Seasonal-Changes in the Habitat Distribution of Transient Insectivorous Birds in Southeastern Arizona: Competition Mediated

The distribution and abundance of 26 migratory insectivorous bird species were recorded over an elevational habitat gradient in the Chiricahua Mountains, Arizona for the spring and fall migratory seasons. Most of the species used this area only during migratory passage, and 54% exhibited significant shifts in the habitats occupied from spring to fall. The majority (69%) of species also exhibited significant changes in density within habitats between seasons. Using pairwise correlations of bird densities from 7 habitat types and 2 seasons, I identified 5 groups that contained species whose seasonal distributional patterns were similar to one another but independent and distinct from members of the other 4 groups. Despite independence among groups in the seasonal patterns of habitat distribution, the combined density of all species was significantly positively correlated with a measure of food availability taken from each of the habitat types in each migratory season. Consequently, the spring-to-fall change in insect density within each habitat also was significantly correlated with the seasonal change in bird density over each of the habitat types. The hypotheses that best explain these correlations include that in which competitive adjustments among the migratory birds enable a close match to food resource availability and that whereby noncompetitive adjustments occur in response to the diversity (itself correlated with food abundance) of food types available

Seasonal Variation in the Foraging Behavior of Some Migratory Western Wood Warblers

I observed the foraging behavior of four warbler species (Dendroica petechia, Oporornis tolmiei, Geothlypis trichas, and Wilsonia pusilla) in the summer in Wyoming and in the winter in Nayarit, Mexico. Of six variables (absolute foraging height, relative foraging height, vegetation density, horizontal foraging position, feeding method, and foraging substrate) believed to be potentially important in distinguishing the warbler species ecologically, the two foragingheight variables provided the greatest separation of the four species in both summer and winter. An analysis of the behavioral similarity of each species from summer to winter revealed that feeding method was the least changed behavior and that absolute foraging height involved the greatest behavioral flexibility. The behaviors that are most flexible are possibly the least well tied to the birds\u27 morphology and are also the ones that have been shown by other workers to reveal the effects of competitors through niche shifts. Therefore, ecological relationships among coexisting species (in terms of overlaps or positions in niche space) may never be fully derivable from morphological information alone

A Description of Mixed-Species Insectivorous Bird Flocks in Western Mexico

Insectivorous bird flocks were observed in all types of forested habitats during the nonbreeding season in western Mexico. The species composition of flocks changed markedly and predictably among five categories of habitat type. The average number of species per flock in lowland habitats was 4.7, while a mean of 18.6 species participated in highland flocks, ranking the latter among the most species-rich flocks in the world. The mean proportion of the local insectivorous species that participated in mixed-species flocks was significantly greater in the highlands (61.3%) than in the lowlands (24.6%). About half of the flock participants in both undisturbed lowland and highland habitats were north temperate migrants, ranking west Mexican flocks among the most migrant-rich in the world as well. In highland flocks, the maximum number of individuals per attendant species was generally two to three, but there were often six to twelve individuals belonging to each of several nuclear species. The lowland deciduous forest flocks seemed to lack nuclear species

The Ecological Importance of Severe Wildfires: Some Like it Hot

Many scientists and forest land managers concur that past fire suppression, grazing, and timber harvesting practices have created unnatural and unhealthy conditions in the dry, ponderosa pine forests of the western United States. Specifically, such forests are said to carry higher fuel loads and experience fires that are more severe than those that occurred historically. It remains unclear, however, how far these generalizations can be extrapolated in time and space, and how well they apply to the more mesic ponderosa pine systems and to other forest systems within the western United States. I use data on the pattern of distribution of one bird species (Black-backed Woodpecker, Picoides arcticus) as derived from 16 465 sample locations to show that, in western Montana, this bird species is extremely specialized on severely burned forests. Such specialization has profound implications because it suggests that the severe fires we see burning in many forests in the Intermountain West are not entirely “unnatural” or “unhealthy.” Instead, severely burned forest conditions have probably occurred naturally across a broad range of forest types for millennia. These findings highlight the fact that severe fire provides an important ecological backdrop for fire specialists like the Black-backed Woodpecker, and that the presence and importance of severe fire may be much broader than commonly appreciated