New distributional records for the endagered worthen's sparrow Spizella wortheni in San Luis Potosí, Mexico

Nuevos registros de distribución del gorrión de Worthen Spizella wortheni en San Luis Potosí, México

Julio C. Canales-Delgadillo,¹ Leonardo Chapa-Vargas,¹ Jorge A. Carlos Gómez² y Jachar Arreola Aguirre²

¹ División de Ciencias Ambientales, Instituto Potosino de Investigación Científica y Tecnológica A. C. Camino a la Presa San Jose 2055. C.P. 78218, San Luis Potosí, México. <julio.canales@ ipicyt.edu.mx> Corresponding author: <lchapa@ipicyt.edu.mx>

Recibido: 21/03/2014.
Aceptado: 23/02/2015.

RESUMEN

El rango de distribución restringido y el pequeño tamaño poblacional del gorrión de Worthen han ocasionado que esta ave sea considerada una especie amenazada en México y a nivel internacional. Aquí reportamos nuevos registros de esta ave los cuales expanden su rango de distribución conocido en los últimos cuarenta años y abren la posibilidad del uso de hábitats adicionales que no habían sido documentados anteriormente. Por lo tanto es importante realizar esfuerzos para documentar posible actividad reproductiva en estos hábitats.

Palabras clave: Gorrión de Worthen, Altiplano, conservación, manejo, rango de distribución.

Among the Mexican Avifauna, the Worthen's sparrow (Spizella wortheni) is considered one of the rarest passerines. Its distribution range has been severely reduced presumably as a consequence of land use changes (Wege et al. 1993), which have modified the structure and function of ecosystems associated with arid and semiarid areas of the Mexican High Plateau.

Recent studies have focused on the genetic structure of the remaining populations, and how this bird copes with habitat loss (Canales-Delgadillo et al. 2012). The previous results, in addition to reporting behavioral habits of the species, suggested the possibility of the existence of unknown populations in other areas. The Worthen's sparrow, as other birds from arid and semiarid areas, shows nomadic behavior (Canales-Delgadillo et al. 2012). This implies that individuals constantly move across landscapes searching for areas having the best conditions to carry out all their activities, including feeding, mating, nest construction, egg laying and hatching (Anderson 1980; Dean, 1997, 2004). This behavior could facilitate the establishment of populations in areas not previously occupied, abandonment and recolonization of previously occupied sites and partial isolation of these newly established populations for a number of generations. Nevertheless, these processes could also benefit the species by promoting gene flow between populations and the increase of the overall genetic variability. On the other hand, nomadism might also complicate the identification of the entire distribution range of the species.

Here we report new distributional findings of the Worthen's sparrow in San Luis Potosi, Mexico. These records constitute the southernmost sightings recorded in the Mexican High Plateau in the last 40 years.

We conducted bird surveys from August to November 2012 and from March to November 2013 in the municipalities of Catorce and Charcas, in an area known as the "Zona Ixtlera" of San Luis Potosi (Fig. 1) that is part of Wirikuta, a protected area which possess a high biodiversity value because endangered plant species from the Cactaceae family and others occur. Nine sampling sites were established within three ejidos locally known as "Guadalupe Victoria", "Presa de Santa Gertrudis" and "La Cardoncita" based on vegetation characteristics (Table 1). Vegetation types in the region include 1) "Izotal": rosetophil shrublands dominated by arboreal elements of the Yucca genus and a combination of creosote bush (Zarrea tridentata) and tarbush (Fluorensia cernua) as the dominant species at the shrub layer, and 2) microphylous shrublands dominated by creosote bush in which the tree layer has been eliminated; for several decades, local inhabitants extracted the fiber from Yucca trees located in the immediacies of small villages to manufacture ropes. More recently, this activity has been abandoned, but intense goat grazing now inhibits tree establishment (Authors pers. obs.).

Bird surveys consisted of operating 21 12 m x 2.5 m mist nets during three consecutive days and conducting line transects of approximately 500 m in length in each of the nine study sites from sunrise until approximately 1100 GMT/UTC.

We heard and sighted three Worthen's sparrow males during the morning of 10 and 11 May 2013. All three individuals were located in an area of Izotal dominated by Yucca spp. and creosote bush where the shrub height ranged from 1.4 to 1.6 m. The same dates we captured three individuals in nets; two on 10 May, and one on 11 May. Two more Worthen's sparrows were captured at the same site on 25 and 27 October, 2013, and one more individual was captured in the same site a year later, on 19 June 2014. These captures allowed us to confirm the identity of the species (Fig. 2). We measured and weighed the captured birds and conducted visual exploration for sexing as recommended by Ralph et al. (1993) (Table 2). One of the captured individuals in May had an enlarged cloacal protuberance, suggesting possible reproductive activity. All three captured individuals were marked with aluminum leg bands, provided by the Secretariat of the Environment and Natural Resources (SEMARNAT), and released. On subsequent surveys, a single Worthen's sparrow male was heard and sighted in the afternoon of 1 June 2013 in an area of microphylous shrubland which has been degraded by Yucca spp. extraction and goat overgrazing. This area is dominated almost exclusively by creosote bush but with scattered presence of tarbush. On 10 July 2013, we sighted three more Worthen's sparrows in an area located near the ejido "La Cardoncita". These birds were singing males perched on Yucca spp. trees and creosote bushes on both sides of a rural road.

Our findings have several implications regarding habitat associations of the species and to prioritize conservation planning. First, the Worthen's sparrow may be able to use more habitat types than previously reported (see Scott-Morales et al. 2008; Canales-Delgadillo & Scott-Morales 2012). This species is considered to be closely associated to habitats where Mexican prairie dog (Cynomys mexicanus) colonies exist, and therefore this bird was regarded as highly restricted to the shrub-grassland areas that these mammals inhabit (Scott-Morales et al. 2008). The records we report here, and those made by Valdés and Riuz correspond to sites where there are neither colonies of the Mexican prairie dog nor areas of association between grasslands and shrubs. This result opens the possibility that shrublands dominated by Yucca spp. and/or tall individuals of creosote bush and tarbush could provide habitat for this endangered species. However, whether Izotal and microphylous shrubland landscapes with moderate to intense disturbances created by anthropogenic activities in the region provide habitat for this species is a hypothesis that still needs to be evaluated. Our sightings may have been related to various possibilities such as nomadic movements of some individuals, dispersal irruptive movements, range expansion resulting from anthropogenic disturbances, or simply because in the past not enough ornithological surveys had been conducted in the region. The only evidence we have so far in favor of the range expansion due to anthropogenic disturbances consists of the presence of singing males and one individual with a cloacal protuberance which suggests there could be more breeding sites than those currently known (Canales-Delgadillo et al. 2007; Garza de-León et al. 2007; Canales-del Castillo et al. 2010), and that the records presented here were made both within the breeding and non-breeding seasons, but no direct breeding evidence has been obtained so far. Secondly, the presence of the Worthen's sparrow in Wirikuta enhances the importance of this protected area significantly. The Worthen's sparrow joins more than 60 resident and migratory bird species that inhabit in the area (Garza-Hurtado 2011). Finally, increasing our understanding of actual current distribution range and location of possible unknown populations is necessary in order to prioritize conservation actions. Efforts to conserve and effectively manage this area should be implemented in order to preserve the habitat needed by this endemic bird and other species.

ACKNOWLEDGMENTS

We thank the useful comments and suggestions of two anonymous reviewers to improve this report and to the public database eBird for publishing information on the records made by Rene Valdés and Cinthya Ruiz Heredia.

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