Influence of season and frequency of fire on Henslow's sparrows (Ammodramus henslowii) wintering on Gulf Coast pitcher plant bogs

The Auk 120(1):96-106, 2003

ABSTRACT.-Henslow's Sparrow (Ammodramus henslowii) is a grassland bird that has suffered drastic population declines for over 30 years. Declining populations can be largely attributed to loss of breeding habitat, but loss of wintering habitat associated with longleaf pine (Pinus palustris) communities, especially pitcher plant (Sarracenia spp.) bogs, along the Gulf Coast also may be a contributing factor. Fire is critical for restoration and conservation of remaining longleaf pine communities, but the influence of fire on wintering Henslow's Sparrows has not been evaluated. We examined the influence of season and frequency (time since burning) of fire on use of pitcher plant bogs by Henslow's Sparrows wintering in the Conecuh National Forest, Alabama, and Blackwater River State Forest, Florida, during winters of 1999-2000 and 2000-2001. Density of Henslow's Sparrows was greatest on bogs the first winter after burning. Although significant effects for season of burning were not found, bogs burned during winter typically hosted Henslow's Sparrows for only one winter, whereas bogs burned during the growing season hosted sparrows for at least three winters. Growing-season fires may be more beneficial than dormant-season fires and will prevent forced abandonment of bogs burned during winter. Frequency of seed stalks of grasses and density of forbs were the most influential vegetation parameters affecting occurrence of Henslow's Sparrows at pitcher plant bogs. Henslow's Sparrows were found on bogs as small as 0.06 ha, but were found on bogs >0.25 ha more frequently than on smaller bogs. We conclude that burning pitcher plant bogs on an annual or biennial basis during the growing season will maximize the benefits to both wintering Henslow's Sparrows and the host of other organisms associated with those unique communities. Received 18 December 2001, accepted 12 September 2002.

THE LONGLEAF PINE (Pinus palustris) ecosystem once dominated the coastal plain of the southeastern United States and extended from Virginia to Texas (Wahlenberg 1946). Forests within that ecosystem were characterized by large, widely spaced longleaf pines with a dense ground cover of grasses and forbs (Chapman 1932). Frequent fires resulting from a high incidence of lightning strikes are the primary agent responsible for the diversity and structure of longleaf pine forests (Chapman 1932). Disruption of natural fire regimes and conversion to other land uses have resulted in the loss of 95% of that once extensive ecosystem (Outcalt and Sheffield 1996), making it one of the most heavily affected of all forested ecosystems (Noss 1989, Simberloff 1993). Concurrent with the loss of longleaf pine forests, populations of many species characteristic of those forests (e.g. Red-cockaded Woodpecker [Picoides borealis]) have declined and have become threatened with extinction.

Henslow's Sparrow (Ammodramus henslowii) is a migratory bird species that breeds in moist grasslands of the central and eastern United States (Hyde 1939) and migrates to the southeastern United States where it primarily spends the winter in open pine savannas and pitcher plant (Sarracenia spp.) bogs (Plentovich et al. 1998; 1999), microhabitats characteristic of the longleaf pine ecosystem. Between the years of 1966 and 1996, populations of breeding Henslow's Sparrows declined at an average rate of 8.8% per year (Peterjohn and Sauer 1999). Although loss and degradation of breeding habitat is probably the major factor contributing to decreasing populations of Henslow's Sparrows, loss of winter habitat also may be important (Askins 1993). For example, Brooks and Temple (1990) suggested that reduced overwinter survival of a migrant population of Loggerhead Shrikes (Lanius ludovicianus) that breeds in Minnesota and winters along the Gulf Coast probably contributed to population declines more strongly than factors occurring on the breeding grounds. Gulf Coast pitcher plant bogs, a major wintering habitat for Henslow's Sparrows (Plentovich et al. 1999), have been reduced to

A major threat to the remaining pitcher plant bogs is alteration of natural fire regimes (Folkerts 1982, Frost et al. 1986). Traditionally, land managers have used fire primarily during the winter (dormant season), but evidence suggests that natural fires most often occurred during late spring and summer (growing season) which coincides with the season of greatest electrical storms (Robbins and Myers 1992). In addition, frequency of fire is a critically important factor. Walker and Peet (1983) found maximum species richness of plants on pitcher plant bogs burned annually, and Frost et al. (1986) stated that those habitats might require fire on a near annual basis.

Similarly, bogs not burned regularly may become unsuitable for some bird species. Plentovich et al. (1999) found that wintering Henslow's Sparrows abandoned bogs in Alabama that went without disturbance for more than four years. Furthermore, Henslow's Sparrows showed strong site fidelity during winter, suggesting that fire might have a strong negative effect on populations if burning occurred during winter (Plentovich et al. 1998). However, no studies have addressed directly the influence of season and frequency of burning on abundance of Henslow's Sparrows wintering on pitcher plant bogs. Thus, the objective of our study was to examine the influence of season and frequency (i.e. time since burning) of fire on abundance of Henslow's Sparrows.