Alberto Macías-Duarte

Factors Influencing Habitat Use by Migratory Grassland Birds in the State of Chihuahua, Mexico

ABSTRACT. Breeding populations of grassland birds are declining across North America. Alteration of wintering-ground habitat in Mexico may be a significant causal component in these declines, considering the rapid changes in land use occurring there. We investigated the influence of precipitation, habitat structure, and plant community on the abundance and distribution of grassland passerines in the state of Chihuahua, Mexico. We conducted bird and vegetation surveys during 1998–2007 in 65 plots at two study areas 75 km apart, Sueco and Tinaja Verde. Grassland guild species were the dominant component of the wintering bird community at both study areas. Passerine density at Sueco during winter was directly related to cumulative rainfall from the preceding December to September, a factor known to be strongly correlated with grass production. Herbaceous and woody vegetation structure and habitat type influenced occurrence of migratory grassland birds at the plot level. Nine of 11 study species occurred in a greater proportion of plots at Sueco than at Tinaja Verde, even though grasslands at the latter area were apparently in better condition. Whereas we may have overlooked key factors differentiating habitat quantity in the two study areas, some of the evidence suggests that Sueco is on a migratory route and Tinaja Verde is not. The future of desert grasslands in Chihuahua is in peril because of the ongoing wide-scale conversion to agriculture there and climate-change predictions of lower rainfall in coming decades.

REPRODUCTION, PREY, AND HABITAT OF THE APLOMADO FALCON (FALCO FEMORALIS) IN DESERT GRASSLANDS OF CHIHUAHUA, MEXICO

Abstract A breeding population of Aplomado Falcons (Falco femoralis) remains in the desert grasslands of Chihuahua, Mexico, despite their extirpation in the nearby southwestern United States. We monitored falcon reproduction at 35 territories during 1996-2002, a period of severe drought in the region. To test the influence of prey abundance and vegetation on falcon reproduction, we surveyed avian prey abundance through plot counts in 1998-2002 and characterized vegetation at each plot. Aplomado Falcon productivity declined from 1.57 fledglings per occupied territory in 1997 to 0.63 in 2002 at the larger of two study areas, a trend consistent with cumulative effects of consecutive years of low rainfall. Reproduction in the smaller area remained low throughout the study. Both productivity and incubation start date were significantly associated with prey bird abundance. Summer rain most likely influenced falcon reproduction by affecting seed abundance and therefore abundance of granivorous prey birds the following winter and spring. Falcons nested in open grasslands with sparse woody vegetation, an adaptation likely related to higher prey vulnerability and fewer predators. However, important prey birds were positively correlated with woody-plant density, which suggests that proximity of shrublands increased nest-site suitability.

Association of Habitat Characteristics with Winter Survival of a Declining Grassland Bird in Chihuahuan Desert Grasslands of Mexico

ABSTRACT. Grassland bird populations have shown persistent declines over the past four decades in North America. Possible explanations for the declines include decreased winter survival because of habitat deterioration. To address this hypothesis, we evaluated the relationship between habitat structure and winter survival of Vesper Sparrows (Pooecetes gramineus) in Chihuahuan Desert grasslands of northern Mexico. We radiotagged and monitored the survival of 102 individuals during the winters of 2009 and 2010. We obtained visual estimates of vegetation structure and composition at every individuals radiotelemetry location. We used an exponential regression model to estimate daily survival rates and determine the association between habitat structure and survival. We estimated a daily winter survival probability of 99.1% (95% confidence interval: 97.4–99.7%) for Vesper Sparrows in both years. Our survival analysis suggests that habitat structure is an important predictor of winter survival. Average grass height and shrub height were positively related to Vesper Sparrow survival. Our results suggest that grassland bird populations may be negatively affected by poor grassland conditions during the winter and that low winter survival may be an important factor in population declines. Winter habitat conditions in the Chihuahuan Desert grasslands are shaped primarily by grazing and climate, highlighting the need to improve range management, especially in Mexico, as a means to reverse persistent population declines of grassland birds.

