Bird Community Dynamics Research Articles

Long-Term Research and the Dynamics of Bird Populations and Communities

Long-Term Research and the Dynamics of Bird Populations and Communities

Long-Term Research and the Dynamics of Bird Populations and Communities

Long-Term Research and the Dynamics of Bird Populations and Communities

Forest Fragmentation and Bird Community Dynamics: Inference at Regional Scales

With increasing fragmentation of natural areas and a dramatic reduction of forest cover in several parts of the world, quantifying the impact of such changes on species richness and community dynamics has been a subject of much concern. Here, we tested whether in more fragmented landscapes there was a lower number of area-sensitive species and higher local extinction and turnover rates, which could explain higher temporal variability in species richness. To investigate such potential landscape effects at a regional scale, we merged two independent, large-scale monitoring efforts: the North American Breeding Bird Survey (BBS) and the Land Use and Land Cover Classification data from the U.S. Geological Survey. We used methods that accounted for heterogeneity in the probability of detecting species to estimate species richness and temporal changes in the bird communities for BBS routes in three mid-Atlantic U.S. states. Forest breeding bird species were grouped prior to the analyses into area-sensitive and non-area-sensitive species according to previous studies. We tested predictions relating measures of forest structure at one point in time (1974) to species richness at that time and to parameters of forest bird community change over the following 22-yr-period (1975–1996). We used the mean size of forest patches to characterize landscape structure, as high correlations among landscape variables did not allow us to disentangle the relative roles of habitat fragmentation per se and habitat loss. As predicted, together with lower species richness for area-sensitive species on routes surrounded by landscapes with lower mean forest-patch size, we found higher mean year-to-year rates of local extinction. Moreover, the mean year-to-year rates of local turnover (proportion of locally new species) for area-sensitive species were also higher in landscapes with lower mean forest-patch size. These associations were not observed for the non-area-sensitive species group. These results suggest that landscape structure may influence forest bird communities at regional scales through its effects on the total number of species but also on the temporal rates of change in community composition. Evidence for higher rates of local extinction and turnover in more fragmented landscapes suggests that bird communities function as metapopulations at a regional scale, and points out the importance of colonizations and recolonizations from surrounding landscapes to local community dynamics. Further, our results illustrate that the methods used to estimate the community parameters can be a powerful statistical tool in addressing questions relative to the dynamics of communities.

Vegetation and bird community dynamics in fragmented coppice forests

Richness and composition of plant and bird communities were used to evaluate the importance of seven within‐forest factors on biodiversity in fragmented coppice forests in south‐western France. Plants and birds were sampled on the same 98 plots. Plant abundance–dominance was measured on 400 m 2 plots. Birds were sampled using a 50‐m fixed radius point‐count method in the early and late periods of the breeding season. The selected factors were: years since logging (YSL), retention of standards (STANDARD), ownership (OWNER), distance to the nearest edge (EDGE), tree richness (TREE), density of vegetation in low (ST 2 ) and high (ST 16 ) strata. Paired comparisons of richness were carried out between YSL classes, according to other variables. The respective influence of each variable was evaluated using adapted CCA (canonical correspondence analysis) on plant and bird composition. Plant richness varied strongly between YSL classes, was independent of STANDARD, increased with TREE and was higher in collective owned forests. Bird richness seemed to be independent of YSL and was higher in low YSL plots with standards. Bird and plant richness was not correlated. Plant community composition was mainly explained by YSL/ST 16 on the first axis and by OWNER on the second axis. STANDARD and OWNER explained bird community composition. OWNER must be considered as a synthetic variable. This could be useful in adapting forestry practices for a better management of biodiversity, taking into account the very different taxonomic groups involved.

Scrub-Successional Bird Community Dynamics in Young and Mature Longleaf Pine-Wiregrass Savannahs

We investigated how management for habitat conditions to support the endangered red-cockaded woodpecker effects the biodiversity of the breeding bird community associated with those habitats. Habitat is created by thinning, burning and mid-story control of hardwoods in mature longleaf stands. In addition, similar habitat structurally can be found in recently harvested areas. We tested the hypothesis that diversity and abundance, as well as survival and reproduction would be greater in mature stands. However, mature stands used for recruitment always had fewer species (36/31) than recently harvested areas (54/55). All species that occurred in recruitment stands also occurred in mature stands. No differences in survival rates were found between mature and recent cuts for Bachman's sparrow and indigo bunting.

