is listed as a federally threatened species and a California Species of Special Concern. Previous studies suggest humans, dogs and corvids are sources of disturbance to plovers on public beaches. During 2014 to 2019, these disturbance factors were examined at Robert W. Crown Memorial State Beach in Alameda, California. In decreasing order of impact, the beach using public, corvids, and dogs were found to be the major stressors to over wintering plovers. Both the public and corvids respectively, resulted in disturbance and avoidance behaviors by plovers nearly 40% of the time. In 2015, the District created the Plover Protection Zone (PPZ) by installing symbolic fencing, signage, and establishing a volunteer team to monitor plovers and educate the public. In 2016, the potential prey abundance within the plover protection zone and areas directly north and south were examined using core samples and sticky traps. Statistical analysis showed a significant difference in the amount of macro-invertebrate prey available in the area used by the plovers as compared to other locations. Habitat choice and prey availability are vital to wintering shorebird. During this study, the wintering population of western snowy plovers increased from six to over 54 individuals.

Egg size variation in a long-lived polyandrous shorebird in the context of senescence and breeding phenology

Anisogamy is a central component of sex role evolution, however, the effect of female-female mating competition on egg size variation in polyandrous species is unclear. Moreover, egg size may also be shaped by age-dependent trade-offs between reproductive investments and somatic maintenance that are responsible for senescence. Here we investigate how mating behaviour and senescence are associated with egg size variation in female snowy plovers (Charadrius nivosus). Snowy plovers are long-lived shorebirds (longevity record: 20 years) that often produce several nests each year, with females either sequentially changing partners between breeding attempts or remaining monogamous between attempts. We examined how age, seasonality, body size, and mating behaviour relate to within- and between-female variation in egg volume using repeated measures collected over a 15-year period. We found no evidence of reproductive senescence in egg volume in snowy plover females. Rather, egg volume, polyandry, and re-nesting were strongly linked to breeding phenology: early breeding females had a higher likelihood of being polyandrous or replacing failed clutches, yet these individuals laid smaller eggs likely due to physiological limitations associated with the early season. Older individuals and local recruits secured the earliest breeding opportunities in the season suggesting that prior experience could give an edge in the female-female competition for mates. Larger females laid the largest eggs, as expected, but there was no relationship between body size and lay date - implying that size may not provide an advantage in female-female competition. Our findings highlight the existence of several direct and indirect constraints on female reproductive investment that likely shape individual variation in lifetime reproductive success. Future research investigating reproductive senescence of wild populations should consider mating system dynamics when examining variation in reproductive investment.

Reply to ‘Commentary CeutaOPEN, individual-based field observations of breeding snowy plovers Charadrius nivosus’

In their commentary, Székely et al. (SA) question our publication of a long-term dataset on threatened Snowy Plovers Charadrius nivosus breeding at Bahía de Ceuta, Mexico. The commentary was submitted to Scientific Data but rejected by the editors, who based on the reports of two reviewers concluded that our data descriptor had reasonably acknowledged prior work and was written scientifically ethical and suitable given the limitations of the publication’s format. Most importantly, we note that SA do not raise any fault with the main content of the paper – the data and their accompanying description – providing further assurance that these data can be analysed or reanalysed with confidence. However, SA write that the data descriptor has “omissions and inaccurate statements, and fails to acknowledge key contributions to their project”. We reject these criticisms and maintain that i) we were within our full rights to publish these data and, ii) the data descriptor is accurate.

Using Decision Science for Monitoring Threatened Western Snowy Plovers to Inform Recovery

Western Snowy Plovers (Charadrius nivosus nivosus) are federally listed under the US Endangered Species Act as Threatened. They occur along the US Pacific coastline and are threatened by habitat loss and destruction and excessive levels of predation and human disturbance. Populations have been monitored since the 1970s for distribution, reproduction, and survival. Since the species was federally listed in 1993 and a recovery plan was approved under the US Fish and Wildlife Service in 2007, recovery actions have resulted in growing populations with increased presence at breeding and wintering sites throughout their Pacific Coast range. This success has created logistical challenges related to monitoring a recovering species and a need for identifying and instituting the best monitoring approach given recovery goals, budgets, and the likelihood of monitoring success. We devised and implemented a structured decision analysis to evaluate nine alternative monitoring strategies. The analysis included inviting plover biologists involved in monitoring to score each strategy according to a suite of performance measures. Using multi-attribute utility theory, we combined scores across the performance measures for each monitoring strategy, and applied weighted utility values to show the implications of tradeoffs and find optimal decisions. We evaluated four scenarios for weighting the monitoring objectives and how risk attitude affects optimal decisions. This resulted in identifying six strategies that best meet recovery needs and were Pareto optimal for cost-effective monitoring. Results were presented to the US Fish and Wildlife Service, responsible for monitoring as well as for consideration to ensure consistent monitoring methods across the species’ range. Our use of structured decision-making can be applied to cases of other species once imperiled but now on the road to recovery.

Detectability and Abundance of Snowy Plovers at Salt Plains National Wildlife Refuge, Oklahoma

In the past two decades, Salt Plains National Wildlife Refuge has been increasingly recognized as important habitat for both breeding and migratory shorebirds. North American snowy plovers Charadrius nivosus in particular rely on the nearly 5,000 ha salt flat at Salt Plains National Wildlife Refuge, which thousands use as breeding and stopover habitat. Elsewhere on the Southern Great Plains, decadal declines up to 75% within snowy plover subpopulations have been documented and attributed to vegetation encroachment, increased rates of nest predation, and decreased availability of fresh surface water. Despite many attempts to estimate this species’ abundance across the continent, to date, no known attempt at distance sampling of snowy plovers has occurred. To address this paucity of data, we assessed feasibility of distance sampling methods to accurately estimate snowy plover abundance and detectability. Distance sampling surveys (2017-2018) indicated high detection probability (P = 0.80) and the population abundance estimate across the salt flat extrapolated to 3,307 individuals. The distance sampling population abundance estimate is lower than population abundance estimates determined by two previous studies within the last decade but far greater than 2,105 estimated for a study in 2006. Overall, distance sampling snowy plovers at Salt Plains National Wildlife Refuge proved to be an effective addition to pre-established survey protocols but further investigation is needed to compare accuracy and precision of methods used in this study, annual surveys conducted by Salt Plains National Wildlife Refuge, and other potential snowy plover surveys.

