GEOGRAPHIC VARIATION IN BROWN-HEADED COWBIRD (MOLOTHRUS ATER) PARASITISM ON DICKCISSELS (SPIZA AMERICANA) IN GREAT PLAINS TALLGRASS PRAIRIE

Auk, The, Apr 2005 by Jensen, William E, Cully, Jack F Jr

ABSTRACT

The incidence of brood parasitism by Brown-headed Cowbirds (Molothrus ater; hereafter "cowbirds") within host species typically reflects the continental pattern in cowbird abundance across North America, where parasitism is heaviest in the Great Plains. However, we found considerable variation in cowbird parasitism on Dickcissel (Spiza americana) nests within a subregion of the Great Plains (the Flint Hills), where the highest levels of cowbird parasitism on grassland bird nests had been previously reported. Local parasitism frequencies on Dickcissel nests varied latitudinally across the Flint Hills, ranging from 0% to 92% of nests parasitized. Interestingly, we found no obvious patterns in habitat or host attributes that were associated with this steep geographic gradient in brood parasitism. Cowbird parasitism on Dickcissel nests was not correlated with the vertical density of local prairie vegetation, mean nest distance to edge, proportion of forest to grassland habitat surrounding study sites (≤5 to 10 km), geographic variation in host abundance, or Dickcissel density or nest initiation dates. Parasitism frequencies and intensities (number of cowbird eggs per parasitized nest) on Dickissel nests were only significantly related to variation in local female cowbird density. Dickcissel clutch size and apparent fledging success were negatively correlated with local cowbird parasitism levels. Geographic patterns in cowbird abundance within and among regions should be considered when establishing conservation areas for grassland birds or other cowbird hosts of concern. Received 24 October 2003, accepted 18 November 2004.

Key words: brood parasitism, Brown-headed Cowbird, Dickcissel, geographic variation, Molothrus ater, Spiza americana.

Variación Geográfica en el Parasitismo de Nidos de Spiza americana por Molothrus ater en Praderas de Pastos Altos de la Gran Planicie

RESUMEN.-La incidencia del parasitisme de cría por parte de Molothrus ater en sus especies hospederas típicamente refleja el patrón continental de abundancia de este parásito a través de América del Norte, donde el parasitismo es más intense en la región de la gran planicie. Sin embargo, encontramos variación considerable en el parasitisme de los nidos de Spiza americana dentro de una subrregión de la gran planicie (las colinas de Flint), en donde los niveles máximos de parasitismo de cría por parte de M. ater sobre aves de pastizal habían sido documentados previamente. Las frecuencias locales de parasitismo sobre los nidos de S. americana variaron latitudinalmente a través de las colinas de Flint, en un rango entre 0% y 92% de nidos parasitados. Resulta interesante que no encontramos patrones obvios en atributos del hábitat o del hospedero asocìados con este marcado gradiente geográfico en el parasitismo de cría. El parasitismo de los nidos de S. americana no estuvo correlacionado con la densidad vertical de la vegetación local de pradera, la distancia media al borde, la proporción de bosque-pastizal en el ambiente que rodeaba los sitios de estudio (5 a 10 km), la variación geográfica en la abundancia del hospedero, la densidad poblacional de S. americana o la fecha de iniciación de la nidificación. Las frecuencias e intensidades (número de huevos de M. ater por nido parasitado) de parasitismo sólo estuvieron relacionadas significativamente con la variación en la densidad local de hembras de M. ater. El tamaño de la nidada y el éxito de emplumamiento aparente de S. americana estuvieron correlacionados negativamente con los niveles locales de parasitismo. Los patrones de variación geográfica en la abundancia de M. ater entre regiones y dentro de éstas deben ser tenidos en cuenta al establecer áreas de conservation para aves de pastizal y otros hospederos de esta especie parásita.

BIRDS OFTEN EXHIBIT continent-wide range distributions, yet considerable geographic variation in abundance, as exemplified in continental distribution maps of bird abundance from surveys such as the North American Breeding Bird Survey (BBS) (Price et al. 1995). Such geographic variation in abundance may be attributable to variation in local niche availability (Brown et al. 1995). Biogeographic variation in abundance within species also may be related to magnitudes of interspecific interactions (Wiens 1989), such as competition, predation, and brood parasitism.

The Brown-headed Cowbird (Molothrus ater; hereafter "cowbird") is a North American brood parasite that exhibits geographic variation in abundance, which parallels geographic variation in its parasitism frequency on hosts (Hoover and Brittingham 1993, Smith and Myers-Smith 1998, Thompson et al. 2000, Herkert et al. 2003). Geographic variation in cowbird parasitism is especially prominent among grassland-nesting birds. In parts of the Midwest, grassland-nesting songbirds are rarely parasitized (Strausberger and Ashley 1997, Kershner and Bollinger 1998, Robinson et al. 1999, Winter 1999, Peer et al. 2000); whereas those same species are heavily parasitized in the center of the cowbird's distribution in the Great Plains (Elliort 1978, Zimmerman 1983, Koford et al. 2000), where cowbirds are most abundant (Peterjohn et al. 2000). Variation in cowbird parasitism on grassland birds is more clearly related to variation in regional cowbird abundance than the size of remnant grassland patches (Herkert et al. 2003) or intraspecific behavioral responses of hosts to cowbird parasitism (Peer et al. 2000).