Impact of flowering on bird community dynamics in some central Victorian eucalypt forests

Many species of Eucalyptus in Australia provide copious amounts of nectar during their reproductive seasons. The nectar is used by many animal species but especially by birds, insects and some bats, which act as pollinators. One of the major features of eucalypt flowering in southern Australia is the patchy, asynchronous flowering of different species, which appears to drive mass nomadism of nectarivorous birds among regions and among habitats. Here we explore whether flowering asynchrony or climate is primarily responsible for the influxes and effluxes of vast numbers of nectarivorous birds in central Victoria, Australia. By using a structured sampling program, we show that winter flowering by red ironbark Eucalyptus tricarpa is the most likely agent controlling avian‐nectarivore dynamics rather than climatic differences among regions. Densities and species richness of nectarivores, and numbers of nectarivory events, are all closely related to measures of flowering intensity. However, non‐ nectarivores, such as insectivores and granivores, show no relationships with either habitat or region. We discuss how dependence on a patchily distributed but highly rewarding resource such as nectar influences population densities and community structure in birds.

Numts: A Challenge for Avian Systematics and Population Biology

CONFER, J. L. AND K. KNAPP. 1981. Golden-winged Warblers and Blue-winged Warblers: The relative success of a habitat specialist and a generalist. Auk 98:108-114. FICKEN, M. S., AND R. W. FICKEN. 1968. Territorial relationships of Blue-winged Warblers, Goldenwinged Warblers, and their hybrids. Wilson Bulletin 80:161-172. GILL, F. B. 1980. Historical aspects of hybridization between Blue-winged and Golden-winged warblers. Auk 97:1-18. GILL, F. B. 1997. Local cytonuclear extinction of the Golden-winged Warbler. Evolution 51:519-525. GILL, F. B., AND W. E. LANYON. 1964. Experiments on species discrimination in Blue-winged Warblers. Auk 81:53-64. GILL, F. B., AND B. G. MURRAY, JR. 1972. Discrimination behavior and hybridization of the Bluewinged and Golden-winged warblers. Evolution 26:282-293. HANDS, H. M., R. D. DROBNEY, AND M. R. RYAN. 1989. Status of the Golden-winged Warbler in the northcentral United States. United States Fish and Wildlife Service, Twin Cities, Minnesota. HILL, N. P., AND J. M. HAGAN III. 1991. Population trends of some northeastern North American landbirds: A half-century of data. Wilson Bulletin 103:165-182. HOLMES, R. T., T. W. SHERRY, AND F. W. STURGES. 1986. Bird community dynamics in a temperate deciduous forest: Long-term trends at Hubbard Brook. Ecological Monographs 50:201-220. LITVAITIS, J. A. 1993. Response of early successional vertebrates to historic changes in land use. Conservation Biology 7:866-873. MURRAY, B. G., JR., AND F. B. GILL. 1976. Behavioral interactions of Blue-winged and Golden-winged warblers. Wilson Bulletin 88:231-254. RAPPOLE, J. H., AND M. V. McDoNALD. 1994. Cause and effect in population declines of migratory birds. Auk 111:652-660. SAUER, J. R., J. E. HINES, G. GOUGH, I. THOMAS, AND B. G. PETERJOHN. 1997. The North American Breeding Bird Survey home page, version 96.3. Patuxent Wildlife Research Center, Laurel, Maryland. SCHEIDER, F. 1959. Warblers in southern New York. Kingbird 9:13-19. SMITH, C. R., D. M. PENCE, AND R. J. O'CONNOR. 1993. Status of Neotropical migratory birds in the northeast: A preliminary assessment. Pages 172-188 in Status and management of Neotropical migratory birds (D. M. Finch and P. W. Stangel, Eds.). United States Forest Service General Technical Report RM-229, Newton Corner, Massachusetts. WILL, T. C. 1986. The behavioral ecology of species replacement: Blue-winged and Golden-winged warblers in Michigan. Ph.D. dissertation, University of Michigan, Ann Arbor. WITHAM, J. W, AND M. L. HUNTER, JR. 1992. Population trends of Neotropical migrant landbirds in northern coastal New England. Pages 85-95 in Ecology and conservation of Neotropical migrant landbirds (J. M. Hagan III and D. W Johnston, Eds.). Smithsonian Institution Press, Washington, D.C.

