Despite extensive research on this species in the last several decades, many aspects concerning its life history and population ecology remain unknown. Much of the available information from the breeding areas comes from research conducted at one large forested site in New Hampshire (see Holmes 2007, Holmes 2011, Holmes and Likens 2016), although recent work at the Nantahala National Forest in North Carolina has begun to broaden our understanding of this species in the southern portion of its breeding range (e.g., Stodola et al. 2009, Stodola et al. 2010, Cline et al. 2016). Little is known about survival of hatch-year birds between fledging and migration or about dispersal, especially between the natal site and where birds settle for their first breeding season. Similarly, although site tenacity to both breeding and wintering sites is strong, the connectivity between breeding and wintering localities is still unclear. Stable isotope studies as pioneered with Black-throated Blue Warblers by Chamberlain et al. (Chamberlain et al. 1997) and Rubenstein et al. (Rubenstein et al. 2002) elucidated some of these relationships, but new methods involving geolocators and other newer technologies will be needed to explore this topic further. Although most adult mortality occurs during migration (Sillett and Holmes 2002), the major causes of this mortality remain unknown. Habitat-specific demography in the winter has not been adequately examined. Do individuals wintering in different habitats or geographic regions vary in their ability to maintain body condition and/or survive and how will these be affected by changing environmental conditions, especially climate change?
Most studies to date have dealt with populations in large tracts of relatively undisturbed forests, but the effects of habitat change, including human-caused fragmentation and degradation, still require further study, including the potential impact of brood parasites in some parts of the range. Similarly, the potential direct or indirect impacts of pests and pathogens, such as the hemlock adelgid (Lovett et al. 2006) and emerald ash borer (Koenig et al. 2013), have yet to be assessed. Finally, Black-throated Blue Warblers appear to be adaptively plastic in their ability to initiate breeding based on the timing of leaf-out, even as spring temperatures have increased at Hubbard Brook. The limits of this plasticity are unknown. Moreover, the growing season in much of the species’ breeding range, is extending, i.e., starting earlier in spring and lasting longer in autumn (Richardson et al. 2006, Lany et al. 2016). Further research is therefore needed to understand how the species will respond to future climate change on both breeding grounds and winter quarters.