Aves de pastizal invernando en México: relaciones de densidad y hábitat//Wintering grasslands birds in Mexico: density and habitat relationships

Las poblaciones de aves de Norteamérica, y en particular las de pastizales, están disminuyendo. En un área de pastizales del noroeste de México, entre 2009 y 2011, estimamos la densidad invernal de tres especies de pastizal: Centronyx bairdii (BAIS), Ammodramus savannarum (GRSP) y Anthus spragueii (SPPI) y su relación con los atributos de la vegetación y cobertura de suelo. Los promedios de densidad calculados para BAIS, GRSP y SPPI fueron 50.8, 76.7 y 12.6 (aves/km2) respectivamente, estas densidades son mayores que las estimaciones en la mayoría de las áreas de invernada en el Desierto Chihuahuense. El test de Kruskal-Wallis, demuestra que existen algunas diferencias en el uso de hábitat entre las tres especies. Las estimaciones de la cobertura y altura de la vegetación para cada especie mostraron algunas diferencias entre años (2010, 2011). Los valores de covariación de Gregorius (C), muestran que las especies estudiadas tienen una correlación significativa (P <0.025) con seis variables de atributos de vegetación invernal (altura y cobertura de pasto y hierba, suelo desnudo, y cobertura de arbusto). Los resultados de este estudio permiten reconocer que las tres especies de aves estudiadas requieren áreas de pastizal en buena condición. Por ello, es necesario establecer estrategias de conservación que mitiguen la alteración de los pastizales del Desierto de Chihuahuense.ABSTRACTBird populations in North America and in particular, grassland populations, are declining. We estimated winter density in three grassland focal species: Centronyx bairdii (BAIS), Ammodramus savannarum (GRSP) and Anthusspragueii (SPPI) and its relationship with vegetation cover between 2009 and 2011, in a grassland area of northwestern Mexico. The density averages calculated for BAIS, GRSP and SPPI were 50.8, 76.7 and 12.6 (birds / km2) respectively, are higher than previous estimations in most wintering areas in the Chihuahuan desert. The Kruskal-Wallis test shows that there are some differences in habitat use between the three species. Estimates of vegetation cover and height for each species showed some differences between years (2010 and 2011). Gregorius covariation values (C) indicate that the studied species have a significant correlation (P <0.025), with six variables of winter vegetation attributes (height and grass and grass cover, bare soil, and bush cover). The results found allow us to recognize that these birds require grassland areas in good condition, so it is necessary to establish conservation strategies that might help mitigate the alteration of the grasslands of the Chihuahuan desert.

Trends in Bird Densities at a Remnant Fescue Grassland in Saskatchewan

Populations of grassland birds in North America have declined greatly in the past five decades. Hypothesized drivers of decline include habitat loss, fragmentation, and adverse impacts from human activities. At a remnant fescue grassland in Saskatoon, Saskatchewan numbers of Savannah Sparrow (Passerculus sandwichensis), Western Meadowlark (Sturnella neglecta), Brewer’s Blackbird (Euphagus cyanocephalus), and Vesper Sparrow (Pooecetes gramineus) have been stable. Numbers of clay-colored Sparrow (Spizella pallida) have increased since the 1960s. Sprague’s Pipit (Anthus spragueii), Upland Sandpiper (Bartramia longicauda), and Burrowing Owl (Athene cunicularia) are no longer present. Baird’s Sparrow (Ammodramus bairdii), Bobolink (Dolichonyx oryzivorus), and Horned Lark (Eremophila alpestris) have become irregular visitors. In the past 50 years, 91 species have been observed displaying territorial behaviour, feeding, nesting, or migrating at this remnant native grassland. With encroaching urban development and increased human influence at the prairie and surrounding area, the impacts on the bird communities at the site in the future are unknown.