Worthen's Sparrow (Spizella wortheni) in the Northern Mexican Plateau

THE SOUTHWESTERN NATURALIST 53(1):91–95 MARCH 2008 WORTHEN’S SPARROW (SPIZELLA WORTHENI) IN THE NORTHERN MEXICAN PLATEAU LAURA SCOTT-MORALES,* JORGE NOCEDAL, MAURICIO COTERA, AND JULIO CANALES-DELGADILLO Facultad de Ciencias Forestales, Universidad Autónoma de Nuevo León, Carretera Nacional km 145, 67700 Linares, Nuevo León, México (LSM, MC, JCD) Centro Regional Durango, Instituto de Ecologı́a, A.C. Km 5 Carretera Durango-Mazatlán esq. Blvd. Los Remedios s/n, 34100 Durango, Durango, México (JN) Present address of JN: Platte River Whooping Crane Trust, 6611 West Whooping Crane Drive, Wood River, NE 68883 *Correspondent: lscott@fcf.uanl.mx ABSTRACT—Worthen’s sparrow (Spizella wortheni) is probably one of the least known sparrows of North America. Its breeding range is uncertain, as there are questionable records from several localities in Mexico and from the type locality near Silver City, New Mexico, in the United States. Currently, the only unquestionable records are from Coahuila and Nuevo León. Here we describe new localities for this species from the state of Nuevo León and confirm already known localities from Coahuila. These new records were gathered during a bird-monitoring program within colonies of the Mexican prairie dog (Cynomys mexicanus) in northeastern Mexico. We surveyed seven grassland areas and recorded Worthen’s sparrow at four of them; Los Angeles, La Perforadora, La Soledad, and El Guerrero. Our records add more detail to the known distribution of this species and our behavioral observations suggest they breed in all four localities. RESUMEN—El gorrión de Worthen (Spizella wortheni) es quizá una de las especies de gorrión menos conocida de América del Norte, más aún su distribución es incierta, ya que existen registros cuestionables en diferentes localidades en México y la localidad tipo en Silver City, Nuevo México, USA. Actualmente los registros confirmados provienen sólo de los estados de Coahuila y Nuevo León. Nuestros registros confirman esta especie en el estado de Coahuila y se presentan nuevas localidades para el estado de Nuevo León. Los registros nuevos se obtuvieron durante un programa de monitoreo de aves en sitios habitados por el perrito llanero mexicano (Cynomys mexicanus) en el noreste de México. Se monitorearon 7 áreas de pastizales y se obtuvieron registros del gorrión de Worthen en cuatro de ellas: Los Angeles, La Perforadora, La Soledad y El Guerrero. Nuestros registros añaden más detalle a la distribución conocida del gorrión de Worthen y nuestras observaciones de comportamiento sugieren que se reproduce en las 4 localidades. Worthen’s sparrow (Spizella wortheni) is probably one of the least known sparrows of North America (Wege et al., 1993; Rising, 1996; Beadle and Rising, 2002). Its breeding range is uncertain as there are dubious records from the Mexican states of Veracruz and Puebla, which were listed in the original breeding distribution (Miller et al., 1957). More recently, those records were regarded as belonging to migratory birds (Wege et al., 1993; American Ornithologists’ Union, 1998), although Howell and Webb (1995) assumed those populations likely were extirpated. In addition, there is another described subspecies (S. w. browni) from western Zacatecas (Webster and Orr, 1954), but it is not recognized (Howard and Moore, 1991). Even the type locality of Silver City, New Mexico, is considered as accidental (American Ornithologists’ Union, 1998) as it lies well outside of its former distributional range. The type specimen, described by Ridgway (1884), is the only record in the United States and has never been recorded again (Wege et al., 1993; BirdLife International, 2000). The more-likely distribution is southern Coahuila, western Nuevo León, northwestern San Luis Potosı́ as the main range with an isolated population in western Zacatecas. At present, the only unquestionable records are from Coahuila and Nuevo León (Wege et al., 1993; Behrstock et al., 1997). All confirmed The Southwestern Naturalist swna-53-01-12.3d 3/1/08 00:59:55 91 92 vol. 53, no. 1 The Southwestern Naturalist records for Worthen’s sparrow in the past 10 years are from Las Esperanzas and San Rafael in Nuevo León, and Tanque de Emergencia in Coahuila (Wege et al., 1993; Behrstock et al., 1997; BirdLife International, 2000). All recent records are from gypsophilous grasslands (i.e., grasslands tolerant of