Landscape Dynamics of Bird Communities in Relation to Mass Flowering in Some Eucalypt Forests of Central Victoria, Australia

The large-scale dynamics of bird communities in central Victoria, Australia, were studied in relation to eucalypt flowering by recording community changes throughout the annual cycle in sites north and south of the Great Dividing Range (GDR). Two kinds of forest habitat were considered, one of which flowers profusely in winter ('ironbark') while the other flowers little but over the warmer months ('stringybark'). There were eight sites, two of each habitat in both the northern and southern regions. In the absence of flowering in ironbark sites during summer, the avian communities of both kinds of habitat are virtually the same in each region. However, the sets of communities in each region differ significantly from one another at this time. With the onset of winter flowering, avian community composition in ironbark habitats in each region diverges greatly from the non-flowering ironbark and stringybark communities, but converges back to those communities as flowering declines. Most of these changes are due to the influx of high densities of nectarivores, especially honeyeaters and lorikeets. Such influxes in stringybark habitats did not occur, probably because the flowering intensity was at most an order of magnitude less than that in ironbark sites. The results show the dramatic impact that eucalypt flowering has on avian communities in south-eastern Australia, and the influence of habitat differences. Thus, although nectarivores move into the northern region of the study area to capitalize on winter flowering, they show high habitat specificity and virtually avoid the non-flowering stringybark habitats. The results are discussed in terms of landscape and regional dynamics of birds and the possible interactions among avian sub-communities (the 'nectarivores' and 'non-nectarivores').

Bird Community Dynamics along Primary Successional Gradients of an Amazonian Whitewater River

We used spot-map methods to census birds in two plots (40 ha and 80 ha) representing the early and middle stages of primary succession generated by the meandering of the Manu River in southeastern Peru in 1983 and 1985. We distinguished seven distinct successional stages beginning with the first plant communities growing on open beaches (Tessaria: stage 1) and proceeding through mature floodplain forest (stage 7). Each successional stage was more structurally and floristically complex. One plot was dominated by early successional vegetation (stages 1-5) and the other was domi- nated by middle-to-late stages (5-6). We supplemented spot-map data with mist netting to census nonterritorial species, principally nectarivores and frugivores. Because the ear- liest successional stage (Tessaria) covered only small (<3 ha) areas on any given me- ander tongue, we censused six additional stands to characterize their bird communities. Species richness increased along the successional gradient, but not uniformly. Struc- turally simple Tessaria stands (stage 1) contained more diverse communities of breeding birds than the next two successional stages, which appeared to have few resources avail- able to birds. After stage 3, breeding species richness increased by 31-71 species per successional stage as a distinct canopy layer formed (stage 4), fruiting trees became more available (stages 5-6) and the vertical structure of the forest became fully developed (stage 7: 71 more species than stage 6). Early successional bird communities (stages 1- 3) were dominated by a few very abundant insectivores and omnivores, but wanderers from the adjacent forest plus Nearctic and austral migrants seasonally outnumbered breeding residents. Many species that breed in Tessaria stands were restricted to just one or two of the seven stands censused. The middle stages of succession had the highest estimated richness, abundance, and biomasses of nectarivores and frugivores. Nectarivore abundance reflected the huge stands of Heliconia that dominate the understory of mid- successional stages. Bark foragers reached their highest abundance in late successional stages as the dominant canopy tree species of middle successional stages (e.g., Cedrela odorata) began to die off, but remained standing. Middle successional stages lacked many of the insectivores characteristic of more structurally complex mature forest. Among the missing species were understory Mockers, ant-followers, ground foragers, and many arboreal insectivores that participate in canopy flocks. Species of early and middle stages tended to be more abundant on average than those of mature forest. Early-/and late-successional bird communities had few species in common, and many congeners segregated along this successional gradient. Most species characteristic of early successional vegetation, however, were also found in many other kinds of natural and anthropogenic disturbances. The Tessaria beaches of the Manu lacked most species endemic to the same successional stage on the larger rivers in the Amazon system. Mid-successional stages had relatively fewer species restricted to them, but some of these species are rare and endemic to southwestern Amazonia. Many char- acteristic species of floodplains reached their peak abundances in stages 4-6. Mid-suc- cessional stages, therefore, may play a greater role in contributing to the unusually high species richness of western Amazonia than had previously been recognized. These results point to the importance of preserving natural, undammed meandering rivers in conserving the avian diversity of Amazonia. Logging and agricultural devel- opment of mid-successional floodplain forests probably has a negative impact on regional bird communities. Not all species of the early and middle stages of succession have adapted to human-generated secondary succession. Recovery of the full diversity and richness of tropical bird communities following disturbance may take several hundred

