In the Great Lakes region, numbers of the Piping Plover (Charadrius melodus) are gradually increasing, but the population of <100 pairs is still at risk of extinction and remains sensitive to fluctuations in adult survival. Given the central role of adult survival in population recovery, an accurate estimate of this quantity is essential. Using a Barker model and 11 years of mark-resighting data of breeding and nonbreeding Piping Plovers (1998–2008), we estimated true adult survival (S), probability of detection of breeding birds (p), probability of detection nonbreeding birds (R), and site fidelity (F). Adults' survival rate averaged 0.76 but with considerable annual process variation (σ = 0.05) and strong evidence of decline over this period. The probability of detection of breeding birds was near perfect (p = 1.00, 95% CI 0.97–1.00), but probabilities of detection of nonbreeders were substantially lower, although they increased from approximately 0.25 to 0.50 through the study period because of increased effort in observation and reporting. The birds' fidelity to the area of the Great Lakes in which they were monitored was high ( = 0.91, 95% CI 0.66–0.98), with evidence of an increase through time that we attribute to increased intensity of monitoring. Evidence for a decline in true adult survival over nearly a decade suggests a pressing need for research on the timing and causes of adult mortality in this population.

The post-fledging period represents a time of considerable energetic constraint and selection pressure for juvenile songbirds. Despite its potential importance for survival, this period remains one of the least studied stages of the annual cycle. Using radio telemetry and taking advantage of unique topographical characteristics at three spatially separated study sites, we assessed four hypotheses regarding the function of broad-scale, landscape-level movements for juveniles: (1) commencement of migration, (2) habitat optimization, (3) selection of a breeding territory, and (4) formation of a homing target. For the Blackpoll Warbler (D. striata), we found that movement was directed to the southeast at all three study sites but individuals were also associated with river valleys, providing support for the homing-target and habitat-optimization hypotheses. For the Yellow-rumped Warbler (Dendroica coronata), we found that movement was directed toward and down river valleys, following the unique orientation of the valleys at each study site, supporting the habitat-optimization hypothesis. Our results provide important insights into the function of broad-scale landscape-level movements for juvenile songbirds prior to migration, as well as their relative importance to individuals' success, both immediate and future.

Delaware Bay is an important spring stopover site for the Western Atlantic Red Knot (Calidris canutus rufa), which has suffered a recent population decline. Stable-isotope evidence suggests Red Knots from different wintering areas partially segregate at the site, with short-distance migrants concentrating on the Atlantic coast of New Jersey rather than in Delaware Bay itself. The Red Knot has been little studied on the Atlantic coast, and our objectives were to determine the extent to which birds captured on the coast used Delaware Bay and to characterize their habitat use and behavior. In 2006, we radio-tracked birds captured on an Atlantic beach and collected behavioral data and prey samples for comparison to those at randomly selected plots within Red Knot habitat. Depending on tidal stage, the distribution of Red Knot locations was skewed toward either Atlantic marshes or beaches. It was skewed away from Delaware Bay on the falling tide. Red Knots observed in Delaware Bay were foraging, whereas birds on the Atlantic coast were equally likely to forage or rest on rising and high tides. On the Atlantic coast, Red Knot locations had significantly more prey items than did random points, but this was not so on Delaware Bay beaches. Our results support the idea of partial segregation between Red Knots stopping on New Jersey's Atlantic coast and those in Delaware Bay, at least in some years. Whatever the cause of this partial segregation, conservation of Red Knots depends on protecting a complex of stopover habitats.

Bachman's Sparrow (Peucaea aestivalis) is listed as a species of conservation concern throughout its range and therefore an important candidate for demographic studies. We estimated probabilities of males' annual survival by monitoring a large color-marked population for 4 years by using a combination of (1) mark-resighting within a primary study area and (2) surveys outside the primary area to document emigration. We used multi-strata models to assess probabilities of survival, detection, and movement between the primary study area and the surrounding landscape. In the top models included in our confidence set, probabilities of annual survival were ≥0.68. Probabilities of detection and dispersal varied among the top models but averaged ≥0.86 and ≤0.14, respectively, for individuals in the primary study area. We used a survival probability of 0.68 in combination with published information on the species' productivity to construct a two-stage Leslie matrix. Survival of adult males had the greatest proportional influence on population projections (elasticity = 0.52), while survival of juveniles and annual productivity had equal influence (elasticities = 0.24). Our results suggest that off-plot surveys and multistrata models provide an efficient method for estimating probabilities of survival of Bachman's Sparrow.

