Raven Abundance Research Articles

DRC-1339 Egg Baits: Preliminary Evaluation of Their Effectiveness in Removing Ravens

I measured the preliminary effects of chicken egg baits treated with DRC-1339 on removing common ravens for the purpose of reducing raven depredation of prairie grouse nests in northeastern Nevada. Greater sage grouse and Columbian sharp-tailed grouse are game species that are declining in distribution and abundance throughout their historic range. Reduced nest success due to egg depredation by ravens is thought to be an important cause of nest failure. Ravens are subsidized nest predators that have substantially increased in abundance throughout their range within the past 25 years. During 2002-2004, I removed ravens from a treatment site using chicken egg baits treated with DRC-1339 to protect sage grouse and a nascent population of reintroduced Columbian sharp-tailed grouse in northeastern Nevada. I performed raven surveys at the treatment site and 2 control sites, located 22 and 53 km northwest of the treatment site, to measure the effects of using DRC-1339. Preliminary analyses indicate chicken egg baits treated with DRC-1339 significantly reduced raven densities at the treatment site, and change in densities over time was different at the treatment site than the control sites (F = 10.212,59, P < 0.001). Also, anthropogenic resource subsidies (i.e., power-lines, roads, agricultural fields, and public landfills) appeared to influence raven abundance. If lethal removal of ravens is implemented as a short-term management action to protect areas from ecological or economical damage, the technique described here is valuable to managers.

COMMON RAVEN JUVENILE SURVIVAL IN A HUMAN-AUGMENTED LANDSCAPE

Anthropogenic resource subsidies have contributed to the dramatic increase in the abundance of Common Ravens (Corvus corax) in the western Mojave Desert, California, during the past 30 years. To better understand the effects of these subsidies on raven demography, we examined whether survival to juvenile departure from the natal territory could be predicted by a set of environmental and morphological variables, such as nest proximity to anthropogenic resources and juvenile condition. We captured 240 juvenile ravens over 2 years and marked them prior to fledging. Nest proximity to anthropogenic resources and earlier fledging dates significantly predicted raven juvenile survival to departure from the natal territory. The best-fitting mark-recapture models predicted postdeparture survival as a function of time since fledging, nest proximity to anthropogenic resources, and year hatched. The positive effect of nest proximity to anthropogenic resources influenced postdeparture survival for at least 9 months after fledging, as revealed by the mark-recapture analysis. Annual survival was 47% for first-year, 81% for second-year, and 83% for third-year birds. Our results support the hypothesis that anthropogenic resources contribute to increasing raven numbers via increased juvenile survival to departure as well as increased postdeparture survival. We expect raven numbers to grow in concert with the growing human presence in the Mojave Desert unless raven access to anthropogenic resources is diminished. La Sobrevivencia Juvenil de Corvus corax en un Paisaje Mejorado por Humanos Resumen. Los subsidios de recursos antropogénicos han contribuido al aumento dramático de la abundancia del cuervo Corvus corax en el occidente del desierto de Mojave durante los últimos 30 años. Para entender los efectos de estos subsidios en la demografía del cuervo, examinamos si la sobrevivencia hasta la salida de los juveniles del territorio natal podría ser predicha por un conjunto de variables morfológicas y ambientales, tales como su cercanía a los recursos antropogénicos y la condición de los juveniles. En dos años capturamos 240 cuervos juveniles y los marcamos antes que desarollaran plumas de vuelo. La cercanía del nido al punto de subsidio más cercano y las fechas tempranas de emplumamiento predijeron significativamente la sobrevivencia de los juveniles hasta su salida del teritorio natal. Los modelos de marcaje y recaptura más adecuados predijeron la sobrevivencia posterior a la salida del territorio como una función del tiempo desde el momento de emplumamiento, la cercanía al punto más cercano de subsidio y el año de nacimiento. Según el análisis de marcaje y recaptura, el efecto positivo de la cercanía a subsidios antropogénicos influye en la sobrevivencia después de la salida del territorio hasta por lo menos 9 meses después del emplumamiento. La sobrevivencia anual fue de 47% en el primer año, de 81% en el segundo año y de 83% para aves en su tercer año. Nuestros resultados apoyan la hipótesis de que los recursos antropogénicos contribuyen al crecimiento de los números de cuervos aumentando la sobrevivencia juvenil antes y después de su salida de los territorios natales. Esperamos que los números de cuervos aumenten en relación con la presencia humana en el desierto de Mojave a menos que su acceso a los recursos antropogénicos sea disminuido.