Temporal Variation in Birds and Fruits Along an Elevational Gradient in Costa Rica

We studied temporal fluctuations in fruit production by plants and in populations of understory fruit—eating birds at three elevations (50, 500, and 1000 m) in Costa Rican wet forests over a 12—16 mo period to investigate effects of resource variation on bird movements and community structure. We used mist nets to monitor changes in frugivore abundance, migration patterns, and breeding and molting cycles. We sampled understory fruits of each forest concurrent with studies of frugivores. Both frugivores and fruit exhibited considerable seasonal variation in abundance. Peak frugivore capture rates occurred during peak periods of ripe fruit abundance. Altitudinal migrants left lower montane (1000 m) forest during periods of fruit scarcity and wee present in lowland (50 m) and foothill (500 m) forest when ripe fruit was abundant. Migrants, both altitudinal and temperate, accumulated fat before migration, and perhaps (for altitudinal migrants) in anticipation of breeding. Some residents also put on fat before breeding. Breeding was seasonal at all forests and occurred when ripe fruit abundance was low. Results of this study indicate that birds may track changes in resource abundance. Thus, variation in resource abundance influences dynamics of bird communities, both in terms of species composition and abundance. Further, results illustrate the importance of viewing communities from different scales; dynamics at a local scale (e.g., one elevation) can be influenced by changes in conditions (e.g., fruit abundance) elsewhere. That some species regularly moved along elevational gradients implies that preservation of many species and of the biotic integrity of entire systems may require conservation of large, connected blocks of suitable habitat.

Bird community structure and dynamics during spring migration in selected habitats of northern Italy

Abstract Bird community structure and dynamics were studied in seven habitats of northern Italy during three periods. Respectively 87 and 70 species were detected in 1984 and 1985 (a total of 98 species for 1984 + 1985). Of these, 45% belonged to permanent resident species. This category was dominant in abandoned mountain farmland, shrublands, young wood and abandoned chestnut orchard. The tree cover was negatively correlated with bird diversity. The permanent resident and transient migrant species had the highest interseason turnover. Abandoned chestnut orchard showed the highest value of specific dominance (322). The species with the widest habitat breadth (> 0.80) were: blackcap (Sylvia atricapilla), blackbird (Turdus merula), great tit (Parus major) and greenfinch (Carduelis choris). Based on the relative abundance of single species abandoned chestnut orchards, shrubland and young wood were the habitats with the most similar bird community. Seven species had an acoustic repertoire breadth > 0.60. The ...

