Winter Use of South Florida Dry Prairie by Two Declining Grassland Passerines

Abstract

Resumen. Las aves que obligatoriamente viven en pastizales están sufriendo algunas de las disminuciones poblacionales más pronunciadas de todos los paserinos de América del Norte. Pocos estudios han tratado los patrones de uso de hábitat de estas especies mientras están invernando en el sudeste de Estados Unidos. La pradera seca del sur-centro de Florida constituye una de las mayores áreas remanentes de pastizal continuo dentro de esta región y durante el invierno mantiene un grupo diverso de aves migratorias que obligatoriamente viven en pastizales. El uso actual del suelo de esta región ha alterado el régimen natural del fuego del ecosistema y ha desplazado la comunidad vegetal de graminoideas y herbáceas de hoja ancha hacia una sobreabundancia de especies leñosas como Sereona repens. Simultáneamente, la distribución histórica del hábitat ha decrecido con el desarrollo urbano y la transformación para agricultura. Para entender cómo estos cambios afectan a las aves que invernan en los pastizales, documentamos la comunidad invernal de aves de las praderas secas y evaluamos los efectos de las características del hábitat y del tiempo transcurrido desde un incendio en la presencia de Ammodramus savannarum pratensis y Cistothorus platensis, dos residentes invernales. Muestreamos las aves a lo largo de transectas y empleamos un enfoque teórico de la información para seleccionar los modelos que mejor predijeron la presencia de las especies. El tiempo desde el incendio fue la variable que mejor predijo a A. s. pratensis en ambos años de nuestro estudio. La ocupación por parte de esta especie fue seis veces más probable si las transectas habían sido quemadas en el transcurso del año anterior. Cistothorus platensis se vio favorecida por intervalos más prolongados entre los incendios y su respuesta a las covariables de hábitat difirieron en los dos años. Estos resultados enfatizan la necesidad de que la pradera seca sea manejada con intervalos naturales (1-3 años) de retorno del fuego para mantener hábitat invernal para las aves de pastizal que están declinando.Abstract. grassland-obligate birds are undergoing some of the steepest population declines of all North American passerines. Few studies have addressed these species' patterns of habitat use in their winter ranges in the southeastern United States. The dry prairie of south-central Florida constitutes one of the largest areas of contiguous grassland remaining within this region and during winter supports a diverse group of migratory grassland-obligate birds. Modern land-use of this region has altered the ecosystem's natural fire regimes and shifted the vegetative community away from graminoids and forbs toward overabundance of woody-stemmed species such as the saw palmetto (Sereona repens). Simultaneously, the prairie's historic range has decreased with urban development and conversion to agriculture. In order to understand how these changes affect overwintering grassland birds, we documented the dry prairie's winter bird community and evaluated the effects of habita...

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“…, Butler et al. ). Methods varied by sampling period, with point counts during summer breeding seasons when birds are vocal and often readily visible, whereas flush and line transects are considered to be more efficient methods for sampling migratory or wintering birds (Igl and Ballard , Fletcher et al.…”

“…, Butler et al. ). We conducted all surveys from sunrise until 3 h after sunrise during favorable weather conditions (i.e., no precipitation and wind <16 km/h) to minimize the effects of weather on detections and recorded percent cloud cover, temperature, average wind speed, and humidity with a Kestrel 3000 pocket weather meter (Nielsen‐Kellerman, Boothwyn, Pennsylvania, USA).…”

“…Due to logistical constraints, spring surveys were not conducted in 2011; all other survey periods were sampled for the 3-yr duration of the study (January 2011-December 2013). We used multiple sampling methods to detect birds including flush transects, point counts, and line transects (Ralph et al 1993, Butler et al 2009). Methods varied by sampling period, with point counts during summer breeding seasons when birds are vocal and often readily visible, whereas flush and line transects are considered to be more efficient methods for sampling migratory or wintering birds (Igl and Ballard 1999, Fletcher et al 2000, Butler et al 2009).…”

“…We used multiple sampling methods to detect birds including flush transects, point counts, and line transects (Ralph et al 1993, Butler et al 2009). Methods varied by sampling period, with point counts during summer breeding seasons when birds are vocal and often readily visible, whereas flush and line transects are considered to be more efficient methods for sampling migratory or wintering birds (Igl and Ballard 1999, Fletcher et al 2000, Butler et al 2009). We conducted all surveys from sunrise until 3 h after sunrise during favorable weather conditions (i.e., no precipitation and wind <16 km/h) to minimize the effects of weather on detections and recorded percent cloud cover, temperature, average wind speed, and humidity with a Kestrel 3000 pocket weather meter (Nielsen-Kellerman, Boothwyn, Pennsylvania, USA).…”

“…We randomly assigned transect orientation and started all flush transects from the eastern-most endpoint to prevent observers from facing the sun during surveys. Two observers pulled a 25-m rope with noisemakers (1-L bottles containing rocks attached to the rope every 6 m) across the vegetation, with a third observer near the center of the rope serving as the data recorder; all observers identified flushed birds to species using visual, aural, and flight cues (Butler et al 2009). When existing vegetation precluded efficient rope-dragging (vegetation > 1.5 m in height), each edge observer walked approximately 12 m from the center observer and made noise with hand-held noisemakers to flush any birds.…”