high concentrations of gypsum) mixed with arid scrubs. Here, we describe new localities from the states of Coahuila and Nuevo León. All these new localities were discovered while conducting a bird monitoring program in Mexican prairiedog (Cynomys mexicanus) colonies (Scott-Morales et al., 2004). MATERIALS AND METHODS—We conducted bird surveys during 2002–2005 within colonies of the Mexican prairie dog in seven grassland areas in northeastern Mexico. Occasionally, we recorded Worthen’s sparrow in gypsophilous grasslands, mixed with desert scrub. Generally, the area is between 24u709–25u159N and 100u159–101u209W. Elevation was 1,850–2,000 m. We used point counts on line transects to monitor populations of birds. Transects were 1,000 m in length and centered on clonies of prairie dogs. At each transect, point counts were made every 200 m and lasted 10 min. RESULTS—We recorded Worthen’s sparrow at four of seven grassland areas surveyed; Los Angeles, La Soledad, La Perforadora, and El Guerrero (Fig. 1). We recorded breeding behavior, such as counter-singing behavior by territorial males and food carrying by one pair, although we never saw these birds feeding young. We recorded 8 birds at El Guerrero, 17 at Los Angeles, 27 at La Perforadora, and 2 at La Soledad. Worthen’s sparrows showed a clear preference for ecotones between gypsophilous grasslands and desert scrub dominated by creosotebush (Larrea tridentata) and tarbush (Flourensia cernua). At La Perforadora, this sparrow was associated with scattered patches of one-seeded junipers (Juniperus monosperma) within grasslands. Description of El Guerrero—Vegetation at this site was composed of two strata; the shrub layer was mostly creosotebush and tarbush at heights of 1.5–2.0 m, with an understory of Parthenium incanum, Lycium berlandieri, and Krameria ramosissima. The shrub layer was open with canopy plants rarely touching each other. The herbaceous layer was dominated by the rhizomatous buffalograss (Buchloe dactyloides) and burrograss (Scleropogon brevifolius), with ground cover of #22%. Grasses usually were patchily distributed, although they sometimes formed a continuous carpet. Some cacti, such as cholla (Opuntia imbricata), added heterogeneity to the habitat. Description of La Soledad—Desert scrub was throughout the plains or in hills with gentle slopes and also was dominated by creosotebush and tarbush, with shrub cover of 15–20%. Gypsophilous grasslands extended over the center of the valley and were composed of grasses that rarely were taller than 15 cm. Dominant grasses were the endemics Muhlenbergia villiflora var. villiflora and Bouteloua chasei. Some common species were burrograss and fluffgrass (Erioneuron pulchellum 5 Dasyochloa pulchella). Description of La Perforadora—Dominant grasslands were the endemic Muhlenbergia villiflora var. villiflora, associated with burrograss and buffalograss with ground cover of 20–25%. At the edge of the grasslands, a rosette scrub developed in coarser soils, with species such as Yucca carnerosana, Dasylirion texanum, Nolina caespitifera, Agave lecheguilla, and A. striata; plant cover rarely exceeded 15%. At some points in the hills, there were dense patches of pine-juniper woodland (Juniperus monosperma–Pinus pseudostrobus) in association with oaks (Quercus striatula), but never covering .15% of the area. Description of Los Angeles—At the intermountain valley, dominant plant communities were native grasslands mixed with stem-succulent scrub with species such as prickly-pear. Native common grasses were buffalograss (Buchloe dactyloides), blue gramma (Bouteloa gracilis), sand muhly (Muhlenbergia arenicola), creeping muhly (Muhlenbergia repens), hack (Stipa clandestina), vine mesquite (Panicum obtusum), three-awn (Aristida), and wolftail (Lycurus phleoides). Bunch-grasses were mostly distributed on slopes of the mountains, forming a relatively homogenous plant community with dense plant cover of 25–35% that decreased as slope and height on hills increased. Dominant bunchgrasses were side-oats gramma (Bouteola curtipendula), hairy gramma (B. hirsuta), and species of Muhlenbergia and Nasella. Yucca dominated high slopes of mountains with heights of 4–5 m; Yucca carnerosana was the most-typical species. At some less-steep slopes, some stem-succulent and rosette-shrubs were dominant with a herbaceous layer of introduced grasses. The Southwestern Naturalist swna-53-01-12.3d 3/1/08 00:59:56 92 March 2008 Scott-Morales et al.