Treesearch

In his review of the overall population status of the southwestern willow flycatcher, Unitt (1987) concluded that "Even if a few hundred pairs persist in New Mexico, the total population of the subspecies is well under 1000 pairs; I suspect 500 is more likely." Since Unitt's review, a substantial amount of information has been collected rangewide on the distribution and abundance of the southwestern willow flycatcher. The 1992 petition to list the flycatcher under the Endangered Species Act and the subsequent proposal by the U.S. Fish and Wildlife Service in 1993 to list the subspecies as...

The continued survival of the southwestern willow flycatcher (Empidonax traillii extimus) is threatened indirectly by the loss, modification, and fragmentation of riparian habitat, and directly by factors that impact the survival and reproductive success of flycatchers. Because the impact of habitat loss on small populations can be particularly severe, we first discuss some of the population-level effects that may be influencing flycatcher population dynamics. We then review some contemporary cases of habitat loss and discuss other factors potentially impacting the flycatcher. The effects...

The extent of riparian habitat in the southwestern United States during the past 100 years appears to have been reduced by modern land development, urban expansion, and a general increase in human populations (Finch 1996; Shaw and Finch 1996). Thousands of acres of river flood plain along the Rio Grande and Colorado rivers have been cleared for agriculture, construction of housing, and industrial development. River channels have been diverted and contained by dams, levies, and other water control structures in order to provide irrigation and drinking water, and to protect this investment...

The distribution and abundance of a species across a landscape depends, in part, on the distribution and abundance of appropriate habitat. If basic resource needs such as food, water, and cover are not present, then that species is excluded from the area. Scarcity of appropriate habitat is generally the key reason for the status of most rare and endangered species. An understanding of an endangered species’ habitat characteristics is crucial to effective management, conservation and recovery.

The willow flycatcher (Empidonax traillii) breeds across much of the conterminous United States and in portions of extreme southern Canada. As might be expected in such a wide-ranging species, willow flycatchers in different portions of the range exhibit differences in appearance, song, and ecological characteristics. The intent of this chapter is to provide information on the breeding-season ecology of the southwestern subspecies, E.t. extimus. However, most ecological studies to date have dealt with other willow flycatcher subspecies. Relatively few studies have been published on E.t....

The willow flycatcher (Empidonax traillii) is a Neotropical migrant that breeds in North America, but winters in Central and northern South America. Little specific information is known about migration and wintering ecology of the southwestern willow flycatcher (E. t. extimus) (Yong and Finch 1997). Our report applies principally to the species as a whole and not just to the endangered subspecies. In this chapter, we describe timing of migration, migration routes, stopover behavior and ecology, winter distribution, and migration and winter habitats of the willow flycatcher species complex....

An understanding of the basic demography of a species is necessary to estimate and evaluate population trends. The relative impact of different demographic parameters on growth rates can be assessed through a sensitivity analysis, in which different parameters are altered singly to assess the effect on population growth. Identification of critical parameters can allow managers to focus their efforts on factors most likely to increase populations.

Brown-headed cowbirds (Molothrus ater) are obligate brood-parasites, that is, female cowbirds lay their eggs in nests of other species. If the cowbird eggs are accepted, the host pair may raise the young cowbird, often at a reduction of the hosts' reproductive success. Cowbird females are also known to remove host eggs and nestlings from nests, which may also affect the reproductive success of the hosts (Smith 1981, Scott et al. 1992, Sealy 1992). It is difficult to assess the impact of parasitism both because reproductive success operates at many levels and because the hosts have defenses...

We used field data, topographical information (elevation, slope, aspect, landform), and Landsat Thematic Mapper imagery to model forest vegetative types to a 10-m resolution on the Kaibab National Forest in northern Arizona. Forest types were identified by clustering the field data and then using a decision tree based on the spectral characteristics of a Landsat image and topographical information to predict the forest types. Significant variables in the models included raw basal area and proportion of basal area by species. Use of additional variables (canopy closure, understory vegetative...

Birds with both eastern and western distributions occur in the Black Hills of western South Dakota. This forest is mostly ponderosa pine (Pinus ponderosa) and is managed for timber. Logging alters forest characteristics and the bird community. We studied habitat relations of breeding songbirds at the stand- and site-level scales in ponderosa pine and quaking aspen (Populus tremuloides)/paper birch (Betula papyrifera) forest. Sixty bird species were observed ≤50 m from count points. Species richness was greater in aspen/birch than in ponderosa pine. Species richness was generally lower in...