In many species, immature individuals are less proficient at foraging than are adults, and this difference may be especially critical during winter when survival can be at its minimum. We investigated the foraging proficiency of adult and immature American Oystercatchers (Haematopus palliatus) during the nonbreeding season. Oystercatchers forage on prey that must be handled with specialized skills, so age-related differences in foraging behavior may be expected. We found that adults spent more time searching than did immatures, a trend toward immatures taking longer to handle prey than did adults, and immatures more often handling prey unsuccessfully than did adults. Feeding rates and diet composition did not differ by age class. We posit that the immature birds traded off longer handling times with shorter searching times and that ultimately the abundant prey in the region may contribute to the ability of immature birds to feed at rates similar to those of adults.

The quality of winter territory can have important consequences for migratory songbirds throughout the year. In the American Redstart (Setophaga ruticilla), a warbler in which plumage maturation is delayed, yearling males winter in a variety of habitat types that vary in quality. Little is known regarding which physical traits allow some yearlings to occupy higher-quality sites. Here, we measured eight variables characterizing the plumage and morphology of yearling males in two habitats that differ in suitability to determine which aspects of phenotype predict winter habitat occupancy. Yearlings wintering in high-quality mangrove habitat in Jamaica had more extensive adult-like black plumage on their breast than those in low-quality scrub. No other phenotypic differences associated with winter habitat were detected. Additionally, yearling males arriving earlier on the breeding grounds in Ontario had more extensively black breasts than those arriving later. Previous studies using stable carbon isotopes have linked adult male American Redstarts' date of arrival in the breeding range with quality of their winter habitat. Our findings indicate an association between the extent of adult-like plumage and habitat occupancy, suggesting that variation in yearling males' appearance may be correlated with their ability to compete for high-quality habitat.

The realized niches of closely related species must differ if these species are to co-exist stably. The Gentoo (Pygoscelis papua) and Chinstrap Penguins (P. antarctica) breed concurrently in the Scotia Sea and Antarctic Peninsula regions. To identify species- and site-specific foraging niches, we compared the two species' foraging niches, including diet and foraging locations, at two sites in the South Shetland Islands with contrasting marine habitats. Both species fed primarily on Antarctic krill (Euphausia superba), though fish was also a component of the Gentoo Penguin's diet. Gentoo Penguins foraged closer to shore than did Chinstrap Penguins. Gentoo Penguins foraged during the day, while Chinstrap Penguins foraged throughout the diel cycle, sometimes traveling farther from shore on overnight foraging trips. In most cases, these species-specific foraging patterns were consistent with trends seen elsewhere in the region. However, within each species, site-specific differences in foraging niches were still evident. Overall, we observed that Gentoo Penguins traveled within consistent and limited distances from their colonies, but their diets varied by site in both meal mass and content. Chinstrap Penguins, in contrast, had a uniform diet of krill at both sites but varied the distance from shore and times at which they foraged.

Although the onset of incubation prior to completion of the clutch leads to developmental asynchrony of the embryos, waterfowl eggs tend to hatch synchronously (within 3–24 hr). The mechanisms waterfowl use to synchronize development of embryos and allow for synchronous hatching are still not well understood. A variety of mechanisms have been proposed, including vocalization by embryos, egg size, egg constituents, and developmental asynchrony at hatching. Empirical evidence for any of these mechanisms, however, is limited for species that lay large clutches (>6 eggs). We hypothesize that the eggs' position within the clutch may synchronize development of embryos of dabbling ducks that lay clutches large enough that central and peripheral eggs may be distinguished. We tested this hypothesis by comparing the developmental stage of central and peripheral eggs and testing for a relationship between the order in which an egg was laid and its position (central or peripheral) within the clutch. We found that eggs laid later were central more often than expected if their distribution were random and that during the first 12 days of incubation central eggs tended to be less developed than peripheral eggs.

