Justification of Red List Category
Since 1987, conservation action has successfully increased the population of this species. Numbers exceeded 500 singing males in 1994 following doubling of suitably aged habitat between 1987 and 1990. Numbers continue to increase, but its population remains small, hence its classification as Near Threatened. Should current trends continue, the species may be eligible for downlist in the near future.
The population size increased beginning in 1990, with latest estimates from 2015 putting it at around 2,365 singing males (Michigan Department of Natural Resources 2015). The population is estimated at 4,500-5,000 mature individuals (D. Ewert in litt. 2016), which equates to 6,750-7,500 individuals, rounded here to 6,800-7,500 individuals.
Following major declines in c.1900-1920 and 1961-1971, numbers have recovered from 167 singing males in 1974 and again in 1987, to 2,365 singing males in Michigan in 2015, the highest since surveys began in 1951 (Michigan Department of Natural Resources 2015; Cooper et al. 2019b).
Almost the entire population of Setophaga kirtlandii breeds in north and central Michigan (Anon. 2008), with small numbers breeding in Michigan's Upper Peninsula and Wisconsin, U.S.A. and Ontario, Canada (Eskelsen 2007; Richard 2013, 2014). Breeding habitat underwent significant past declines, but is now considerably more extensive than the 18 km2 occupied in 1994 (Nelson and Buech 1996). It has a small winter range in the Bahamas, Turks and Caicos Islands (to U.K.), and Cuba; recent studies demonstrate that most overwinter in the central Bahamas islands of Eleuthera, San Salvador, Cat and Long Islands (Cooper et al. 2017, 2019a). There were major declines in c.1900-1920 and 1961-1971 (Haney et al. 1998), with the population numbering just 167 singing males in 1974 and 1987 (Anon. 1996). Numbers have recovered to 2,365 singing males in Michigan, Wisconsin and Ontario in 2015 (Michigan Department of Natural Resources 2015), the highest since surveys began in 1951 (National Wildlife Refuge Association in litt. 2006); 97.5% of the singing males were in the lower peninsula of Michigan. Migration routes were recently described, with important stopover areas in Florida, Georgia, Ohio and along the coasts of North and South Carolina (Cooper et al. 2017). Finally, it was recently determined that the species exhibits weak migratory connectivity between wintering and breeding grounds, with individuals wintering on any one island migrating to locations across the entire breeding range (Cooper et al. 2018).
Its optimal breeding habitat is large (>80 acres), fire-maintained homogeneous stands of 1-5 m tall jack pines Pinus banksiana on sandy soil (Mayfield 1992; Sykes 1997; Anon. 2008). However, greater than 75% of habitat is now planted through species-specific management efforts (Donner et al. 2008). Birds arrive to the breeding grounds from early May to early June (Rockwell et al. 2012; Cooper et al. 2017) and eggs are laid in late May and June (Curson et al. 1994) with hatching occurring ~14 days later and fledglings leaving the nest in July (Bocetti et al. 2020). Birds depart on fall migration from late September through late October (Cooper et al. 2017). It winters in early-successional disturbed habitat (Wunderle et al. 2010) consisting of natural and secondary scrub, and saline/upland ecotone (Sykes and Clench 1998; Wunderle et al. 2010, 2014). Stands of Caribbean pine, P. caribbaena were believed to be important winter habitat (Haney et al. 1998), but more recent surveys indicate that warblers use almost exclusively scrub habitats (Cooper et al. 2019a). It feeds on arthropods and abundant fruit during winter (Wunderle et al. 2010, 2014). Some birds move from patch to patch in the wintering grounds as food supplies are depleted and areas dry out, eventually concentrating in small patches where they maintain small and overlapping home ranges (Wunderle et al. 2014). Following dry winters, birds arrive relatively late on the breeding grounds, which is associated with lower reproductive success and survival (Rockwell et al. 2012, 2017).
