An examination of climate and land-use change as drivers of population dynamics in breeding bird populations

Anthropogenic land-cover change and climate change are the major drivers of the steep loss of avian biodiversity in past decades. Loss of avian biodiversity is predicted to result in the reduction of ecosystem services and ecological functions. Identifying avian population changes and the drivers of these trajectories is essential for effective species conservation. I documented changes and potential drivers of change for birds in South Korea and the Pacific Northwest of the United States. Because a key determinant of the population ecology for migratory birds is survival during migration and over-wintering, I also quantified migratory paths and wintering grounds for a climate-sensitive bird species. In Chapter 2, I modeled occupancy dynamics of 52 species of common breeding landbirds in South Korea to provide the first quantitative population status assessment in the region and identify the broad-scale drivers of occupancy dynamics while accounting for imperfect detection survey effort. Twenty species (38%) were declining in average occupancy, and seven of them were declining more than 40% between 1997-2005 and 2014-2019. Notable examples are Black-capped Kingfisher (Halcyon pileata; 95% decline from average occupancy of 0.69 in the first period), Yellow-rumped Flycatcher (Ficedula xanthopygia; 66% decline from average occupancy of 0.19in the first period), and Brown Shrike (Lanius cristatus; 59% decline from average occupancy of 0.17 in the first period). Species that migrate farther and species with higher initial occupancy declined more severely than short-distance migrants, residents, and less common species in general. A comparison with IUCN red list categories for these species revealed that only one species (Brown Shrike) showed quantitative evidence for population declines, and no species were recognized as threatened by the IUCN. Only one species (Chinese Sparrowhawk, Accipiter soloensis) is recognized as Vulnerable by the Korean Government. Using the IUCN criteria, six species could be re-categorized as Endangered at least in the national level, and one species, the Black-capped Kingfisher could be categorized as Critically Endangered, at least for South Korea. Many of these species are near-endemic to the Korean Peninsula or to East Asia; thus their status in South Korea may represent a large proportion of their global population. In the Chapter 3, I investigated how old-growth forest vegetation can reduce negative effects of climate warming for breeding bird species using seven-year (2011-2018), fine-scale avian point count data and associated microclimate and vegetation data from the H.J. Andrews Experimental Forest, western Cascades, Oregon. Old-growth forests can buffer under-canopy temperatures and provide higher compositional and structural diversity than simple-structured second-growth forests. I tested two non-mutually exclusive hypotheses based on previous broad-scale studies from the region. First, the microclimate buffering hypothesis suggests that old-growth forests can provide microrefugia for breeding bird populations under the warming climate. And second, the insurance hypothesis explains that higher forest composition and structural diversity in old-growth forests can provide greater resource availability than forests with a simple structure. Eight of twenty species had more positive population trends at sites with cooler microclimates. The cooler microclimates provided by old-growth forest structure, in addition to topographic effects, dampened the negative effect of regional warming on population trends. In contrast, we found weak evidence of the insurance effect of forest composition and structure. For some species, vegetation characteristics changed the slope of the microclimate effect on bird population trends, but with low levels of statistical confidence. For example, the negative effect of microclimate on population trends of Wilson’s Warbler (Cardellina pusilla) and Red Crossbills (Loxia curvirostra) were positively mediated in areas with more diversity in vegetation composition. These results, in summary, suggest that the conservation of old-growth forests and promotion of old-growth characteristics in second-growth forests can potentially reduce the negative effect of warming on declining bird populations in the western coniferous forests. One of the declining species identified in chapter 3, Hermit Warbler (Setophaga occidentalis), was negatively associated with warmer temperatures in our study, as well as in previous studies. Hermit Warblers are long-distance migrants that spend most of their annual cycle outside of their breeding grounds. However, there is a paucity of information on the migration ecology and migratory connectivity for this species. I tracked 22 individual male birds from six breeding grounds to their wintering locations and back to their breeding range. Hermit Warblers appear to exhibit diffuse migratory connectivity with an overlapping chain migration pattern. Hermit Warblers have relatively short breeding seasons and migrate to montane regions in California during the post-breeding molt. Diffuse migratory connectivity may imply that breeding population declines are more likely to be attributable to breeding ground environmental change (e.g., forest degradation and climate change) unless there is spatially consistent broad-scale deterioration in habitat conditions across most of the wintering range. In summary, my study further advances knowledge for avian biodiversity conservation, with particular focus on the demography of breeding bird populations in Oregon, USA, and South Korea, and migration ecology of a declining long-distance migrant songbird species in the Washington, Oregon and California, USA. Such science-based, quantitative species status assessment proved important to correctly identifying sharply declining species. Also, by using information on fine-spatial scale population dynamics and their drivers, I show how old-growth structure and composition can serve to reduce the negative impact of climate warming on some forest bird populations in the Pacific Northwest. Lastly, understanding migratory connectivity and migration ecology can help to identify potential drivers of population change in migratory species.