Theory predicts that parents should invest less in dependent offspring with lower reproductive value, such as those with a high risk of predation. Moreover, high predation risk can favor reduced parental activity when such activity attracts nest predators. Yet, the ability of parents to assess ambient nest-predation risk and respond adaptively remains unclear, especially where nest-predator assemblages are diverse and potentially difficult to assess. We tested whether variation in parental investment by a multi-brooded songbird (Brewer's Sparrow, Spizella breweri) in an environment (sagebrush steppe) with diverse predators was predicted by ambient nest-predation risk or direct experience with nest predation. Variation among eight sites in ambient nest-predation risk, assayed by daily probabilities of nest predation, was largely uncorrelated across four years. In this system risk may therefore be unpredictable, and aspects of parental investment (clutch size, egg mass, incubation rhythms, nestling-feeding rates) were not related to ambient risk. Moreover, investment at first nests that were successful did not differ from that at nests that were depredated, suggesting parents could not assess and respond to territory-level nest-predation risk. However, parents whose nests were depredated reduced clutch sizes and activity at nests attempted later in the season by increasing the length of incubation shifts (on-bouts) and recesses (off-bouts) and decreasing trips to feed nestlings. In this unpredictable environment parent birds may therefore lack sufficient cues of ambient risk on which to base their investment decisions and instead rely on direct experience with nest predation to inform at least some of their decisions.

Birds that migrate to the neotropics have experienced dramatic population decreases over the last few decades. Although the transformation of their wintering habitats has been related to such decreases, several neotropical migrants are tolerant of human disturbances and winter in human-altered habitat. We surveyed a neotropical city broadly to evaluate how migrant birds respond to urbanization, finding that urbanization has a negative effect on communities of migrant birds: species richness and density of migrants decreased with urbanization. Although we recorded a significant decrease in the total number of individual neotropical migrants toward the center of the city, we found no differences among species when we assessed possible effects of the distance from the city's edge by species. Only tree cover had a positive significant relationship with both species richness and bird abundance. Our results show that urbanization can have an even greater negative effect on communities of neotropical migrant birds than for those of resident birds. However, some migrants that used the urban habitats we studied are of conservation concern, indicating that urbanization can act as a two-edged sword, affecting several migratory species negatively while offering winter habitats to others.

The magnitude of differentiation in vocal signals examined in a phylogenetic context has helped uncover cryptic diversity in subsocine passerines such as the antbirds (Thamnophilidae). On the basis of variation in 14 acoustic traits of loudsongs and a phylogenetic hypothesis inferred from a mitochondrial gene tree, we investigated species limits in a group of trans-Andean Myrmeciza antbirds with a convoluted taxonomic history (M. l. laemosticta, M. l. palliata, M. nigricauda, and M. berlepschi). Diagnostic differences in three vocal traits suggest that the two subspecies of the Dull-mantled Antbird (M. l. laemosticta and M. l. palliata) should be treated as distinct species. We present evidence that only two diagnosable vocal characters may be sufficient for two populations to be considered distinct species in this group. The vocal results are consistent with the molecular phylogenetic analysis, which revealed a long independent history of isolation of M. l. laemosticta from M. l. palliata, roughly congruent with the divergence observed between species currently recognized in this assemblage. We propose elevating the Magdalena Antbird (M. palliata) to species rank and thus that four species be recognized in the M. laemosticta complex.

The Black Oystercatcher (Haematopus bachmani) is an intertidal obligate along North America's Pacific coast and a species of high conservation concern (population size 8900–11 000 individuals). Understanding birds' movements and space use throughout the annual cycle has become paramount in the face of changing environmental conditions, and intertidal species may be particularly vulnerable to habitat change due to anticipated sea-level rise associated with climate change and increasing coastal development. Conservation of the Black Oystercatcher is hindered by a lack of information on the species' nonbreeding distribution, seasonal movements, and habitat connectivity. Using satellite (n = 19) and VHF (n = 19) radio transmitters, we tracked Black Oystercatchers from five breeding sites (Vancouver Island, British Columbia; Kodiak Island, Prince William Sound, Middleton Island, and Juneau, Alaska) through one and one half annual cycles (May 2007–Dec 2008). We documented medium- to long-distance migration (range of migration distance 130–1667 km) in three populations (Prince William Sound, Middleton Island, and Juneau) and year-round residency in two others (Kodiak and Vancouver Island). We observed variation in the timing and length of migration by study site, and individual birds demonstrated fidelity to breeding and nonbreeding sites. We did not observe strong migratory connectivity. Migratory oystercatchers distributed themselves widely along the coasts of British Columbia and southeast Alaska during winter. Results provide baseline information on the Black Oystercatcher's movements and space use throughout the annual cycle.