Allelic diversity and patterns of selection at the major histocompatibility complex class I and II loci in a threatened shorebird, the Snowy Plover (Charadrius nivosus)

Background Understanding the structure and variability of adaptive loci such as the major histocompatibility complex (MHC) genes is a primary research goal for evolutionary and conservation genetics. Typically, classical MHC genes show high polymorphism and are under strong balancing selection, as their products trigger the adaptive immune response in vertebrates. Here, we assess the allelic diversity and patterns of selection for MHC class I and class II loci in a threatened shorebird with highly flexible mating and parental care behaviour, the Snowy Plover (Charadrius nivosus) across its broad geographic range. Results We determined the allelic and nucleotide diversity for MHC class I and class II genes using samples of 250 individuals from eight breeding population of Snowy Plovers. We found 40 alleles at MHC class I and six alleles at MHC class II, with individuals carrying two to seven different alleles (mean 3.70) at MHC class I and up to two alleles (mean 1.45) at MHC class II. Diversity was higher in the peptide-binding region, which suggests balancing selection. The MHC class I locus showed stronger signatures of both positive and negative selection than the MHC class II locus. Most alleles were present in more than one population. If present, private alleles generally occurred at very low frequencies in each population, except for the private alleles of MHC class I in one island population (Puerto Rico, lineage tenuirostris). Conclusion Snowy Plovers exhibited an intermediate level of diversity at the MHC, similar to that reported in other Charadriiformes. The differences found in the patterns of selection between the class I and II loci are consistent with the hypothesis that different mechanisms shape the sequence evolution of MHC class I and class II genes. The rarity of private alleles across populations is consistent with high natal and breeding dispersal and the low genetic structure previously observed at neutral genetic markers in this species.

Polyandry and senescence shape egg size variation in a precocial bird

Identifying age-dependent trade-offs between reproductive effort and survival in wild organisms is central for understanding the evolutionary mechanisms of senescence. According to the disposable soma theory, early-life energy investments in reproduction compromise late-life investments in somatic maintenance – leading to senescence. Once thought to only be detectable in captive populations, senescence has recently been documented by several longitudinal studies of wild organisms. However, some reproductive traits that are used to quantify senescence may also be shaped by other age-dependent processes such as mating tactics. For instance, polygamy, which is often positively associated with age, may lead to reduced gamete size due to trade-offs between gamete quantity and quality. Here we investigate how mating behaviour and senescence are associated with reproductive trade-offs of female snowy plovers (Charadrius nivosus). Snowy plovers are long-lived shorebirds (longevity record: 19 years) that produce several nests each year, with females either being polyandrous and sequentially changing partners between breeding attempts or remaining monogamous between attempts. We examined how age, seasonality, and mating tactics affect within-female variation in egg volume using repeated measures collected over a 14-year period. Our results provide clear evidence for senescence in snowy plover females starting at three years of age. Furthermore, females laid smaller eggs in years when they were polyandrous compared to years when they were monogamous, with early- and late-season clutches having the smallest eggs. We suggest that individual female reproductive performance is regulated by flexible mating tactics and age- and season-dependent effects. Our findings highlight the existence of multiple trade-offs for female reproductive investments that likely shape individual variation in lifetime reproductive success. We encourage future research investigating reproductive senescence of wild populations to incorporate age-dependent reproductive investments in light of mating system dynamics.Impact SummaryWhy do organisms senesce at older ages? Life-history theory predicts that early life investments in reproduction compromise future investment opportunities for somatic repair, which leads to senescence. Earlier works assumed that senescence was only observable in captive populations due to the high degree of extrinsic mortality experienced by wild organisms. However, with the expansion of longitudinal studies collecting repeated measures from wild individuals, recent works have identified key insights into the selective processes driving patterns of senescence in nature. Here, we use a 14-year longitudinal mark-recapture dataset of snowy plovers (Charadrius nivosus) breeding in a wild subtropical population in western Mexico to investigate age-dependent trade-offs between female reproductive effort and performance. The snowy plover is a long-lived shorebird characterized by a flexible polyandrous mating system. This rare breeding behaviour represents a unique background for investigating senescence in light of between and within-individual variation in reproductive investment. Our study focused on age-dependent dynamics of egg volume – a convenient measure of a female’s intrinsic reproductive investment, particularly in birds with precocial chicks. We explored age-related correlates of within-female egg volume dynamics while controlling for among-female variation and the confounding effect of trait-dependent (dis)appearance of young (or old) females in the population. Our results show that egg volume expresses an early-life increase followed by a late-life decrease, indicative of reproductive senescence. Furthermore, females tended to lay smaller eggs in years during which they were polyandrous than in years when they were monogamous. Notably, polyandry was not associated with age. As expected, egg volume was highly repeatable within females and clutches. We conclude that age-dependent reproductive effort is an important component driving between- and within-individual variation in reproductive performance of facultatively polygamous species. We encourage future works studying reproductive senescence of wild populations to incorporate age-dependent reproductive investment and mating tactics.