Short-Term Dynamics of Cavity-Nesting Bird Communities in Disjunct Floodplain Habitats

We studied turnover rates in cavity-nesting bird communities during 1983 and 1984 in 34 cottonwood-willow (Populus-Salix) habitat fragments along the North Platte River in southeastern Wyoming. Our principal goal was to determine whether the equilibrium theory of island biogeography was applicable to this system. We tested two hypotheses: (1) equilibrial turnover of cavity-nesting bird species does not occur in disjunct floodplain habitats, and (2) no significant relations exist between absolute turnover rate (ATR) and habitat-fragment features. Paired-t tests and a one-sample sign test corroborated equilibrial turnover for the group of fragments collectively, but examination of the hypothesis at the level of individual fragments did not support the equilibrium model. Simpleand partialregression coefficients indicated ATR was positively associated with fragment size, the reverse of what equilibrium theory predicts; ATR, however, was negatively related to distance to nearest streamside habitat fragment (analogous to degree of isolation), which is consistent with the theory. ATR was significantly related to five fragment features in a validated multiple regression model: amount of edge/hectare, size of fragment, frequencies of snag classes 4 (56 to 70 cm dbh) and 6 (>85 cm dbh), and the presence of palustrine wetland bordering each fragment. Short-term dynamics of cavity-nesting bird communities in the system we studied are not completely predictable from the equilibrium theory. Fragment features appear to be the important deterministic forces affecting species richness and dynamics, but stochastic immigration and extinction may also be operative. Application of equilibrium island-biogeographic theory to conserve the system we analyzed is inappropriate. Instead, recommendations that recognize the deterministic influences we have identified should be used.

Impact écologique du feu sur l’avifaune et gestion du paysage en France méditerranéenne

A research programme was carried out in the Eastern Pyrennees in order to study the impact of fire on the fauna, and particularly the avifauna, in several Mediterranean habitats. Continued without any interruption for 10 years, this programme allowed to measure the variations of several quantitative parameters of the communities (richness and diversity) and of the populations (relative abundances of ca. 50 species), and to elaborate quantitative models permitting some predictions on the postfire dynamics of bird communities.

Bird Community Dynamics in a Temperate Deciduous Forest: Long‐Term Trends at Hubbard Brook

Changes in species composition and abundance of birds breeding in an unfragmented temperate deciduous forest in New Hampshire, USA, were studied intensively during 16 consecutive breeding seasons, 1969—1984. The number of species breeding in the 10—ha study area in any one year varied from 17 to 28, and averaged 24. Total numbers of individuals breeding on the 10—ha plot ranged from 214 to 89, with many species (70%) declining during the 16—yr period. Overall, there was significant positive covariation among population trends of all species, suggesting a major, perhaps single, factor affecting population levels, such as weather or food supply. No two species, however, had identical patterns of change across all 16 yr. Thus, populations in this forest fluctuated largely independently of one another, suggesting a different combination of regulatory factors for each species. The major factors pulsing or stressing bird populations in the field included: (1) changes in food abundance due largely to irruptions of defoliating Lepidoptera (most bird species, but especially vireos and warblers), (2) harsh late spring and summer weather (Scarlet Tanager, American Redstart), (3) changes in habitat structure related to forest succession (Least Flycatcher, Philadelphia Vireo), (4) interspecific interactions, particularly interference competition (Least Flycatcher—American Redstart, Red—eyed Vireo—Philadelphia Virreo), and (5) mortality during winter for both resident and migrant populations (Hermit Thrush, Dark—eyed Junco, permanent resident species such as woodpeckers and nuthatches. Some, but not all, irruptions of defoliating Lepidoptera significantly influenced many bird populations in this forest and contributed to the observed positive covariation in abundances. These irruptions produced pulses of food for breeding birds, but occurred at long and variable time intervals. Between outbreaks, food may regularly limit reproductive output of these forest birds, and low food abundance, along with the effects of predators, weather, and other mortality factors, contributes to high variability in bird reproductive success, which ultimately influences population size. Competition may also become important during these periods of food scarcity. We therefore propose that birds in these temperate deciduous forests experience periods of prolonged food limitation, interrupted by relatively brief periods of superabundant food, a pattern contrasting with that described for other temperate systems (e.g., by Wiens 1977). From this examination of bird community dynamics on one site over many years, it is apparent that each species responds to its environment in a unique way, as determined by a variety of influences on its populations. Some of these operate on a local scale (e.g., vegetation structure, food abundance, interspecific competitors), while others function at regional (e.g., some weather effects) and global (e.g., winter events) geographic scales. These influences also differ as to their temporal persistence and predictability, and thus are difficult to detect in short—term studies. This pluralistic view of community structure for birds in temperate forests represents a compromise between nonequilibrial and competitionist models, and argues against the existence of a tightly organized community at any one spatial or temporal scale.