In the southeastern U. S., longleaf pine savannas, important habitat for grassland birds, are subject to hurricanes and subsequent management. The intensity of tropical storms is predicted to increase with climate change, heightening the importance of understanding the effects of storm disturbance and salvage logging on species of concern such as Bachman's Sparrow (Peucaea aestivalis), a pine-savanna specialist. To determine how Bachman's Sparrow occupancy varies with the age and salvage history of pine stands, and to identify the vegetation and post-hurricane habitat features influencing stand occupancy, we surveyed Bachman's Sparrows in Mississippi savannas affected by Hurricane Katrina. Our point counts and vegetation surveys covered 89 stands over two breeding seasons, beginning one year after salvaging. Bachman's Sparrow occupancy was best predicted by increasing graminoid cover and density, with evidence of a quadratic effect reversing the relationship at high levels, decreasing tree density, also with a quadratic effect, decreasing shrub cover and density, and increasing abundance of downed tree crowns and upturned root balls. Occupancy was higher in mature stands, regardless of salvage, and in stands of seedlings and saplings, but lower in middle-aged stands. Our results suggest that disturbance from Katrina may have benefited Bachman's Sparrows by thinning trees, creating perches in the form of downed tree crowns, from which males sing, and creating refugia from predators in upturned root balls. Three years after the disturbance, salvage logging appeared to have no effect on occupancy, but such logging could be detrimental if it alters ground-layer vegetation severely.

The effects of climate change have been detected in numerous biological systems. Study of phenology, including the timing of bird migrations, has proven to be an effective tool for understanding the degree to which plants and animals are affected by climate change. In this study, we analyzed records of birds' arrival at Concord, Massachusetts, over 157 years, compiling the longest known record of bird-arrival dates in North America. Using records of bird arrivals by American philosopher and naturalist Henry David Thoreau for 1851–1854 and the well-known ornithologists William Brewster for 1886 and 1900–1919 and Ludlow Griscom for 1930–1931 and 1933–1954, we examined whether birds are shifting their arrival times in response to a warming climate. Concord resident Rosita Corey provided a set of recent observations for the years 1956–1973 and 1988–2007. When we considered all 22 species of migratory songbirds we analyzed together, we found no average change in arrival date over time, though when we analyzed each species separately, we found that three species are arriving significantly earlier and four species are arriving later. The arrival dates of eight species are significantly correlated with temperature, seven of these species arriving earlier during warmer years. At Concord in general, birds' arrival times are apparently less responsive to temperature than are plants' flowering times, a disparity that has the potential to lead to ecological mismatches in this ecosystem. This study demonstrates the challenges of using nontraditional natural-history data in climate-change research.

Previous studies have shown that Eurasian Reed Warblers (Acrocephalus scirpaceus) are able to distinguish the Common Cuckoo (Cuculus canorus) from other intruders at the nest and that mobbing is an effective defense measure against brood parasitism. Here we studied the nest-defense behavior of Eurasian Reed Warblers facing four successive exposures to a Common Cuckoo to investigate whether a previous experience of interacting with a brood parasite may play a role in shaping the host's defenses in further encounters. Nesting warblers significantly increased their aggressive behavior from the first to the second presentation of a dummy Common Cuckoo and then sustained their response at the same intensity. The intensity with which the birds tested mobbed the dummy decreased both as the season progressed and with the time of the day. Multiple encounters with the dummy, however, did not increase the warblers' propensity to make recognition errors, i.e., to reject their own eggs in the absence of a Common Cuckoo egg in the nest. We discuss possible explanations of the increased intensity of nest defense with respect to the positive-reinforcement hypothesis and known patterns of nest attendance in the Eurasian Reed Warbler.