Restoring Aplomado Falcons to the United States

Abstract Captive-bred fledgling Aplomado Falcons (Falco femoralis) were released along the coastal plain of southern Texas (839 birds from 21 sites during 1993–2004) and in the Chihuahuan Desert of western Texas (637 birds from 11 sites in 2002–2011) and southern New Mexico (337 from 10 sites during 2006–2012). The Texas coastal releases yielded two nesting populations: 15–18 pairs near Brownsville and 15 pairs on two islands near Rockport. Habitat in these areas is extensive open savanna, the ancestral condition over most of the region. Today, it is almost entirely dominated by farmland and brushland, the latter harboring the Great Horned Owl (Bubo virginianus), a major predator of the falcons. Conversely, releases in the Chihuahuan Desert have been unsuccessful in establishing a wild population in either area. Although 8–10 pairs were present in West Texas in 2009, by 2011 only one pair remained, and none were found in 2012, the apparent consequence of severe drought. A single pair documented in New Mexico in 2011 was associated with artificial feeding of prey birds. We concluded that the conservation and expansion of Aplomado Falcon populations on the Texas coastal plain will require the protection and management of existing breeding territories, and the creation and management of more brush-free savanna. Persistent drought, the reduction of prey populations, and high rates of mortality from raptor predation appear to preclude the reestablishment of Aplomado Falcons in western Texas or New Mexico. DEVOLVIENDO A FALCO FEMORALIS A LOS ESTADOS UNIDOS Se liberaron volantones de Falco femoralis criados en cautiverio a lo largo de las planicies costeras del sur de Texas (839 aves de 21 sitios durante 1993–2004) y en el desierto de Chihuahua del oeste de Texas (637 aves de 11 sitios durante 2002–2011) y del sur de Nuevo México (337 aves de 10 sitios durante 2006–2012). Las liberaciones en la costa de Texas produjeron dos poblaciones que anidaron: 15–18 pares cerca de Brownsville y 15 pares en dos islas cerca de Rockport. El hábitat de esta área se compone de una extensa sabana abierta, lo que representa la condición ancestral de casi toda la región. Actualmente se encuentra casi completamente dominada por tierras de labranza y matorrales; este último alberga a Bubo virginianus, uno de los mayores depredadores de halcones. Por el contrario, las liberaciones en el desierto de Chihuahua no fueron exitosas en el establecimiento de poblaciones silvestres. Aunque se encontraron 8–10 pares en el oeste de Texas en 2009, para el 2011 sólo se registró un par y no se encontró ninguno en 2012, como consecuencia aparente de una sequía severa. Una sola pareja documentada en Nuevo México en 2011 estuvo asociada con la alimentación artificial de aves de presa. Concluimos que la conservación y la expansión de poblaciones de F. femoralis en las planicies costeras de Texas requerirán de la protección y el manejo de los territorios de cría existentes, y de la creación y manejo de sabanas libres de matorrales. La sequía persistente, la reducción de las poblaciones de presas y las altas tasas de mortalidad debidas a la depredación por parte de rapaces parecen impedir el restablecimiento de poblaciones de F. femoralis en el oeste de Texas o Nuevo México.

The Imminent Disappearance of the Aplomado Falcon from the Chihuahuan Desert

Desaparicion Inminente de Falco femoralis Del Desierto Chihuahuense Seguimos la ocupacion territorial de Falco femoralis durante un periodo de 12 anos (2003–2014) en el centro de Chihuahua, Mexico, bajo condiciones de sequia severa y destruccion de su habitat de anidamiento. La conversion de los territorios de cria de los halcones a tierras agricolas ha destruido hasta ahora 17 de los 35 territorios de cria ocupados por parejas de F. femoralis conocidos al menos una vez desde el ano 2000. Solo una pequena fraccion territorio de los restantes fue ocupada en 2014. La desaparicion de los halcones de los territorios de cria antiguos que se mantienen intactos sugiere un deficit de reclutamiento causado por tasas reproductivas bajas relacionadas con una disminucion general en la calidad de los pastizales y en la cantidad de presas. La persistencia de unas pocas parejas sugiere que la poblacion se mantiene actualmente por la elevada supervivencia de los adultos mas que por su fertilidad. Esta tendencia, asi como...

Seed size, bill morphology, and handling time influence preferences for native vs. nonnative grass seeds in three declining sparrows