Ecological Relations and Social Interactions of Philadelphia and Red-Eyed Vireos

-The ecological and social interactions of the interspecifically aggressive Red-eyed (Vireo olivaceus) and Philadelphia (V. philadelphicus) vireos were studied during four breeding seasons in the Hubbard Brook Experimental Forest, New Hampshire. The two species here have extensively overlapping territories but occupy different vegetational strata during most of the breeding season. The aggressiveness of the Philadelphia Vireo towards the Red-eyed Vireo varies within a breeding season, being most frequent and intense during the periods of greatest vertical overlap. The aggressiveness of Red-eyed Vireos towards Philadelphia Vireos varies between years, being most frequent and intense in years of high Red-eyed Vireo population density. In one year no aggression was apparent between the two vireos. During some aggressive encounters, the smaller, subordinate Philadelphia Vireo used what appeared to be distraction displays when Red-eyed Vireos came close to their young. I hypothesize that resource competition may be the underlying cause of the aggression between these two ecologically similar species, largely because the most intense and frequent interspecific interactions occurred during periods of maximum overlap in foraging microhabitat. A comparison of these findings with those in another study illustrates the importance of forest height and vegetation structure in facilitating their coexistence. The ecological and evolutionary significance of interspecific aggression in birds has been the subject of controversy in recent years (Orians and Willson 1964, Cody 1969, 1974, Murray 1971, 1976). Support for the adaptiveness of interspecific aggression (cf. Cody 1974) has come from Rice's (1978a, b, c) study of the Red-eyed (Vireo olivaceus) and Philadelphia (V. philadelphicus) vireos where they occur together in the woodlands of northeastern Ontario. In this region the two species occupy exclusive non-overlapping territories, the smaller (12 g) Philadelphia Vireo successfully defending territories against its larger (17 g) congener. Rice (1978a) considered this horizontal separation to be a form of ecological segregation, because in nearly all other respects (habitat, use of vertical strata, foraging behavior) the two species were found to be essentially identical. In a study of the ecological relations of these same two species in a late successional forest in New England, I have observed that their territories overlap extensively and that interspecific aggression occurs at certain times during the nesting cycle. In this paper I examine the interaction of these two vireos at this site, compare the results to those of Rice (1978a, b, c) and consider reasons for the aggression between the two species. STUDY AREA AND METHODS This study was conducted in the 3,076-ha Hubbard Brook Experimental Forest, West Thornton, New Hampshire. In this late successional northern hardwoods forest the canopy layer (13-27 m) is dominated by sugar maple (Acer saccharum), American beech (Fagus grandifolia) and yellow birch (Betula alleghaniensis) with occasional white ash (Fraxinus americana). The sapling layer (2-12 m) is relatively sparse (Sherry 1979) and consists primarily of beech, sugar maple and some striped maple (A. pensylvanicum). The shrub layer (0-2 m) is a very dense and heterogeneous mixture of hobblebush (Viburnum alnifolium), striped maple, mountain maple (A. spicatum), sugar maple and beech. Most of my data were gathered on or near a 10-ha study plot at an elevation of 600 m. This plot is marked by a 50 m by 50 m grid, and has been the site of an intensive study of bird community dynamics and struc. ture since 1969 (Holmes and Sturges 1975, Holmes et al. 1979a). From late May through late July during 1975-1978 I visited this plot nearly every day for a total of over 200 days and 1,200 hours. Unless otherwise stated, all observations used in this paper were made on or within 100 m of the study plot. Other sections of the forest, especially those with Philadelphia Vireos, were also visited regularly each summer. To characterize the ecological and social relations of the two species, I measured their spacing, microhabitat use, habitat selection and reproductive success, each of which is treated separately below. Spacing. I mapped territories and estimated population densities of vireos on the study plot by methods similar to those of Holmes and Sturges (1975). Briefly, I made cumulative maps of all individuals heard, seen and/or captured in mist-nets and of their nest locations hen known. An average of 65% of the Red-eyed Vireos (REVs) on the 10-ha plot were color-banded by

Temporal variation in habitat structure and shrubsteppe bird dynamics.