The Rusty Blackbird (Euphagus carolinus) breeds across the boreal forest zone of North America and winters throughout the eastern United States. Over the past four decades, the North American Breeding Bird Survey and the Christmas Bird Count have shown high rates of population decline ranging from approximately 5 to 12% per year. Regional surveys suggest declines and range retractions in the southern boreal zone. Analyses of historical accounts suggest that the Rusty Blackbird's abundance has been dropping steadily for over a century. A number of hypotheses have been proposed to explain the decline. The species relies on wooded wetlands throughout the year, so loss and degradation of these habitats—particularly in the winter range—is a prime suspect. Blackbird-control programs may have contributed. In recent decades, habitat disturbance, global warming, and environmental contamination in the boreal zone may have taken their toll on breeding populations. In 2005, the International Rusty Blackbird Working Group was formed to develop research efforts toward understanding the mysterious decline. This special section presents the group's research findings—the first on the species' use of breeding and winter habitat, reproductive success, parasite prevalence, patterns of molt, and migratory connectivity. Data on the levels of methylmercury in tissues and the role of timber management on reproductive success are intriguing. We outline research needed for assessment of the roles of various factors in causing the decline of the Rusty Blackbird.

The Rusty Blackbird (Euphagus carolinus) has declined dramatically across its range in North America since at least the 1960s, but the causes for this decline are unknown. We measured ratios of stable hydrogen isotopes (δD) in feathers collected from Rusty Blackbirds wintering in the Mississippi Alluvial Valley (n = 255 birds) and the coastal plain of South Carolina and Virginia (n = 281 birds), 2005–2009, to estimate the region of origin of birds wintering west and east of the Appalachians, respectively. We also measured δD values in feathers from all available museum specimens collected from 1879 to 1990 in these same two regions (n = 190 birds). Isotopic values support migratory connectivity in this species with breeding populations in the western and central boreal forest migrating through a central or Mississippi flyway and those breeding in the eastern boreal forest migrating to a winter range east of the Appalachians. We detected little long-term change in the breeding origins of modern and historical populations wintering east and west of the Appalachians. However, we found short-term temporal variability in the breeding origins of birds wintering on the coastal plain from 2007 to 2009. The migratory divide suggests that efforts at management should be tailored to at least eastern and western subpopulations on both the breeding and wintering grounds. Our approach can be applied to a broad range of migratory species in North America and on other continents.

Recent evidence suggests that mercury exposure has negative effects on the health of songbirds, and species that forage in wetlands may be at a greater risk of bioaccumulation of mercury than are those of other habitats. We examined mercury concentrations in blood and feathers from the wetland obligate and rapidly declining Rusty Blackbird (Euphagus carolinus) from five regions across North America: three wintering areas in the contiguous United States and breeding areas in the western boreal forests of Alaska and the Acadian forests of northeastern North America. In blood, mercury concentrations in Rusty Blackbirds from the Acadian forest (geometric mean 0.94 µg g^-1; n = 59) were >3× than in those from Alaska (0.26 µg g^-1; 107). Wintering birds had blood mercury levels approximately an order of magnitude lower than those of breeding birds (0.07 µg g^-1; 332). In feathers, mercury concentrations in samples from the Acadian forests exceeded published minimum levels for adverse effects on birds (8.26 µg g^-1; 45) and were 3× to 7× those observed from the other regions. The mercury concentrations we report in blood and feathers of the Acadian forest population of the Rusty Blackbird are among the highest reported for wild populations of passerines at sites without a known local source of mercury. Mercury should be considered as a potential contributor to the species' dramatic population decline in New England and the Maritime provinces and in other areas where bioavailability of mercury is high.

Animals are subject to ecological traps when anthropogenic changes create habitat that appears suitable but when selected results in decreased fitness. The Rusty Blackbird (Euphagus carolinus) breeds in boreal wetlands and has declined by 85–95% over the last half century. We studied nest-site selection and daily nest-survival rate (DSR) of 43 Rusty Blackbird nests in northern New England and evaluated whether regenerating logged areas adjacent to wetlands created ecological traps. Although nesting adults avoided high-canopied forests and selected areas with dense balsam fir (Abies balasmea) 1 to 3 m high, those characteristics were not associated with DSR. Conversely, the frequency of speckled alder (Alnus incana) and sedges (Cyperaceae) in the nest plot varied with DSR, suggesting that the risk of predation of nests within wetlands was lower. DSR also varied with past logging; nests in plots not harvested recently were 2.3× more likely to fledge young than nests in plots harvested within 20 years. When logging extends to the edges of or into wetlands, the subsequent dense regenerating conifers appear to attract Rusty Blackbirds to nest closer to or within these human-altered uplands, exposing their nests to increased predation not typical of unaltered wetlands. Three surrogates for habitat preference did not differ by timber-management history, suggesting that the birds do not prefer habitats that increase their fitness. Rusty Blackbirds nesting in harvested wetlands may be subject to “equal preference” ecological traps, and we suggest that buffers 75 m wide around the perimeter of suitable wetlands should increase DSR.

