Sensitivity to Disturbance at Nest and Roost Sites
No experimental tests of the effects of disturbance on nest site selection or nesting pairs. Females often abandon a nest site if disturbed during nest building, but once eggs are laid and incubation commences, they tolerate human visits, even the presence of nearby cameras and videorecorders for documenting behavior (Nagy and Holmes 2005a, Kaiser et al. 2014, RTH, NLR).
Shooting and Trapping
Pesticides and Other Contaminants/Toxics
Little information for this species. Mercury (total and methyl mercury) loads significantly above background levels (Keller et al. 2014, Jackson et al. 2015), and contamination probably increases with elevation (Townsend et al. 2014). Whether the observed loads are affecting behavior or demography is unknown, but such effects are unlikely as loads are well below those known to cause harm in other songbird species (Edmonds et al. 2012). Measurable levels of organophosphates (especially chlorpyrifos) obtained from feet of individuals found dead after colliding with windows or other lighted structures in Toronto, Canada, attributed to overwintering in pesticide-treated agricultural areas in Central America or Caribbean (Alharbi et al. 2016).
Degradation of Habitat: Breeding and Overwintering
Breeding period. This species is area-sensitive, occurring mainly in forest tracts larger than 100 ha (Robbins et al. 1989a). In Ontario and New Brunswick, area-sensitivity reported (Betts et al. 2007, Desrochers et al. 2010). Does not usually occur commonly in young clearcuts or second growth, but readily uses stands with group-selection cuts (NLR) and does not avoid transmission line corridors (Anderson et al. 1977d). Becomes frequent once canopy becomes well developed and gaps allow the development of shrubs, usually > 50 yr following clearcutting (Holmes 1990, RTH). Appears to be about equally common in both managed and unmanaged northern hardwoods forests (Welsh and Healy 1993, Buford and Capen 1999). Densities are not significantly affected by selective-logging (Webb et al. 1977, Holmes and Pitt 2007) or by forest degradation ("maple decline") in Ontario (Darveau et al. 1992), as long as there is a dense shrub layer or patches of dense shrubs, and relatively complete canopy cover (Jobes et al. 2004). Beech bark disease, however, reduces habitat suitability as beech saplings reduce shrub density, which in turn leads to lower Black-throated Blue Warbler numbers and lower reproductive success for those individuals that do settle in areas of low shrub density (Holmes et al. 1996).
Males can be lured by song playbacks into small clearings and open areas, and readily cross roads, thus capable of crossing gaps or fragmented areas (Harris and Reed 2001, Harris and Reed 2002b). Breeding males translocated from home territories took more time and were less likely to return to their territories as forest cover decreased in the landscape (Belisle et al. 2001). In Maine, more older (after-second-year) birds occurred in forest interiors and these had higher productivity than younger (second-year) individuals breeding near edges (Harris and Reed 2002a). However, the same study recorded higher pairing success and food density in shrubbier habitats along edges, and concluded that males at edge and interior appeared to have similar probabilities of producing fledglings (Harris and Reed 2002a). In New Brunswick, no significant differences in adult densities and reproductive success occurred among forests modified by logging (Bourque and Villard 2001). Reported to be intolerant of development (Clark et al. 1984), and point count and mist net abundances were positively related to fragment size in western Massachusetts (Kluza et al. 2000). This species is more likely to occur in large forest patches in New Brunswick (Betts et al. 2007). Thus, distribution may be constrained by habitat characteristics, and forest fragmentation above a threshold level probably affects both occurrence and abundance.
Habitat loss and degradation during migration is likely occurring in multiple ways. Forest habitat loss due to land use change (Drummond and Loveland 2010, Hansen et al. 2013); increasing use of architectural glass (Pariafsai 2016) and wind turbines (Arnold and Zink 2011, Erickson et al. 2014) resulting in bird collisions (Cusa et al. 2015); growth of predator populations, particularly domestic and feral cats (Loss et al. 2012); proliferation of lighted communication towers (Gehring et al. 2009, Longcore et al. 2013). Last, climate change is increasing variability of spring weather, including severe frosts (Schwartz et al. 2013, Kim et al. 2014) after insect emergence may be reducing food abundance (NLR). Changes in distribution under different climate change scenarios indicate future range constrictions (Matthews et al. 2011). Quantitative assessment of the relative impacts of these ongoing threats is needed.
Overwintering period. Habitat degradation and change on the overwintering grounds may also be occurring, but evidence is largely indirect (Holmes et al. 1992, Sherry and Holmes 1996b). In Jamaica, this species occurs in a wide variety of habitats, including natural forests, second growth, coffee plantations, orchards, and gardens with fruit trees and other woody vegetation (Confer and Holmes 1995, RTH). No information yet exists, however, on the relative suitability, in terms of overwinter survival and/or body condition, of these different sites, nor specifically on whether or not these habitats are saturated and thus able to absorb individuals displaced by deforestation. Further study of habitat-specific survivorship rates and food supply during the overwintering period is needed to clarify these matters.
Long-term climatic changes, such as those that occur naturally (e.g., El Niño cycle) or through human-influenced global warming, may also affect the distribution and abundance of this species, as well as those of other Nearctic–Neotropical migrants (Rodenhouse 1992, Rodenhouse and Holmes 1992, Sillett et al. 2000, Rodenhouse et al. 2007, Townsend et al. 2016). To date, recruitment and population growth of this species may have benefited from climate change (Townsend et al. 2016), in part, because of its ability to adapt to changes in breeding season phenology (Townsend et al. 2013, Lany et al. 2016). Whether and when such adaptive changes will reach critical thresholds is unknown. Largest future threats associated with climate change may come from drought in overwintering areas that depress food abundance, lowering survivorship and physiological condition at the time of spring migration (Sherry et al. 2005, Smith et al. 2010a, Studds and Marra 2007, Studds and Marra 2011).
Current evidence suggests that management of habitat for this species should focus on the availability of high quality habitat throughout its annual cycle (Rodenhouse et al. 2003, Sheehy et al. 2010). Habitat quality is strongly influenced by food supply in breeding and overwintering areas. Threats to survivorship during migration (as identified above) have yet to be ranked and addressed.