COMMON RAVEN NESTS IN EASTERN KENTUCKY

Although ravens were once widespread throughout Appalachia they are now considered threatened or endangered in many states of the region. We document a nesting pair of common ravens in an area of southeastern Kentucky nearly 50 km northwest of traditional nesting sites. Further, we suggest several factors that may have influenced the patterns of raven abundance in the state and offer management recommendations to assist their recovery.

The effect of management for red grouse shooting on the population density of breeding birds on heather‐dominated moorland

Summary Breeding birds, vegetation and moorland management were surveyed in 320 1‐km squares on 122 estates in upland areas of eastern Scotland and northern England where red grouse shooting is a widespread land use. We assessed whether population densities of 11 species of breeding birds differed between heather‐dominated moorland managed for red grouse shooting and other moorland with similar vegetation. We classified estates that had a full‐time equivalent moorland gamekeeper as grouse moors. The mean density of red grouse shot per year was four times higher and the mean density of gamekeepers was three times higher on grouse moors than on other moors. Rotational burning of ground vegetation covered a 34% larger area on grouse moors than on other moors. Selection of heather‐dominated squares resulted in similar composition of vegetation on grouse moors and other moors (about 76% heath, 12% grass, 8% bog, 2% flush and < 1% bracken on both types). However, grouse moors tended to have less tall vegetation than other moors and differed significantly in some other characteristics of the vegetation, topography and soil type. Densities of breeding golden plover and lapwing were five times higher and those of red grouse and curlew twice as high on grouse moors as on other moors, while meadow pipit, skylark, whinchat and carrion/hooded crow were 1·5, 2·3, 3·9 and 3·1 times less abundant, respectively, on grouse moors. The differences in density between moorland types remained significant (P < 0·001) for golden plover and crow and approached significance (P < 0·10) for lapwing and meadow pipit after allowing for variation among regions. We used Poisson regression models to relate bird density to vegetation cover, topography, climate and soil type. After adjusting for significant effects of these habitat variables, significant differences in bird density between the two moorland types remained for six species, although their magnitude was reduced. Correlations of adjusted bird density with measures of different aspects of grouse moor management provided evidence of a possible positive influence of predator control (assessed using crow density) on red grouse, golden plover and lapwing. The control of crows by gamekeepers is the most probable cause of the low densities of crows on grouse moors. There was evidence of a positive effect of heather burning on the density of red grouse and golden plover and a negative effect on meadow pipit. Multiple Poisson regression indicated that predator control and heather burning had significant separate effects on red grouse density. Significant relationships between adjusted breeding bird densities and the abundance of raptors and ravens were few and predominantly positive. The results provide correlative evidence that moorland management benefits some breeding bird species and disbenefits others in ways that cannot readily be explained as effects of differences in vegetation type or topography. However, experimental manipulations of numbers of some predators and heather burning are required to test these findings.

Buzzards Buteo buteo and Ravens Corvus corax in the uplands of Britain: limits to distribution and abundance

The current breeding ranges of Buzzards Buteo buteo and Ravens Corvus corax in Britain are restricted to the west, and both are relics of former widespread but not necessarily even distributions. Moore (1957) attributed the Buzzard's contraction in range to persecution by gamekeepers, his evidence being the complementary nature of the Buzzard's distribution with those of gamekeepers in the lowlands and grouse moors in the uplands; others have attributed the Raven's decline to the same cause. Is Moore correct and, if so, are these species' ranges still limited by persecution? Here we adopt an approach similar to Moore's, whereby we compare the current geographical patterns of abundance of Buzzards and Ravens in the British uplands with that of an index of grouse moor density. Unlike Moore, however, we take into account climate, land cover, topography and sheep stocking density, as well as grouse moor distribution, using multiple logistic regression modelling. We show that in the uplands the distribution of grouse moors strongly limits that of Buzzards and Ravens but are unable to conclude that this is a result of persecution because there are several competing hypotheses (moor management, food and nest‐site availability) between which we are unable to distinguish.

Environmental Influences on Roost Selection in Wintering Ravens at Churchill, Manitoba, Canada

A large raven (Corvus corax) population was studied over two winters at Churchill, Manitoba, to determine the relationship between roost selection, wind speed, and air temperature. A preferred roost was located in the eaves of an abandoned building which provided protection from prevailing winds. Raven abundance at the preferred roost site had a positive correlation with wind speed and a negative correlation with air temperature. Overall, the number of birds roosting increased with windchill. The preferred roost site may be a critical component in the overwinter survival rate of ravens on the southwestern coast of Hudson Bay.