Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). The population trend now appears to be increasing, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). Therefore, the species has been reassessed as Least Concern.
Rosenberg et al. (2016) estimate the population size to be 390,000 mature individuals.
This species has undergone a non-significant annual increase of 2.68% (6.85% increase to 2.35% decrease) between 2005 and 2015 based on data from the Breeding Bird Survey (Sauer et al. 2017). Longer term trends from Christmas Bird Count data show an annual increase of 9.55% between 1966 and 2017 (T. Meehan in litt. 2018).
Passerculus henslowii breeds in the eastern U.S.A., from Minnesota (primarily in the south-east) east to southern Ontario (Canada) and New York, and extending south-west to north-east Oklahoma, and south-east to north-western Kentucky and West Virginia. Isolated breeding populations also exist in north-east Virginia and north-east North Carolina. It is extirpated or a rare breeder in the north-east U.S. states, from Vermont south to Delaware. It winters in coastal states from south-east North Carolina south to Florida (except for the southern tip) and west to eastern Texas (AOU 1983, Sibley and Monroe 1990). It was once common in unploughed, periodically burnt tall-grass prairie, which originally stretched from Illinois west to Kansas and Nebraska (Rising 1996), but these populations have been thought to have declined, with the greatest declines being in the northern and eastern portions of the range (AOU 1983). Southern populations, however, have tended to increase (Rising 1996), apparently in association with the creation of undisturbed grassland habitat by the Conservation Reserve Program (Rising 1996, Herkert et al. 2002, Cooper 2012).
During the breeding season, the species is typically found in grassland with tall and dense grass, dead trees and thick leaf-litter, although site choice is unpredictable from year to year (Dornak et al. 2013). On their wintering grounds, though, birds prefer recently burned areas (Bechtoldt and Stouffer 2005), with low densities of litter and near-ground vegetation (Carrie et al. 2002, Holimon et al. 2008). Optimal fire intervals may vary regionally, possibly due to different rates of litter accumulation in mesic and drier upland savannas (Palasz et al. 2010). They are often associated with longleaf pine Pinus palustris savanna at this time (Plentovich 1999, Bechtoldt and Stouffer 2005, Thatcher et al. 2006), but also occur in saline soil barrens (Holimon et al. 2004, Holimon et al. 2008). Mowed fields are generally avoided, as are fields with a lot of woody vegetation (Rising 1996, Holimon et al. 2008).
Wetland draining and destruction of grassland habitat through fire suppression, conversion to agriculture or pine plantations and earlier and more frequent cutting of hayfields (Pruitt 1996, Rising 1996, Cooper 2012) all represent threats, and in combination they have caused population declines in the past. The continued investment in fire suppression programmes could result in reduced funding for prescribed fire in longleaf pine savannas and therefore loss of suitable wintering habitat for this species (United States Department of Agriculture 2015). Further, significant additional U.S. Department of Agriculture’s (USDA) Conservation Reserve Program grasslands may be converted to agricultural fields if the high price of corn continues, resulting in loss of breeding habitat (Cooper 2012, Lark et al. 2015). This species has been reported to suffer mortality as a result of collisions with communications towers (Longcore et al. 2013). However, given the ongoing population increase, it is uncertain to what extent these threats could be impacting the species currently.
Conservation Actions Underway
The species is monitored within the North American Breeding Bird Survey and Christmas Bird Count. It is listed as an Endangered species in Canada, and within its U.S. breeding range is considered Endangered in seven states, Threatened in five, and of Special Concern in four. Some grassland conservation activities will be undertaken as part of the national Partners in Flight bird conservation efforts. The Conservation Reserves Program, while not specifically intended to benefit this species, has provided a large area of undisturbed grassland habitat, and Henslow's Sparrow has colonised these fields in many areas. Creation of large areas of undisturbed grasslands through this programme appears to have contributed to recent reversal of long-term population declines (Cooper 2012). It is on the watch list as part of the State of North America's Birds (North American Bird Conservation Initiative 2016).
Text account compilers
Harding, M., Khwaja, N., Isherwood, I., Wege, D., Westrip, J.
Wells, J., Rosenberg, K., Holimon, B., Meehan, T.
BirdLife International (2019) Species factsheet: Passerculus henslowii. Downloaded from http://www.birdlife.org on 17/02/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 17/02/2019.