—Worthen’s sparrow in northern Mexico 93 FIG. 1—Location of grassland areas of the Mexican Plateau where Spizella ortheni was recorded. Dots represent historical records of the species (Wege et al., 1993). The Southwestern Naturalist swna-53-01-12.3d 3/1/08 00:59:56 93 94 vol. 53, no. 1 The Southwestern Naturalist DISCUSSION—Our study confirms the presence of Worthen’s sparrow at three new localities within the northern Mexican Plateau; El Guerrero, Los Angeles, and La Soledad. Additionally, we verified its occurrence at La Perforadora, also known as Tanque de Emergencia, where it had been recorded previously (Wege et al., 1993; Behrstock et al., 1997). The only published records of the distribution of Worthen’s sparrow are from the Mexican states of Coahuila and Nuevo León; i.e., Tanque de Emergencia, San Rafael, and Las Esperanzas (Wege et al., 1993; Behrstock et al., 1997). Our records increase the knownl range of this species in these two Mexican states and our observations on its behavior suggest that it breeds in all four localities, especially at El Guerrero where we recorded evidence of territorial singing and a pair carrying food. Nesting has been described previously at La Perforadora (5Tanque de Emergencia; Wege et al., 1993) and Las Esperanzas (Behrstock et al., 1997), and there was evidence of breeding activity in Zacatecas (Webster and Orr, 1954). Although often described as a specialist of mesquite-juniper-grassland habitat (Wege et al., 1993; BirdLife International, 2000), habitat requirements of Worthen’s sparrow show at least two different habitats for breeding. In the western part of its range, in western Zacatecas, breeding activity was recorded in the pine-oak/mesquitegrassland ecotone at 2,350–2,450 m elevation (Webster and Orr, 1954), whereas in the eastern part of its range, in southeastern Coahuila and western Nuevo León, breeding activity has been recorded more frequently at the yucca-juniper/ grassland ecotone (Wege et al., 1993) and arid shrub-grassland community (Behrstock et al., 1997). Use of other habitats, such as desert scrub, have been recorded only during the non-breeding season (Wege et al., 1993). We detected breeding activity, such as territorial singing and food carrying by a breeding pair, in a desert-scrub habitat dominated by creosotebush and tarbush, especially at El Guerrero. As in previous studies, our observations of Worthen’s sparrows support its preference for ecotones between desert-scrub dominated by tarbush and creosotebush with rather low cover and open grasslands. Furthermore, all known localities for this species on the Mexican Plateau are within the distribution of Mexican prairie dogs (Cynomys mexicanus). This strong association is restricted to intermountain valleys with gentle slopes and elevations of 1,800–2,300 m, at least in the eastern part of its range. Conservation Issues—Habitat destruction in favor of croplands is the most serious threat to the already shrinking range of Worthen’s sparrow. At the western part of its range, transformation of mesquite-juniper-grassland habitat, developed in deep red soils rich in cooper, to crops is a major threat as Zacatecas is the largest bean producer in Mexico. Conversely, the international potato chip industry is acquiring intermountain valley grasslands to grow potatoes, leading to a dramatic loss of habitat, specifically in Nuevo León (ScottMorales et al., 2004). Although Worthen’s sparrow does not inhabit these grasslands where colonies of prairie dogs thrive, this sparrow depends highly on these grasslands and neighboring desert scrub. Based on our preliminary data, future studies will focus on ecology of Worthen’s sparrow at selected localities where they are likely to breed, and also determine the species geographical distribution throughout its range, with special emphasis on western Zacatecas. Field research was supported by The Nature Concervancy (MBP/PW 010703) and the University of Nuevo León (CN 926-04) SEMARNAT-CONACyT (C01-0700). We thank field assistants R. Hernández, C. Doria, F. Heredia, and I. Moran. Constructive criticisms of anonymous reviewers were both helpful and appreciated. LITERATURE CITED AMERICAN ORNITHOLOGISTS’ UNION. 1998. Check-list of North American birds. Seventh Edition. American Ornithologists’ Union, Washington, D.C. BEADLE, D., AND J. RISING. 2002. Sparrows of the United States and Canada. 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