Elizabeth M. Madden

TIME-SPECIFIC VARIATION IN PASSERINE NEST SURVIVAL: NEW INSIGHTS INTO OLD QUESTIONS

Abstract Understanding nest survival is critical to bird conservation and to studies of avian life history. Nest survival likely varies with nest age and date, but until recently researchers had only limited tools to efficiently address those sources of variability. Beginning with Mayfield (1961), many researchers have averaged survival rates within time-specific categories (e.g. egg and nestling stages; early and late nesting dates). However, Mayfield’s estimator assumes constant survival within categories, and violations of that assumption can lead to biased estimates. We used the logistic-exposure method to examine nest survival as a function of nest age and date in Clay-colored Sparrows (Spizella pallida) and Vesper Sparrows (Pooecetes gramineus) breeding in north-central North Dakota. Daily survival rates increased during egg laying, decreased during incubation to a low shortly after hatch, and then increased during brood rearing in both species. Variation in survival with nest age suggests that traditional categorical averaging using Mayfield’s or similar methods would have been inappropriate for this study; similar variation may bias results of other studies. Nest survival also varied with date. For both species, survival was high during the peak of nest initiations in late May and early June and declined throughout the remainder of the nesting season. On the basis of our results, we encourage researchers to consider models of nest survival that involve continuous time-specific explanatory variables (e.g. nest age or date). We also encourage researchers to document nest age as precisely as possible (e.g. by candling eggs) to facilitate age-specific analyses. Models of nest survival that incorporate time-specific information may provide insights that are unavailable from averaged data. Determining time-specific patterns in nest survival may improve our understanding of predator-prey interactions, evolution of avian life histories, and aspects of population dynamics that are critical to bird conservation.

Tree and shrub invasion in northern mixed-grass prairie: implications for breeding grassland birds

Abstract North American grasslands continue to decline in quantity and quality. In the northern mixed-grass prairie, potential edge and fragmentation effects on grassland birds are poorly understood and conclusions are based largely on data from outside the region. Lands in and adjacent to J. Clark Salyer National Wildlife Refuge in north-central North Dakota comprise one of the largest contiguous patches of northern mixed-grass prairie remaining in North America. However, within the region, aspen (Populus tremuloides), willow (Salix spp.), and other woody species have increased, such that continued existence of grasslands is threatened. We examined how breeding grassland birds responded to habitat that has been variably fragmented by encroaching woody vegetation. The probability of occurrence decreased markedly for 11 of 15 bird species (including 3 endemic to the northern Great Plains) as percent woodland, tall shrub, or brush cover increased. Bird species were increasingly affected as the height of woody plants increased from brush to tall shrubs to trees. Grasslands became largely unsuitable for 9 species as woodland cover exceeded 25%. Derived models can be used by land managers to predict the outcome of management actions that alter plant community succession or that restore grasslands degraded by woody invasion.

Nest Survival of Clay-Colored and Vesper Sparrows in Relation to Woodland Edge in Mixed-Grass Prairies

Abstract The quantity and quality of northern mixed-grass prairie continues to decline because of conversion to agriculture, invasion of woody and exotic plants, and disruption of important ecological processes that shape grasslands. Declines in grassland bird populations in North Dakota, USA, have coincided with these largely anthropogenic alterations to prairie habitat. In grasslands of north-central and northwestern North Dakota, woody plants have increased due primarily to fire suppression, extirpation of bison (Bos bison), and widescale planting of tree shelter belts. In northern grasslands, effects of woody vegetation on survival of grassland birds are poorly understood, and conclusions are based mainly on studies conducted outside the region. We examined nest survival of clay-colored sparrows (Spizella pallida) and vesper sparrows (Pooecetes gramineus) relative to the distance nests were located from aspen (Populus tremuloides) woodland edges and relative to other habitat features near the nest. Clay-colored and vesper sparrow nest survival was higher for nests located near woodland edges, nests with greater cover of Kentucky bluegrass (Poa pratensis), and nests more concealed by vegetation. Vesper sparrow nest survival increased as the percent cover of tall shrubs near the nest increased. Based on video-camera data, the 13-lined ground squirrel (Spermophilus tridecemlineatus) was the most common predator of sparrow eggs and young. Thirteen-lined ground squirrels were more common far from woodland edges than near, and this pattern may, in part, explain clay-colored and vesper sparrow nest survival in relation to woodland edges. In contrast to our results, studies conducted in other grassland systems generally report lower nest survival for grassland birds nesting near trees and shrubs. This disparity in results demonstrates the need to identify specific nest predators and their distributions with respect to important habitat features because these data can be important in explaining—and perhaps predicting—patterns of nest predation.

NUMERICAL AND FUNCTIONAL RESPONSES OF MIGRANT BALD EAGLES EXPLOITING A SEASONALLY CONCENTRATED FOOD SOURCE

Abstract We determined the numerical and functional responses of migrant Bald Eagles (Haliaeetus leucocephalus) feeding on spawning kokanee salmon (Oncorhynchus nerka) at Hauser Reservoir, Montana from 1991–1995. Number of Bald Eagles was positively correlated with the fluctuating number of salmon carcasses during four of five years. Immigration and emigration rates were similar across years and were facilitated by several behavioral and physical characteristics of eagles: group foraging, communal roosting, and keen eyesight. Number of subadult eagles showed closer synchrony with density of salmon carcasses than did adult eagles. Eagles scavenging for salmon exhibited a Type II functional response. Handling times of scavenging eagles remained constant across the range of salmon carcass densities, whereas daily attack rates increased. Functional responses of scavenging eagles differed between age groups; adults exhibited a Type I response, whereas that of subadults could not be characterized. Handling times of scavenging adults were constant, but those of subadults increased with salmon density. Attack rates of scavenging adults increased with salmon carcass density. Consumption rates differed between age groups and among eagles using foraging modes of scavenging, stooping, and pirating, which suggests that eagles viewed live and dead salmon as alternative prey types. Bald Eagle scavenging of kokanee salmon was inversely density dependent.

