Setophaga kirtlandii

Phylogenetic analyses of sequences of mitochondrial and nuclear DNA (Lovette et al. 2010) indicate that all species formerly placed in Dendroica, one species formerly placed in Wilsonia (citrina), and two species formerly placed in Parula (americana and pitiayumi) form a clade with the single species traditionally placed in Setophaga (ruticilla). The generic name Setophaga has priority for this clade (AOU 2011).

This species has recovered from historical declines thanks to extensive conservation efforts in its breeding and non-breeding range. The population has more than doubled since 2002 with approximately 2,245 pairs estimated in 2021. The major threats historically impacting this species, habitat loss due to succession and brood parasitism by brown-headed cowbirds (Molothrus ater), have both been virtually eliminated. However, this species remains reliant on conservation efforts to maintain suitable habitat and ensure high rates of fledgling success.

During the breeding season (April-September), the range includes northern Lower Michigan, the Upper Peninsula of Michigan, and northeastern and south-central Wisconsin in the United States, and at least one site in Ontario, Canada (USFWS 2019, Bocetti et al. 2020, Fink et al. 2022). Using records from the breeding range from 2004 onward (the past 20 years) from Global Biodiversity Information Facility (GBIF 2024), the breeding range extent is estimated to be approximately 256,582 km² (GeoCat 2024).

During the non-breeding season (September-April), it occurs primarily on islands of The Bahamas (Bocetti et al. 2020, USFWS 2019). Reports of solitary individuals in Bermuda, Cuba, Dominican Republic, Jamaica, and Mexico are not considered part of its normal non-breeding range (Faanes and Haney 1989, Brewer et al. 1991, Mayfield 1996, USFWS 2012, Cooper et al. 2017, Cooper et al. 2019b, Bocetti et al. 2020, Brunner et al. 2020). There is limited evidence that a small number of individuals spend the non-breeding season in Turks and Caicos (Cooper et al. 2017, Cooper et al. 2019b), but this part of the range was not included due to a lack of specific locality data. Using records from The Bahamas from 2004 onward (the past 20 years) from Global Biodiversity Information Facility (GBIF 2024), the non-breeding range extent is estimated to be approximately 123,101 km² (GeoCat 2024). As the most limited range during its annual cycle, the non-breeding range extent was used in this assessment.

Determining the number of occurrences in Michigan is challenging using standardized criteria due to a constantly shifting breeding habitat mosaic. The occurrences in northern Lower Michigan are likely rather large and, therefore, low in number. At the periphery of the range, there are at least three distinct occurrences in Wisconsin and one in Ontario (USFWS 2019, Bocetti et al. 2020, Kirtland’s Warbler Alliance 2022).

Habitat loss in the breeding range due to succession was considered one of the primary reasons for this species’ decline in the past, but the availability of appropriately-aged forest stands has been secured for the foreseeable future thanks to ongoing long-term management plans in existing habitat and the creation of new habitat in peripheral parts of the range (USFWS 2019, Bocetti et al. 2020, Olah et al. 2022, Van Dyke et al. 2022, Olah et al. 2023). The threat of habitat loss due to succession or development in the non-breeding range is considered low despite much of the habitat not being protected (USFWS 2019). Multiple invasive plant species continue to reduce the amount of native fruit-bearing plants utilized by this species in The Bahamas and Turks and Caicos, although the impacts are not known to be serious (USFWS 2019).

Brood parasitism by brown-headed cowbirds (Molothrus ater) has been considered a threat for decades, but programs to trap and remove them from Kirtland’s warbler breeding sites have successfully reduced this threat to the point that only about 1% of nests were parasitized from 2015-2018 (Cooper et al. 2019a, USFWS 2019, Bocetti et al. 2020, Margenau et al. 2023). As of 2024, the severity of this threat is considered low.

While mortality due to collision with man-made structures is a serious threat for many migratory songbird species, it has only been documented five times for this species (USFWS 2019).

The long-term effects of climate change are projected to have the greatest impact on the non-breeding range. Habitat in the non-breeding range in the Bahamas is projected to decline in both quantity and quality in response to climate change and these factors are projected to cause declines (Brown et al. 2019, USFWS 2019). Damage from increasingly powerful hurricanes can decrease available food sources and habitat but can also produce new habitat in a natural cycle of succession (USFWS 2019). The availability and nutritional quality of native fruits, which are vital energy sources for migration to the breeding grounds in late April and early May, is projected to decline in response to increased temperatures, decreases in annual rainfall, and changes in the timing of rainfall away from the month of March (USFWS 2019). Sea-level rise is not projected to significantly decrease habitat in the coming decades (USFWS 2019). The effects of climate change are not expected to have negative impacts for habitat in the breeding range in the coming decades, although the range could shift northward (USFWS 2019).