Bertin W. Anderson

Use of exotic saltcedar (Tamarix chinensis) by birds in arid riparian systems

Avian use of saltcedar (Tamarix chinensis) along the middle Pecos River was compared with similarly collected data along the lower Colorado River and Rio Grande. Use of saltcedar ranked high among all bird groups in all seasons on the middle Pecos River. In contrast, many species do not occur in saltcedar on the lower Colorado River, while few species winter in saltcedar on the lower Rio Grande. Occurrence of granivores and insectivores during winter in saltcedar on the Pecos River may be explained by seed-producing shrubs and annuals within or adjacent to these habitats. Most breeding birds on the Pecos River are summer visitors. These breeding species, though present, do not occur in saltcedar on the Colorado River despite abundant food resources and occur in intermediate abundances on the Rio Grande. Densities of several summer-visiting insectivores have declined markedly on the Colorado River since the proliferation of saltcedar, whereas they have remained relatively stable in other river valleys to the east. Biogeographical considerations, specifically elevational (climatic) gradients, are suggested reasons for this phenomenon.

Habitat Selection Attributes of an Avian Community: A Discriminant Analysis Investigation

Habitat use characteristics of all bird species occurring in the lower Colorado River valley were studied. Seventy—two transects, each through homogeneous stands of vegetation and together representing all riparian habitat types occurring in the area, were censused three times monthly for 2 yr (1977, 1978) to provide distributional records for all species. From the census data we divided the transects into three groups for each bird species each season: transects where the species was recorded in both 1977 and 1978, transects where the species was absent both years, and transects where the species occurred in only one of the years. We used discriminant analyses to classify the habitat on each transect as suitable or unsuitable for each species, using a set of eight species composition measures, four foliage density measures, and foliage height diversity (FHD). Mean habitat discriminability of all species varied from a low of 86% in summer to a high of 91% in late summer. Most species showed extensive variability among seasons in both the degree of habitat selectivity and the criteria used in habitat selection. This variability reflected both changes in population structure of the species and responses to changing resource and climatic pressures. Ecological attributes of the species were examined to determine which factors shaped the habitat use patterns of each. Feeding guild played only a small role in both the degree and criteria of habitat selectivity, except for highly specialized feeders such as nectivores and frugivores. Species with restricted distributions were usually habitat specialists, but species with widespread distributions included both habitat generalists and specialists. During spring and summer nonresident, short—stay species showed greater habitat selectivity than did extended—stay species, but during nonbreeding seasons both short—stay and extended—stay species showed equivalent habitat selectivity. That both the mix of habitat generalists and specialists and the habitat selectivity of individual species varied seasonally implies that the organization of habitat use of the community is the dynamic product of many interacting factors. The importance of individual habitat attributes was also examined. Species showed preferences for particular habitat attributes significantly more frequently than they avoided habitat attributes. FHD was the most frequently important variable in differentiating areas used from areas not used. Proportions of honey mesquite, cottonwood, and willow in the vegetation were also frequently important variables in habitat selection by avian species, but usually in conjunction with FHD. Foliage density characteristics were less frequently significant, but for species where these measures were important, they tended to be the primary habitat selection criterion. In summer and late summer, habitat selection criteria of individual species seemed independent of the selection criteria of the other species in the community. In other seasons there was evidence of species interactions in these habitat selection patterns. These empirical findings for an entire avian community over all seasons are an advance in our understanding of how habitat use patterns of communities are organized. The extensive seasonal variability in habitat use by avian species and variability of distributions of individual species between years imply that studies of community organization can best be done with large—scale, long—term studies.

Nonlinear relationships between birds and vegetation

We tested for the presence and importance of nonlinear relationships among bird species abundances and vegetation characteristics of the lower Colorado River valley. We transformed vegetation variables into polynomials and then ran multiple regressions, with bird densities as the dependent variables. Polynomial variables were selected in 36% of first steps and 45% of second steps in the regressions. In a significant number of cases there was a response to a polynomial variable but not to the original linear variable. Importance of curvilinear responses varied among bird species. This method offers a more comprehensive examination of responses to envirnomental gradients because unimodal and some skewed relationships can be detected. See full-text article at JSTOR

Avian Community Structure Changes in a Mature Floodplain Forest after Extensive Flooding