We examined the nesting ecology of the Rusty Blackbird (Euphagus carolinus) from 162 nests monitored for nest survival in Alaska and 252 incidental records from Alaska and Canada to identify important habitats for nesting and to test whether low rates of nest success are contributing to populations declines in Alaska. In coastal Alaska and throughout Canada, nests were primarily in conifers (85% of 212 nests). All conifer nests in Alaska and 80% of those in Canada were placed in spruce (Picea spp.), primarily black spruce (P. mariana). In Alaska use of small spruces (<8 cm dbh) was selective and resulted in high reproductive success—nest survival increased with black spruce density, and success of nests in spruce (79%) was higher than that of nests in deciduous vegetation (52%). Survival of nests in taller spruce declined, possibly from predation by red squirrels (Tamiasciurus hudsonicus). Rusty Blackbirds nested near water in interior ( = 8 m) and south-central Alaska ( = 30 m), and small spruces near water appear to be important over much of the species' range. In interior Alaska, however, most nests were in willows (78%), which dominated the vegetation near water. Nest success in Alaska averaged 56%, similar to rates over the same period in New England but higher than those of other North American blackbirds (30–39%). Studies are needed to verify whether nest survival is also high in Canada and to investigate where and why deficits in survival of adults or juveniles may be limiting population growth.

We used double sampling to estimate densities of Rusty Blackbird (Euphagus carolinus) nests among boreal wetlands in Anchorage and Tanana Flats, Alaska, 2007–2008. We also assessed habitat selection by examining the number and location of nests relative to the availability of various wetland types. We rapidly surveyed 78 sample units for adult Rusty Blackbirds and intensively searched for nests in a subset of 55 units to determine actual numbers of nests. Rapid surveys detected 97% of the 75 nests when we restricted counts to pairs and lone females. They overestimated nest numbers by 17% when we included in the counts lone males, which sometimes made long flights within and between sample units. Nest densities in sampled wetlands averaged 2.2 and 3.4 nests km ² in Anchorage and Tanana Flats, respectively, surprisingly similar despite wetlands being rare in Anchorage. An abundance of wetlands made identifying important breeding habitats difficult on the Tanana Flats, where blackbirds used most wetland types in relation to their availability and avoided wetlands with low shrubs. Habitat selection was clearer in Anchorage, where ponds, lakes, and wetlands with emergent vegetation were positive predictors of nest abundance. In this area blackbirds also selected forested wetlands and avoided upland habitats for nest sites. An affinity for open water has been noted throughout the Rusty Blackbird's breeding range. Research is needed to understand whether this affinity is due to specialized food requirements and to assess whether this species is vulnerable to widespread drying of wetlands across boreal Alaska.

We calculated the home ranges and core areas of 13 adult Rusty Blackbirds (Euphagus carolinus) in Maine to determine (1) the area requirements of breeding adults, (2) whether area requirements of the sexes and of colonial and noncolonial individuals differ, and (3) the proportion of the home range and core area that would be protected by a buffer of no logging of 50–100 m around occupied wetlands. Mean home ranges (37.5 ± 12.6 ha) and core areas (11.1 ± 2.8 ha) were large in comparison to those of other breeding icterids, and adults often foraged in multiple unconnected wetlands. Rusty Blackbirds that were part of a loose colony had home ranges and core areas three times larger than those of pairs that nested solitarily, which we speculate may be due to adults following one other to feed on unpredictable emergences of aquatic insects. Home ranges and core areas included a surprisingly small amount of wetland habitat, only 12% and 19% respectively, but adults often foraged in small wet patches (<16 m²) in otherwise upland habitat. The 75-m buffers around wetlands that we recommended in a concurrent study may help protect the Rusty Blackbird's nesting habitat, but such buffers contained less than half the average home range, suggesting that they may be of only limited benefit as a conservation strategy for protecting foraging habitat.