ABSTRACT The invasion of exotic grasses is a potential threat to the winter habitat of migratory grassland birds by reducing native sources of seed food. We compared seed preferences among 3 native (blue grama [Bouteloua gracilis], sideoats grama [Bouteloua curtipendula], and green sprangletop [Leptochloa dubia]), and 3 exotic (Lehmann lovegrass [Eragrostis lehmanniana], buffelgrass [Pennisetum ciliare], and natal grass [Melinis repens]) grass seeds in captive Bairds (Ammodramus bairdii), Grasshopper (A. savannarum), and Savannah (Passerculus sandwichensis) sparrows to investigate factors that determine vulnerability of grassland passerines to exotic grasses in the Chihuahuan Desert. We hypothesized that seed handling time would determine the ability of sparrows to exploit exotic grass seeds, and that larger-billed birds would be able to better exploit a larger variety of seeds, including exotic seeds. We offered seeds in choice and nonchoice trials and determined handling times for the different seed and bird species. The results indicate that handling time in relation to seed size determined seed preferences. Sparrows preferred seeds they could handle more efficiently to maximize energy intake over time. Bairds and Savannah sparrows, with intermediate and small bill sizes, respectively, preferred natal grass seeds of intermediate size and short handling time, and Grasshopper Sparrows, with the largest bill, preferred the larger sideoats grama seeds. Lehmann lovegrass, the smallest seed with intermediate handling time, was avoided by the 3 sparrows. Buffelgrass, the largest seed with the longest handling time, was avoided by Bairds and Savannah sparrows. Blue grama, the second-smallest seed with relative short handling time, was neither preferred nor avoided, and green sprangletop, an intermediate-sized seed with a relatively longer handling time, was avoided in the choice trials but consumed in the nonchoice trials. These results indicate that exotic grass seeds may be a source of food for 3 sparrows wintering in the Chihuahuan Desert, provided they can be handled efficiently. Our results also show, however, that wintering grassland sparrows are probably unable to consume sufficient buffelgrass and Lehmann lovegrass seeds to meet daily energy requirements, indicating that these grasses may be a threat by reducing exploitable sources of seed food.

Spatial patterns in hydrogen isotope ratios in feathers of Burrowing Owls from western North America

ABSTRACT Deuterium (2H) has been used to track movements of land birds, under the assumption that δ2H in precipitation (δ2Hp) and δ2H in bird feathers (δ2Hf) are correlated across broad geographic gradients. The nature of this correlation has been evaluated only in a small percentage of the birds that breed in North America. We sampled Western Burrowing Owl (Athene cunicularia hypugaea) feathers of known origin (nestling feathers) at 36 locations throughout North America (from southern Canada to central Mexico). We used a modification of the “comparative equilibrium” technique of Wassenaar and Hobson (2003) to measure the δ2H of nonexchangeable hydrogen in feather samples. We characterized the strength of the relationship between δ2Hf and amount-weighted mean annual δ2Hp in a raptor that breeds in arid grasslands and deserts throughout western North America. We used a Bayesian hierarchical approach to model the δ2Hf–δ2Hp relationship, accounting for levels of intrinsic and extrinsic variation in δ2Hf. We found a linear relationship between δ2Hf and δ2Hp (δ2Hf = −13.48 + 0.78 δ2Hp; 95% credible interval (slope): 0.55–1.01) and used this relationship to construct a feather deuterium isoscape map. We observed relatively high levels of variation in mean δ2Hf across locations (SD = 11.01‰), due in part to variation in the contribution of precipitation to local food webs, and substantial variation among individuals within locations (SD = 6.68‰). Our data demonstrate that δ2Hf of juvenile Burrowing Owls can be used to infer local amount-weighted mean annual δ2Hp from the location of origin. Deuterium remains a valuable tool for tracking continental-scale raptor movements, with the caveat that researchers must identify and model for potential discontinuities in the δ2Hf–δ2Hp relationship in their inferences. However, isotopic discontinuities, coupled with a high relative abundance of individuals in those areas, can seriously hinder the usefulness of deuterium for identifying the origin of individual birds.

HOME RANGE AND HABITAT USE OF WINTERING VESPER SPARROWS IN GRASSLANDS OF THE CHIHUAHUAN DESERT IN MEXICO

Abstract Grassland birds are declining at an alarming rate and habitat limitation on the wintering grounds may play a major role in population declines. In an attempt to understand habitat needs and constraints of grassland bird species on their wintering grounds, we conducted the first-ever telemetry study to document the movements of a wintering grassland passerine bird in grasslands of the Chihuahuan Desert in northern Mexico. We attached radio-transmitters to 99 Vesper Sparrows in January of 2009 and 2010 and tracked them for up to 51 days. We estimated vegetation characteristics at foraging, roosting and random locations. We used kernel density estimators of the utilization distribution for each individual sparrow to estimate home range size. We found differences in average home range size between 2009 and 2010 (108.46 ± 36.43 ha and 30.91 ± 4.74 ha, respectively). Home ranges showed high levels of intraspecific overlap, with average pairwise overlap of 0.41, 0.70 and 0.79 among individuals at three study sites, as measured by the Bhattacharyyas affinity. Grasses and shrubs were shorter in foraging locations than in random sites within Vesper Sparrow core habitat, but we found no differences in grass cover and shrub cover. We suggest that movements of Vesper Sparrows are mostly limited to <1 km2 during the winter and hypothesize that these movements are subject to constraints by food limitation and predation.