Effects of flooding on the vegetation and avian communities of the Bill Williams River, Arizona, were evaluated from 1976 to 1983. High water flows in 1978 through 1980 caused the death of 99% of all Fremont cottonwoods (Populus fremontii) and 64% of all Goodding willows (Salix gooddingii) on a 120-ha area near the confluence with the Colorado River. By 1982, cattails (Typha spp.) had become the dominant vegetation. Groundand canopy-dwelling avian insectivores and cavity nesters decreased, whereas passerine marsh insectivores and rail-like species increased. Water released from Alamo Dam resulted in loss of the last remaining large stand of mature cottonwood-willow habitat in the lower Colorado River valley, causing declines in numbers of some bird species possibly threatened with local extirpation. Need for flood management procedures to conserve vegetation is paramount when beleaguered habitats are involved. J. WILDL. MANAGE. 51(2):495-502 Creation of reservoirs and channelization of rivers have extensively altered plant and animal communities along many major water courses. In some areas controlled water releases in spring and early summer have resulted in a spread of native riparian tree species, as along the Platte River in Colorado and Nebraska (Williams 1978), thus increasing riparian wildlife in that area. However, in other areas, water flow management has been detrimental to both native vegetation and wildlife, as along the Missouri River (Anderson 1971, Johnson et al. 1976). Such problems are encountered also in the Southwest, where a prolific exotic tree, Chinese tamarisk (Tamarix chinensis), largely has replaced native tree species in many areas (Horton 1977). The lower Colorado River and its tributaries are such areas and serve as dramatic testimonials to the ecological consequences of heavily managed riverine systems. Examination of historical records shows that until the mid-1800s, the lower Colorado River was lined with gallery forests consisting of cottonwoods and willows. Channelization of the river, cutting of trees for firewood, fires, and land clearing for agriculture removed most of these forests (Ohmart et al. 1977). Cessation of natural floods and high soil salinities have prevented the return of these communities. Today, most Fremont cottonwood and Goodding willow areas that remain in the lower Colorado River valley are decadent and form mixed stands with exotic tamarisk, screwbean mesquite (Prosopis pubescens), honey mesquite (P. juliflora glandulosa), and arrowweed (Tessaria sericea). Only along the Bill Williams River does the cottonwood-willow forest remain relatively intact in the lower Colorado River mainstem and adjacent tributary deltas. A water release from Alamo Dam (near the confluence of the Bill Williams and Big Sandy rivers, Mohave County, Arizona) of unusually long duration flooded a 120-ha stand of cottonwood and willow on the Bill Williams River at its confluence with the Colorado River during October 1978-April 1981. Management of native vegetation is presently given very low priority in flood control procedures. This study was conducted to determine the effects of longterm flooding in a riparian forest and to denote subsequent changes in the avifauna. Results from this study will be useful in planning management options during future flooding events. Special thanks are due R. W. Engel-Wilson, M. J. Kasprzyk, J. M. Jackson, and D. Krueper for organizing and assisting with the fieldwork. D. E. Busch provided water flow data and insightful views into the workings of water management agencies. K. V. Rosenberg, A. W. Laurenzi, and V. C. Hink critically reviewed the manuscript and helped with interpolation of preand during-flood bird and vegetation data. S. M. Cook and J. R. Durham provided editorial assistance, and C. D. Zisner typed the manuscript. The research was funded partly by the U.S. Bur. Reclamation, Contract 7-07-30-V0009.

Seasonal Habitat Selection by Birds in the Lower Colorado River Valley

Patterns of seasonal variation in use of specific habitats by birds were investigated using monthly census data from 110 transects in riparian vegetation along the lower Colorado River from Arizona to California. Specific hypotheses were tested about changes in habitat breadth, timing of such changes, and the use of habitats which were preferred in seasons of maximum density. We generally found support for the hypotheses that habitat breadth narrows in seasons of supposed resource restriction, and that the narrowing occurs before the winter season. However, species with maximum winter populations tended to show the same kinds of trends in habitat use as were shown by species with summer maximum populations. Resource depletion does not provide a full explanation for habitat breadth restriction. Alternative factors, especially changes in population structure, are suggested, although at this point explanations must be undertaken on a piecemeal basis. Habitats preferred in the breeding season were found not to be consistently preferred in other seasons, implying that conclusions on habitat optimality may not be appropriate from studies during only part of a year.

Turnovers in Species Composition of Avian Communities in Contiguous Riparian Habitats

We examined 4 yr of intensive census records for 72 line transects in contiguous riparian vegetation. Across all transects the number of bird species differing between pairs of suc- cessive years followed a random distribution. For any season, from one-quarter to over one-third of the species differed. There were weak tendencies for specific plant communities and seral stages to have high or low turnover rates. The proportion of avian species in a community changing between years was lower in summer than in other seasons but was similar between years for any specific season. Locally breeding species showed slightly more stable communities than did nonlocally breed- ing species. These high turnover rates in bird community composition between years bring into question assumptions underlying current theories of avian habitat selection, as well as assumptions essential when competition theory is used as the framework to account for community organization. Furthermore, there are substantial empirical similarities between these species turnovers in contig- uous habitats and species turnovers on island and island-like patches of habitat.