The Rusty Blackbird (Euphagus carolinus) has declined by 90% since the 1960s, possibly from conversions of forested wetlands in the southeastern U.S. to agriculture. Because its habitat use during the nonbreeding season had not been quantified, we estimated occupancy rates of Rusty Blackbirds wintering in the lower Mississippi Alluvial Valley in relation to habitat type (four types of bottomland forest and adjacent agricultural fields), tree density, canopy cover, and water cover. In January and February, we surveyed 89 sites eight times during 2006, 117 and 109 sites 10 times during 2007 and 2008, respectively. Occupancy of individual birds, estimated from their presence or absence at a site, was generally high and greater in 2006 (0.71) than during either 2007 (0.43) or 2008 (0.37). Occupancy of flocks, estimated from presence or absence of ≥20 individuals, was highest in 2006 (0.46), lowest in 2007 (0.17), and intermediate in 2008 (0.30), possibly because of water levels fluctuating from year to year. Rusty Blackbirds were not as specialized in habitat use as expected; habitat characteristics did not consistently predict occupancy of either individuals or flocks. Occupancy of individuals increased with canopy cover during 2007, and occupancy of flocks decreased with tree density during 2006. In 2008, occupancy of individuals increased with tree density and was higher in wet bottomland forest (0.51) than in other habitats (0.31–0.35). Further research is needed to determine whether this lack of habitat specificity is widespread and possibly the result of diverse food requirements during the nonbreeding season.

The Rusty Blackbird (Euphagus carolinus) has declined precipitously over the past several decades, and stressors on both the breeding and wintering grounds are suspected causes. Over 3 years, we collected blood samples from breeding birds in Alaska and Maine and from wintering birds in Mississippi and Arkansas to determine the prevalence of hematozoan infections at different times of the year. The prevalence of hematozoa (percent of birds infected) in Alaska was 44% of 43 birds, lower than previously reported from Newfoundland and Ontario (83% of 23 birds). Blood parasites were found among 67% of 12 Rusty Blackbirds sampled in Maine, not different from the prevalence in Newfoundland and Ontario. Leucocytozoon was the most common parasite; few breeding birds were infected with Plasmodium or Trypanosoma sp. During the winter in Mississippi and Arkansas, the overall prevalence of hematozoa was 49% over three years. In winter as in summer, Leucocytozoon was the most commonly encountered parasite (n = 62 birds), followed by Haemoproteus (n = 5), microfilaria (n = 2), Trypanosoma (n = 2), and Plasmodium (n = 1). The prevalence of hematozoa among wintering Rusty Blackbirds was much higher than expected because winter is generally a time when there are few transmissions and in most birds blood parasites are absent from the peripheral circulation. This high prevalence might indicate a nonseasonal relapse of hematozoan infections among wintering birds; possibly due to high levels of stress, which are known to lower the immune response and trigger nonseasonal relapses.

Aging birds often relies on differences in plumage between immatures and adults, and understanding these patterns can improve our ability to discern demographic patterns within populations. We investigated patterns of prebasic molt of the Rusty Blackbird (Euphagus carolinus) in fall at Whitehorse, Yukon Territory, and developed a new technique for aging based on characteristics of the head plumage acquired during prebasic molt. Furthermore, we investigated the possibility of a prealternate molt among wintering Rusty Blackbirds on the basis of captures in Mississippi and museum specimens from across the species' winter range. Finally, we examined how a prealternate molt might affect the aging of winter birds by plumage. Rusty Blackbirds completed their prebasic molt by the end of September, and immature birds had a more prominent eye ring and a paler chin than adults, allowing a reliable age determination. Previously, the Rusty Blackbird was thought to attain its breeding plumage through feather wear exclusively, but we discovered a partial prealternate molt in our examinations of live captures (76% molting) and museum specimens (59% molting). The prealternate molt was observed in all age and sex classes, was concentrated along the feather tracts of the head, and peaked in occurrence from mid-February to mid-March, when nearly 90% of birds were molting. Between mid-December and mid-February, the prealternate molt did not appear to interfere with aging birds in the hand by the pattern of the eye ring and chin in basic plumage. Age determination later in the spring, however, remains to be investigated.