Effects of Prescribed Fire on Vegetation and Passerine Birds in Northern Mixed-Grass Prairie

Abstract Prescribed fire is used widely to manage grasslands on National Wildlife Refuges and other public lands in the northern Great Plains, but its effects on habitat use or production of wildlife in the region are poorly understood. During 1998–2003, we used point counts to examine effects of prescribed fire on vegetation and passerines in a mixed-grass prairie complex in north-central North Dakota, USA (n  =  7 units, each 40–70 ha). Vegetation structure and, to a lesser extent, plant community composition varied with year of study (likely related to changes in annual precipitation) and with number of growing seasons since fire. Fire altered plant structure, especially the amount of residual vegetation, which in turn influenced bird species richness and abundance. The number of indicated pairs for sedge wren (Cistothorus platensis), clay-colored sparrow (Spizella pallida), Le Contes sparrow (Ammodramus leconteii), Savannah sparrow (Passerculus sandwichensis), and bobolink (Dolichonyx oryzivorus) were lowest during the first postfire growing season but generally increased and stabilized within 2–3 postfire growing seasons. Our results support the premise that grassland passerines are well-adapted to frequent, periodic fires, generally corresponding to those occurring prior to Euro–American settlement of the region. Prescribed fire is important for reducing tree and shrub invasion, restoring biological integrity of plant communities, and maintaining or enhancing populations of grassland-dependent bird species. Managers in the northern mixed-grass prairie region should not be overly concerned about reductions in bird abundances that are limited mostly to the first growing season after fire.

Monitoring Native Prairie Vegetation: The Belt Transect Method

decline in quantity and quality because of conversion to other land uses, proliferation of woody vegetation, and invasion by exotic plant species (Samson and Knopf 1994, Bragg and Steuter 1995). Appropriate stewardship of remaining prairies is increasingly critical to preserving their native biodiversity (Ryan 1990, Samson and Knopf 1996). To successfully restore and maintain the ecological integrity of native prairies, restorationists must periodically assess local plant composition to detect vegetation problems (for example, invasive exotic species), help plan management treatments (for example, prescribed fire and grazing), and evaluate results of restoration efforts. Decisions by conservation agencies and organizations to acquire extensive prairie tracts may rest on rapid, yet reliable, assessments of the composition of extant plant communities. Moreover, plant community composition significantly influences the distribution, abundance, and reproduction of native prairie wildlife (Davis and others 1999, Madden and others 2000). The ability to quantify these relationships can help restorationists predict wildlife responses to prairie management. Depending on scale and objectives, the composition of plant species or species groups may be quantified using any of various techniques that assess their frequency of occurrence, percent cover, or density (Elzinga and others 1998). Canopy-coverage estimations (Daubenmire 1959) are often used to measure composition in herbaceous plant communities. Other methods may assess the occurrence of certain individual species and the degree to which those species reflect nondegraded conditions (Swink and Wilhelm 1994, Rooney and Rogers 2002). However, we have found these and other species-level methods ill-suited for broadly addressing fundamental questions of interest to restorationists of the northern mixedgrass prairie. These basic restoration questions seldom require species-level identification. They focus instead on the extent of woody and herbaceous vegetation types, and especially on the amount of native and exotic vegetation. Relatively unintensive assessment methods that can be applied extensively and efficiently are needed for addressing basic prairie restoration issues in the context and scale of this region. Northern mixed-grass prairies set aside for conservation purposes are relatively extensive and remote, while persons charged with their stewardship are scarce. For example, a typical land manager in the U.S. Fish & Wildlife Service’s National Wildlife Refuge System in North Dakota is almost solely responsible for conserving 10,00050,000 acres (4,000-20,000 ha) of mixed prairie-wetland habitat, which often occurs as scores of tracts scattered widely over several counties. Moreover, the prairies typically need frequent, carefully

History and Breeding Ecology of the American White Pelican at Medicine Lake National Wildlife Refuge, Montana

Abstract The American White Pelican (Pelecanus erythrorhynchos) has been breeding at Medicine Lake National Wildlife Refuge (NWR) in northeastern Montana since 1939. We compiled information on the history and ecology of this breeding colony from U.S. Fish and Wildlife Service (USFWS) reports and narratives filed at Medicine Lake NWR. Nesting by American White Pelicans first occurred in 1939, and the recent 10-yr average size (1993-2002) is about 4,000 breeding pairs. Since 1990, the mean number of young fledged pair-1 was 0.51 ± 0.07 (SE, N = 56). Low temperatures, hailstorms, predation, and botulism have killed >50% of young pelicans in some years. American White Pelicans banded at Medicine Lake migrated south, mostly within the Central Flyway. They wintered predominately in Texas, Louisiana and Mexico. Protection of the Medicine Lake colony is important because the breeding colony is the largest in Montana and an important contributor to the eastern metapopulation.