We investigate the role of temporal variation in habitat physiognomy in influencing the dynamics of shrubsteppe bird populations and communities. During a 3-y (1977-1979) study of 14 sites in the northern Great Basin of North America, annual precipitation varied substantially, with one of the driest years on record followed by one of the wettest. This resulted in significant physiognomic variation (increasing height and coverage of vegetation, decreasing horizontal patchiness), mediated largely by changes in the annual elements of the flora, particularly forbs and grasses. Shrub species coverage values, on the other hand, demonstrated no statistically detectable year-to-year changes, nor were they correlated with any physiognomic variation. Despite large scale physiognomic changes, no bird species' abundance varied in a statistically significant fashion; neither could variation in bird abundances be correlated with variation in either physiognomy or shrub species coverages.Multivariate analyses revealed essentially the same patterns as the univariate analyses: substantial changes in physiognomy, few changes in shrub species coverages or bird species abundances, and little correlation of temporal variation among the three data sets. Calculation of the Euclidean distances that sites "moved" in multivariate physiognomic, bird species, or shrub species hyperspaces yields synthetic gradients of annual "turnover" of sites with respect to those data sets. Sites identified as demonstrating high physiognomic turnover were characterized by high coverage of grass and forbs, while low turnover sites had greater coverage of shrubs and higher shrub species diversity. Relatively high bird turnover sites had greater numbers of Western Meadowlarks and Black-throated Sparrows, while more stable sites had high numbers of Brewer's Sparrows. Physiognomically, high bird turnover sites were grassier and had greater total vegetation coverage, while low bird turnover sites had more bare ground and higher horizontal patchiness. A site's position on the avifauna turnover axis, however, was uncorrelated with its position on the physiognomic turnover axis. Shrub species showed virtually no annual turnover.Reanalysis of a previous Principal Components Analysis (PCA) of these same data sets that was applied without regard to year of sampling revealed that the first physiognomic component (41% of the total physiognomic variability) did in fact have a strong temporal element, and that this element was consistent with the changes in univariate characters noted above. No other physiognomic component could be associated with annual variation, nor could any components of parallel bird abundance or shrub species coverage PCA's.Regional patterns indicate that sites tended to be very consistent from year to year in their relationships to one another as defined by their relative locations in either physiognomic or shrub species hyperspaces, but varied independently of one another with respect to their bird species abundances and composition. Examination of the temporal consistency of site relationships between "bird space" and "vegetation space" reveals that bird communities are to a large degree independent of a site's physiognomic position, but instead are strongly associated with its position in shrub species hyperspace.The overall patterns that emerge from these analyses are consistent with the so-called "checkerboard effect" that results from the apparently random annual redistribution of individual birds, and leads to the conclusion that populations of shrubsteppe birds are not existing at maximum density or "carrying capacity." Such observations are consistent with contentions that these populations lack close biological coupling with coexisting species and that interactions among these species (e.g. competition) are likely to play little if any role in the organization of their communities.