Habitat use by Clapper rails in the lower Colorado River valley

-Densities of the Clapper Rail (Rallus longirostris yumanensis) were determined in marshy situations in the lower Colorado River valley in all seasons. We conducted a study to quantify environmental variables important to rails within censused areas along the 450 km of the lower Colorado River north of Mexico in order to learn more about their year-round habitat requirements. Quantified vegetation variables from 40 marsh areas were subjected to principal components analysis; four principal components collectively accounted for 75% of the variance. Habitat breadths of rails were broadest in summer and narrowest in winter. Marshes with the highest rail densities in one season tended to have large rail densities year-round. The converse was true for marshes with low densities. In the first of two analyses, 27 of the marshes (each censused monthly for two years) were used to determine rail associations with the vegetation principal components (PCs). Spearman rank correlations of rail densities with PCs revealed that rails were associated primarily with dense marsh vegetation (PC I) at all seasons. This outcome was tested and confirmed with data from 13 marshes censused during summer 1976 that were not included in the first analysis. One reed (Phrugmites austrulis) and two cattail (Typha domingensis) marshes of moderate foliage density consistently had more rails than expected. One dense cattail marsh consistently had fewer rails than expected. Size of marsh (2-29 ha) and bank slope into the water were apparently unrelated to density of rails per 10 ha. Censuses from this and unpublished recovery team studies suggest a rail population of about 750 birds for the lower Colorado River north of Mexico. The Yuma Clapper Rail (Rallus longirostris yumanensis), restricted to the lower Colorado River drainage system and the Salton Sea in the Imperial Valley of California, is considered an endangered species by the U.S. Fish and Wildlife Service (USFWS). Its numbers have been closely monitored because marsh habitat required by this species is only moderately abundant. In 1972, the USFWS established the Yuma Clapper Rail Recovery Team, which has since annually censused the breeding population along the lower Colorado River. These censuses did not include any quantification of the environmental variables associated with rail densities. Knowledge of the habitat requirements of rails comes largely from detailed, very local, single-year studies with emphasis on the breeding season. Such studies were conducted at Topock Marsh near the northern limit of the range of the Clapper Rail (Smith 1975) and at the western edge of the Salton Sea (Bennett and Ohmart 1978). Studying the habitat of the Clapper Rail over a large portion of its range is important because habitat relationships found in local areas can differ substantially from those over much larger (regional) areas (Wiens 198 1). Habitat relationships found by Bennett and Ohmart (1978) differed markedly from those found by Smith (1975). The rails in those studies inhabited stands of bulrush (Scirpus spp.) or cattail (Typha spp.) in differing degrees. We know of no studies about their use of other vegetation associated with marshes, such as reed (Phragmites australis), loosestrife (Lythrium californicum), and various grasses. In view of the endangered status of the Clapper Rail, data concerning habitat relationships over the entire area are necessary to properly manage the population. Since relationships between various bird species and the vegetation often change seasonally and annually (Rice et al. 1980; Laurenzi et al. 1982; Meents et al. 1982, 1983), data from as many seasons and years as possible should be included when attempting to clarify the relationship between a species and its habitat. In our study we examined Clapper Rail use of marsh habitat along the lower Colorado River for several seasons over a period of three

Man's Influence on Hybridization in Two Avian Species in South Dakota

cussed its taxonomic significance. Some have also commented on mans impact on the vegetation of the Great Plains and its significance to hybridization (e.g., West 1962; Short 1965). Recent changes in the vegetation of South Dakota and possible effects of these changes on hybridization of flickers (Colaptes auratus auratus and C. a. cafer) and orioles (Icterus galbula galbula and I. g. bullockii) are considered here. Nomenclature used conforms to the guidelines established by Short (1969) for hybridizing forms. Flickers and orioles occur throughout much of South Dakota. Hybrids are common in the western half of the state and both hybridizing pairs form zones of hybridization, areas in which parental phenotypes make up 5 per cent or less of the population (Short 1969). Both species occur wherever there are tall or moderately tall trees (towns, farm groves, shelter belts, and along streams and rivers). Dominant trees occurring naturally in such areas include cottonwood (Populus deltoides), American elm (Ulmus americanus), ash (Fraxinus spp.), and river maple (Acer negundo). Orioles were found more frequently than flickers in burr oak (Quercus macrocarpa). Flickers, but not orioles, were found in ponderosa pine (Pinus ponderosa). The requirements of flickers for trees in which they can make holes differ somewhat from the nesting requirements of orioles and help to understand the above observations.

Phainopepla Utilization of Honey Mesquite Forests in the Colorado River Valley

Phainopeplas (Phainopepla nitens) arrive in the lower Colorado River Valley in the fall and remain through the winter and spring. During the winter they eat the fruit of mistletoe (Phoradendron californicum; Rand and Rand 1943, Walsberg 1975, 1977), which primarily parasitizes honey mesquite trees (Prosopis velutina). Phainopeplas breed in the valley in spring and then unlike most winter residents, leave the area during the hot summer months.

Evidence for Social Regulation in Some Riparian Bird Populations

Patterns in the regulation of seven permanent resident riparian bird species were studied in the lower Colorado River valley. Populations may be predator-food regulated or selection might favor dispersal tendencies that keep the population at a given location low enough to avoid resource depletion as well as the attraction of predators. Theory states that socially regulated species should have relatively short breeding seasons and less postbreeding mortality, and should begin population declines later than nonsocially regulated species. Among the seven species studied we found a continuum, with ladder-backed and Gila wood-peckers in agreement with the theoretical criteria of socially regulated species and the cactus wren, verdin, and Gambel quail at the opposite end of the spectrum. The Abert towhee and crissal thrasher showed criteria of both groups, but had more in common with socially regulated species.