Demographic mechanism of a historical bird population collapse reconstructed using museum specimens

Changes in Emberiza bunting communities and populations spanning 100 years in Korea

The rapid decline of a few Emberiza bunting species is increasing conservation concerns, especially in Asia. However, temporal changes in communities and populations of buntings, ones of the most common migratory songbirds in Korea, have not been quantitatively assessed. To understand how the status of buntings has changed over the past 100 years, we collated abundance data from museum collections and bird-banding records between 1910 and 2019. We also used presence-absence data for buntings collected by a nationwide census scheme between 1997 to 2012. Our analysis showed that bunting communities reconstructed from museum-specimen and bird-banding data were not significantly different; however, community composition differed over time. The Meadow (E. cioides), Yellow-throated (E. elegans), Black-faced (E. spodocephala), Rustic (E. rustica) and Chestnut Buntings (E. rutila), which are still common or were once common species, significantly affected the temporal changes in bunting community composition. There were no recent changes in the presence of Rustic and Chestnut Buntings since 1997, but they caused medium-term changes in the bunting community composition, suggesting that there was a sharp to moderate decline in their numbers in the past. The probability of the presence of six bunting species decreased annually, with the most prominent decline in two common breeders, the Meadow (-2.99%/year) and Yellow-throated Buntings (-1.82%/year). This finding suggests that breeding buntings in Korea are under high pressure, as are the migratory buntings. Moreover, despite its recent population decline, the Yellow-throated Bunting was still a major contributor to the community, suggesting that bunting diversity has also been deteriorating while bunting populations are shrinking. Long-term monitoring schemes across their distribution ranges, international cooperation for identifying major threats and key areas of conservation, and law enforcement against illegal hunting and habitat loss are strongly required to mitigate the on-going decline of buntings in Korea and Asia.

Females lead population collapse of the endangered Hawaii creeper

Population collapses result from drastic environmental changes, but the sexes may differ in vulnerability. Collapse of the endangered Hawaii creeper (Oreomystis mana) at Hakalau Forest National Wildlife Refuge resulted from food limitation associated with increased numbers of an introduced bird (Japanese white-eye, Zosterops japonicus), which competes with the creeper for food. Both creeper sexes had stunted bill growth and the greatest change in molt of native species in the community. With a surge in numbers of white-eyes, a recent cohort of adult females had very low survival after breeding, while adult males from the same cohort, and older females and males, continued to have high survival. Lower female survival resulted in a significantly more male-biased adult sex ratio. Recent low female survival was based on a great cost of reproduction, indicated by molt-breeding overlap that was previously avoided, and lower fat during the lengthy fledgling period. The difference in female survival between cohorts was associated with stunted bills from being reared in and then breeding in an increasingly poor food environment. Trend analysis of survey data indicate that the bird is declining throughout the refuge, with males being 72-80% of adults left six years after the white-eye increased. Competition over time was consistent with that previously documented over space on the Island of Hawaii. Adaptive management to recover the bird in this protected area needs to focus on improving both adult female survival and the adult sex ratio.

Reliability of genetic bottleneck tests for detecting recent population declines

The identification of population bottlenecks is critical in conservation because populations that have experienced significant reductions in abundance are subject to a variety of genetic and demographic processes that can hasten extinction. Genetic bottleneck tests constitute an appealing and popular approach for determining if a population decline has occurred because they only require sampling at a single point in time, yet reflect demographic history over multiple generations. However, a review of the published literature indicates that, as typically applied, microsatellite-based bottleneck tests often do not detect bottlenecks in vertebrate populations known to have experienced declines. This observation was supported by simulations that revealed that bottleneck tests can have limited statistical power to detect bottlenecks largely as a result of limited sample sizes typically used in published studies. Moreover, commonly assumed values for mutation model parameters do not appear to encompass variation in microsatellite evolution observed in vertebrates and, on average, the proportion of multi-step mutations is underestimated by a factor of approximately two. As a result, bottleneck tests can have a higher probability of 'detecting' bottlenecks in stable populations than expected based on the nominal significance level. We provide recommendations that could add rigor to inferences drawn from future bottleneck tests and highlight new directions for the characterization of demographic history.

Misleading population estimates: biases and consistency of visual surveys and matrix modelling in the endangered bearded vulture

Conservation strategies for long-lived vertebrates require accurate estimates of parameters relative to the populations' size, numbers of non-breeding individuals (the “cryptic” fraction of the population) and the age structure. Frequently, visual survey techniques are used to make these estimates but the accuracy of these approaches is questionable, mainly because of the existence of numerous potential biases. Here we compare data on population trends and age structure in a bearded vulture (Gypaetus barbatus) population from visual surveys performed at supplementary feeding stations with data derived from population matrix-modelling approximations. Our results suggest that visual surveys overestimate the number of immature (<2 years old) birds, whereas subadults (3–5 y.o.) and adults (>6 y.o.) were underestimated in comparison with the predictions of a population model using a stable-age distribution. In addition, we found that visual surveys did not provide conclusive information on true variations in the size of the focal population. Our results suggest that although long-term studies (i.e. population matrix modelling based on capture-recapture procedures) are a more time-consuming method, they provide more reliable and robust estimates of population parameters needed in designing and applying conservation strategies. The findings shown here are likely transferable to the management and conservation of other long-lived vertebrate populations that share similar life-history traits and ecological requirements.