Comparative Population Bioenergetics of Three Insectivorous Passerines in a Deciduous Forest

At all levels of organization from the individual to the ecosystem, energy resources play a diverse and profound role. The ways in which these resources are used by a species represent the results of natural selection and are expressed in the species' life history patterns and adaptations (King 1974). Therefore, analyses of energy budgets, especially among similar species occupying the same environment, provide a means for understanding the ecological and energetic consequences of different life history phenomena. In addition, determinations of energy utilization and budgeting within species populations are useful in quantifying certain aspects of bird community structure and dynamics (Wiens 1973, Holmes and Sturges 1975, Karr 1975). Over the last two decades, energy budgets have been developed for several bird species occupying a variety of habitats (Orians 1961, Odum et al. 1962, Kale 1965, Brenner 1968, Pinowski 1968, Norton 1973, Custer 1974). No studies prior to ours (Black 1975; this paper), however, have been conducted on the population bionenergetics of insectivorous forestdwelling passerines. In this study, which is part of a larger investigation of bird community dynamics and energetics in the Hubbard Brook Experimental Forest, New Hampshire, we investigated the magnitude and seasonal patterns of time and energy use by three species of passerine birds: the Red-eyed Vireo (Vireo olivaceus), the Least Flycatcher (Empidonax minimus), and the Black-throated Blue Warbler (Dendroica caerulescens). These abundant and characteristic species of northern hardwoods forests differ in habits such as

Dynamics of Bird Communities in the Chihuahuan Desert, New Mexico

To say that communities of organisms are dynamic entities is a truism. Communities of birds-in common with those of other organisms-are in a perpetual state of flux, changing from day to day, season to season, and year to year in a number of properties: overall abundance, species composition, relative abundance of species, and impact on other components of the ecosystem, among others. It is also obvious that a fruitful approach for ecologists seeking to understand factors influencing properties of bird communities might be to follow different ones through time, comparing different properties as they change, while simultaneously following potential causative factors such as climate and food supply. In spite of the logic arguing for such an approach we can find little evidence of attempts to employ it, though many investigations have been made of both short-term and long-term changes in individual species populations (the more significant of these are summarized by Lack 1966). We know of only two published studies of temporal changes in a grassland or desert bird community (Finzel 1964, Webster 1964). Recent, largely unpublished studies made in connection with the International Biological Program will be discussed below.

Bird Community Dynamics and Energetics in a Northern Hardwoods Ecosystem

One of the central goals of ecology is to understand the organization and functional relationships among natural assemblages of organisms and their complex interaction within ecological systems. Although the structures and productivities of terrestrial plant communities and their role as primary producers have been frequently studied (Whittaker 1970), there is little comprehensive information on whole communities of consumer organisms, especially those such as birds living above ground and often high in forest canopies. This is due in large part to the methodological difficulties of obtaining quantitative data simultaneously on the seasonal and annual fluxes in numbers and energy expenditures of a large number of populations. Recent work has demonstrated, however, that with intensive field investigations, bird community structures can be determined quantitatively (Emlen 1971, 1972; this paper) and that the energy expenditure of freeliving birds can be realistically estimated, using information on avian bioenergetics that has recently become available (Holmes & Sturges 1973). Several studies provide information on the structure and energy flow of individual bird populations, e.g. old field sparrows in winter (Odum, Connell & Davenport 1962), breeding red-winged blackbirds, AgelaEus phoeniceus* (Brenner 1968), and marsh wrens, Telmatodytes palustris (Kale 19653, and tree sparrows Passer montanus (Pinowski 1968) on an annual basis. At the community level, Uramoto (1961) and Karr (1968, 1971) have estimated the populations sizes and energy expenditure of birds breeding in deciduous forest in Japan and in Illinois and Panama, respectively, but their results were not discussed in an ecosystem context. Varley (19703 and Phillipson (19733 have modelled energy pathways through a temperate oak woodland, with emphasis on consumer organisms, although their data on the avian community were very incomplete. The most thorough and comprehensive analysis of avian community structure and energetic role is that of Wiens (1973) for grassland bird communities in summer. Inthis paper, we report some results from 51 continuous months of intensive and quantitative studies of bird community dynamics and energetics in the northern hardwoods ecosystems of the Hubbard Brook Experimental Forest in West Thornton, New Hampshire, U.S.A. The major objectives of this paper are to examine the seasonal and annual Ruxes in bird community composition, absolute population densities, biomass, productivities, and energy expenditures, and to consider the place of birds in the energy flow pathways of northern hardwood forests. A report with procedures for estimating