Has Worthen's Sparrow had its chips?

Posted by Irene Ruvalcaba Ortega, Ricardo Canales, José I. González and Stuart Marsden

Stu has recently been in Mexico working with research groups from three universities along with a team from Pronatura (the BirdLife partner) on research into some of the country’s most threatened birds. These included the northern population of the Sierra Madre Sparrow Xenospiza baileyi, the Baja endemic Belding’s Yellowthroat Geothlypis beldingi and the pine-loving Thick-billed Parrot Rhynchopsitta pachyrhyncha (all Endangered).

But first up was the Worthen’s Sparrow Spizella wortheni, a charming but Endangered bird of the shrub-grasslands of the northern states of Coahuila and Nuevo León. A major threat to the sparrow is the expansion of potato-growing in the region. Here, Irene Ruvalcaba Ortega, Ricardo Canales, and José I. González from Laboratorio de Biología de la Conservación y Desarrollo Sustentables (UANL) and I talk about the conservation biology of the species, and a project that we hope to initiate that will help secure its near future. 

The Endangered Mexican endemic Worthen's Sparrow Spizella wortheni (Photo: Ricardo Canales)

Worthen’s sparrow, a relative of the semi-migratory Field Sparrow Spizella pusilla, was first described from New Mexico, USA, in the 1880s – this remains the only record of Worthen’s from outside ‘old’ Mexico. The last 100 years or so has seen its range contract so much that it known now from just a few sites in two of Mexico’s northern states. BirdLife’s assessment is that there may be as few as 100 mature individuals - although they point out that there is scope to find new populations in this vast landscape.

Following discussions at the Universidad Autonóma de Nuevo Leon, where much of the recent research on the species has been carried out, we headed out into the field, joined by Hugo Elizondo, a research assistant who has seen more Worthen’s in the wild than anyone. Work by the group has found several new sites for the species and important new breeding areas (Canales-del Castillo et al. 2010). There may be more undiscovered populations – some areas cannot be visited due to security problems associated with Mexico’s cartels.

The main areas where Worthen's Sparrow are known to occur. New sites are being found, but search efforts are hampered by security issues.

The main habitat of the sparrow is centred around large flat basins, very shallow valleys surrounded by higher ground with Juniper bushes holding Bushtits, and Black-chinned and Black-throated Sparrows. On the flat ground are extensive colonies of the Endangered Mexican Prairie Dog Cynomys mexicanus. Also eating grass are cows, sheep, Black-tailed Jackrabbit and Eastern Cottontails. This is one of those landscapes where everywhere looks the same or every hectare of it looks different – depending on how closely you look. Reconstructing the history of human use of these areas, and identifying the main natural and anthropogenic gradients within the area may be one of the keys to understanding the sparrow’s past, present and future. 

Worthen's sparrow in typical nesting habitat (Photo: Ricardo Canales)

We visited some areas of grassland which are used by flocks of Worthen’s in the non-breeding season. They are nomadic and seem to appear in different areas in different years – this may serve them well as the landscape is constantly changing (e.g. Canales-Delgadillo et al. 2012). Areas are ploughed for growing potatoes, some areas we visited were lush after the rain, others inexplicably dry. Next, we saw 10-15 breeding pairs in a small area of great habitat in La Soledad. Nests were relatively easy to locate, placed 5-30 cm above the ground in the centre of Tarbushes Flourensia cernua and Creosotebushes Larrea tridentata, sometimes with Opuntia sp. Research by UANL suggests that nesting success can be low, perhaps due to predation by snakes, birds etc – intensive study of the factors which affect recruitment should clearly be a key component of further study.

Worthen's nesting habitat at Museo de las Aves de Mexico's La India reserve (Photo: Stu)

Over the two days, we also visited some small reserves, and an area of abandoned crops, which still held a pair of sparrows in a tiny remnant patch of habitat. A collaborator on our planned project will be Museo de las Aves de Mexico www.museodelasaves.org, a museum in the historical city of Saltillo, dedicated to birds. We visited their extensive protected area and research site at La India, where several pairs of Worthen’s were found breeding in a bush-grassland habitat subtly different from that in La Soledad. A comparison of ecology and breeding success in the two differently managed areas would be invaluable.
 
Our immediate aim is to get a Mexican PhD student, supported by Masters and Undergraduate students working on Worthen’s and its habitat.

References

Canales-Delgadillo, J.C., Scott-Morales, L. & Korb, J. (2012). The influence of habitat fragmentation on genetic diversity of a rare bird species that commonly faces environmental fluctuations. Journal of Avian Biology 43: 168-176.

Canales-del Castillo, R., Gonzalez-Rojas, J.I., Ruvalcaba-Ortega, I. & Garcia-Ramirez, A. (2010). New breeding localities of Worthen's Sparrows in northeastern Mexico. Journal of Field Ornithology 81: 5-12.