Loss of breeding habitat was the major threat that led to their near extinction, but this threat has been eliminated following decades of habitat creation. Parasitism by brown-headed Cowbird Molothrus ater was also a major threat in breeding areas (Sykes and Clench 1998). However, because of declining cowbird populations in Michigan, parasitism by cowbirds is not currently a threat. The cause of declines in the cowbird population is not understood and if cowbird populations increase, parasitism may again become a major threat (Cooper et al. 2019b). A cowbird trapping program was reduced between 2015-2017 and then suspended in 2018-2019 due to this decline in cowbird populations and parasitism is currently ~1% (Cooper et al. 2019b). Wintering area threats include droughts and habitat disturbances that eliminate early successional habitats with favoured fruit plants (J. M. Wunderle in litt. 2016). Studies have demonstrated that late winter droughts can eliminate fruit, causing reduction in warbler body condition, annual survival and reduced reproductive success in Michigan (Rockwell et al. 2012; Wunderle et al. 2014; Rockwell et al. 2017). Climate change is a major concern in wintering grounds, especially the potential increase in late winter drought conditions and sea level rise, which threatens the warbler’s coastal habitats as well as limiting availability of freshwater required by warbler’s fruit plants (J. M. Wunderle in litt. 2016; Wolcott et al. 2018; Cooper et al. 2019a); reductions in wintering ground precipitation and hence, habitat quality have been modelled to drive significant S. kirtlandii population declines through carry-over effects (Brown et al. 2017, 2019). Climate change however presents less of a threat in breeding grounds, where conditions are likely to remain largely suitable for jack pine occupancy in the near future (Donner et al. 2018).
Conservation Actions Underway
CMS Appendix I. Management replicating the effects of natural fires has expanded potential breeding habitat considerably to c.120 km2 (N. Cooper in litt. 2020). Efforts are underway to expand the breeding range outside of the Lower Peninsula of Michigan by increasing the size of existing populations in the Upper Peninsula of Michigan, Wisconsin, and Ontario. The Michigan Department of Natural Resources and the United States Forest Service have signed agreements to continue habitat creation (USFWS and MDNR 2015; USFWS et al. 2016). Since 1972, a cowbird trapping programme has reduced parasitism from 70% to 1% (Cooper et al. 2019b). The trapping program was reduced from 2015 to 2017, and eventually suspended in 2018 with no associated increase in the rate of parasitism. The cowbird programme is currently shifting towards a monitoring focus, to ensure that if and when the cowbird population increases to a dangerous level, managers will be aware and can reinstate the trapping programme. Modeling efforts are underway to determine the maximum level of parasitism that the population could withstand and remain sustainable (N. W. Cooper pers. comm). Education and ecotourism initiatives in Michigan include an annual Kirtland's Warbler Festival (Richter 1996). Surveys have been undertaken in the Bahamas since 1998 (Haney et al. 1998) and in the Bahamas and Turks and Caicos Islands from 2010-2016; no further surveys are planned (Cooper et al. 2019a). Migration routes and stopover locations have been evaluated (Cooper et al. 2017). On the wintering grounds, studies are underway to determine the utility of goat farms in creating Kirtland’s Warbler habitat; such sites appear to show promise in maintaining such habitat (Fleming et al. 2019). Breeding success and carry-over effects have been studied by Rockwell et al. (2012) whilst the potential effects of climate change have also been modelled (Brown et al. 2017, 2019; Cooper et al. 2019a). Virtually the entire population breeds within the 219,000-acre Kirtland's Warbler Management Area which includes the Michigan State Forests, the Huron National Forest, and the USFWS Kirtland's Warbler National Management Area.
14.5 cm. Large grey-and-yellow warbler. Male, blue-grey above with diffuse black streaking on the mantle. Black lores and split white eye-ring. Yellow underparts with black streaking on flanks. Female, similar but paler with brown tinge to mantle and lacks black lores. Immature has greyish streaking and spots on throat. Voice Song an emphatic chip-chip-che-way-o.
Text account compilers
Wheatley, H., Everest, J.
Benstead, P., Bird, J., Cooper, N., Ewert, D., Harding, M., Khwaja, N., National Wildlife Refuge Association, Pople, R. & Wunderle, J.
BirdLife International (2021) Species factsheet: Setophaga kirtlandii. Downloaded from http://www.birdlife.org on 20/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 20/10/2021.