Climate Correlates of 20 Years of Trophic Changes in a High-Elevation Riparian System

The consequences of climate change for ecosystem structure and function remain largely unknown. Here, I examine the ability of climate variation to explain long-term changes in bird and plant populations, as well as trophic interactions in a high-elevation riparian system in central Arizona, USA, based on 20 years of study. Abundances of dominant deciduous trees have declined dramatically over the 20 years, correlated with a decline in overwinter snowfall. Snowfall can affect overwinter presence of elk, whose browsing can significantly impact deciduous tree abundance. Thus, climate may affect the plant community indirectly through effects on herbivores, but may also act directly by influencing water availability for plants. Seven species of birds were found to initiate earlier breeding associated with an increase in spring temperature across years. The advance in breeding time did not affect starvation of young or clutch size. Earlier breeding also did not increase the length of the breeding season for single-brooded species, but did for multi-brooded species. Yet, none of these phenology-related changes was associated with bird population trends. Climate had much larger consequences for these seven bird species by affecting trophic levels below ( plants) and above ( predators) the birds. In particular, the climate-related declines in deciduous vegetation led to decreased abundance of preferred bird habitat and increased nest predation rates. In addition, summer precipitation declined over time, and drier summers also were further associated with greater nest predation in all species. The net result was local extinction and severe population declines in some previously common bird species, whereas one species increased strongly in abundance, and two species did not show clear population changes. These data indicate that climate can alter ecosystem structure and function through complex pathways that include direct and indirect effects on abundances and interactions of multiple trophic components

Diversity and Density of Shelterbelt Bird Communities

The number of bird species and the density of each species were monitored in 69 shelterbelts in eastern South Dakota during spring migration and breeding seasons in 1976 and 1977. A total of 44 different species of birds were found during breeding and 68 species during spring migration. Approximately 60 to 80% of the species in a shelterbelt eat insects as part of all of their diet. Most of these bird species are territorial. Usually only 1-2 pairs of a territorial species will reside in any one shelterbelt. This low density is caused by the limited habitat area that shelterbelts provide. Shelterbelts are, essentially, forest islands surrounded by cultivated and natural grasses. Area of the shelterbelt accounted for approximately 60% of the variation in the number of species and density of the bird communities in both season as a result of the “island effect”. The limited food space provided by these forest islands makes ecological isolation among coexisting species necessary for birds to replenish energy stores lost due to migrational flights. The importance of shelterbelt area on species numbers during the breeding season can be partly attributed to the minimum area requirement of territorial pairs during breeding. Some species will not reside in shelterbelts below a minimum size due to the large territory size these birds require. However, minimum area does not explain the upper limit placed on the number of species that will coexist in shelterbelts. Diffuse utilization of the limited food supplies was postulated as setting the upper limit. Bird species that coexisted tended to exhibit different foraging strategies, thus reducing overlap in use of food resources. Implied increases in territory size with increases in the number of coexisting species were found. Theories on species-area models were re-evaluated in terms of competitive saturation. Ability of the species source pool to supply enough competitively different species to saturate the available food space for the smallest islands was postulated as the reason for the high species-area slope found. All species-area relationships were evaluated in terms of one general curve and were considered a sub-section of that curve. Placement on the curve, and consequently, the slope of the species-area relationship, was related to immigration and extinction rates, based on the effective source pool size. The effective source pool size was related to the actual source pool size, the distance of the archipelago from the source pool, and the overall vagility of the species comprising the source pool. The idea of diffuse competition influencing the territory size of bird species was further investigated by mapping territories of yellowthroats, house wrens, and brown thrashers in 2 large shelterbelts. Results indicated that territory size of these species was larger in belts with greater number of coexisting species than in the smallest shelterbelt size colonized by one pair of each species. The variation in community diversity and density unexplained by area was attributed to environmental factors and sampling error. The effect of area was removed. The transformed data were analyzed to provide management alternatives using multiple regression to delineate the environmental factors influencing community diversity and density during both migratory and breeding seasons. A shelterbelt that is subjected to light grazing to eliminate a heavy shrub understory and enhance development of a lust; herbaceous lay considered optimal for both diversity and density. Dense rows of shrubs along the borders of the belts also contributed to an increase in the bi9rd population. Heavy grazing or mowing of the belt after the belt was well established reduced bird diversity. Utilization of tree species that provide open foliage conditions, such as Siberian elm, led to increased bird diversity and density. Multiple regression analyses of the environmental factors influencing 15 of the bird species commonly inhabiting shelterbelts were performed. IN general, the 14 species preferred a shelterbelt configuration similar to that described for the diversity and density measures. In addition, specific preferences of each species suggested ways of modifying the bird community composition. Removal of eastern red cedar may lead to a reduction in noxious species. Planting of green ash, due to it infection by heartrot, and retaining snags enhances the presence of hole nesting species such as house wrens and woodpeckers. Other species preferences are discussed

Avian Life-History Evolution in Relation to Nest Sites, Nest Predation, and Food

Food limitation is generally thought to underlie much of the variation in life history traits of birds. I examined variation and covariation of life history traits of 123 North American Passeriformes and Piciformes in relation to nest sites, nest predation, and foraging sites to examine the possible roles of these ecological factors in life history evolution of birds. Annual fecundity was strongly inversely related to adult survival, even when phylogenetic effects were controlled. Only a little of the variation in fecundity and survival was related to foraging sites, whereas these traits varied strongly among nest sites. Interspecific differences in nest predation were correlated with much of the variation in life history traits among nest sites, although energy trade-offs with covarying traits also may account for some variation. For example, increased nest predation is associated with a shortened nestling period and broth are associated with more broods per year, but number of broods is inversely correlated with clutch size, possibly due to an energy trade-off. Number of broods was much more strongly correlated with annual fecundity and adult survival among species than was clutch size, suggesting that clutch size may not be the primary fecundity trait on which selection is acting. Ultimately, food limitation may cause trade-offs between annual fecundity and adult survival, but differences among species in fecundity and adult survival may not be explained by differences in food abundance and instead represent differing tactics for partitioning similar levels of food limitation. Variation in fecundity and adult survival is more clearly organized by nest sites and more closely correlated with nest predation, species that use nest sites with greater nest predation have shorter nestling periods and more broods, yielding higher fecundity, which in turn is associated with reduced adult survival. Fecundity also varied with migratory tendencies; short-distance migrants had more broods and greater fecundity than did neotropical migrants and residents using similar nest sites. However, migratory tendencies and habitat use were confounded. making separation of these two effects difficult. Nonetheless, the conventional view that neotropical migrants have fewer broods than residents was not supported when nest site effects were controlled

A Conceptual Framework for Clutch-Size Evolution in Songbirds

Causes of evolved differences in clutch size among songbird species remain debated. I propose a new conceptual framework that integrates aspects of traditional life-history theory while including novel elements to explain evolution of clutch size among songbirds. I review evidence that selection by nest predation on length of time that offspring develop in the nest creates a gradient in offspring characteristics at nest leaving (fledging), including flight mobility, spatial dispersion, and self-feeding rate. I postulate that this gradient has consequences for offspring mortality rates and parental energy expenditure per offspring. These consequences then determine how reproductive effort is partitioned among offspring, while reproductive effort evolves from age-specific mortality effects. Using data from a long-term site in Arizona, as well as from the literature, I provide support for hypothesized relationships. Nestling development period consistently explains fledgling mortality, energy expenditure per offspring, and clutch size while accounting for reproductive effort (i.e., total energy expenditure) to thereby support the framework. Tests in this article are not definitive, but they document previously unrecognized relationships and address diverse traits (developmental strategies, parental care strategies, energy requirements per offspring, evolution of reproductive effort, clutch size) that justify further investigations of hypotheses proposed here