Little is known about the timing of migration, migration routes, and migratory connectivity of most of the >230 species of birds that breed at south temperate latitudes of South America and then migrate toward the tropics to overwinter. We used light-level geolocators to track the migration of 3 male and 3 female Fork-tailed Flycatchers (Tyrannus savana) captured on their breeding territories in Argentina. All birds initiated fall migration between late January and late February, and migrated 45 to 66 km day^-1 in a northwesterly direction through central South America to either one or two wintering areas. Five individuals first spent several weeks (in April and May) in western Amazonia (mainly Peru, northwestern Brazil, and southern Colombia) before moving east to spend the rest of the non-breeding season in central Venezuela and northern Brazil. One individual occupied primarily one wintering area in eastern Colombia, northwestern Brazil, and southwestern Venezuela. Fall migration took approximately 7–12 weeks to complete and covered a distance of 2,888–4,105 km. We did not analyze spring migration data because of broad overlap with the austral spring equinox. These results are the first data on wintering locations, migration timing, and routes of individual migrant passerine birds that breed in South America. Given the general lack of similar data for practically all migratory birds that breed in South America, geolocator technology has the potential to revolutionize our understanding of how birds migrate—and the threats they face—on South America's rapidly changing landscape.

Determining the distribution of stopover and overwintering areas of migratory animals is essential for understanding population dynamics and building predictive models. Tree Swallows (Tachycineta bicolor) are small songbirds that breed across North America. Data from Doppler weather radar and eBird indicate that Tree Swallow numbers increase throughout October and November in southeastern Louisiana, but then decrease during December. We thus hypothesized that southeastern Louisiana is a stopover area used by Tree Swallows during fall migration before they move to farther overwintering areas. We tested this hypothesis by attaching light-logging geolocators to Tree Swallows at five breeding sites spanning the species' breeding range from British Columbia to Nova Scotia, and then tracking their fall migration routes, stopover sites, and wintering locations. Of 38 individuals that returned in the following breeding season, 11 birds from three breeding sites (Saskatchewan, Wisconsin, and Ontario) used southeastern Louisiana as a stopover site. Arrival date and duration of stay closely matched observations from both eBird and radar data. From Louisiana, most Tree Swallows continued their migration to one of three wintering sites: peninsular Florida, the Bahamas, or the Yucatán Peninsula, whereas two birds remained until spring within 200 km of the stopover area. Our results (1) suggest that southeastern Louisiana is an extended stopover site for Tree Swallows that originate from a wide geographic range on the breeding grounds; and (2) demonstrate how geolocators, combined with other sources of movement information, reveal habitat use throughout the annual cycle.

We used archival geolocators to track the migration of Red-eyed Vireos (Vireo olivaceus), abundant forest songbirds with significantly increasing breeding-population trends, to identify important stopover and wintering regions. All individuals from a single breeding site (n = 10) wintered in northwestern South America, an extensively forested region, and in spring used a consistent route, crossing the Gulf of Mexico from the Yucatan to Louisiana. Their spring migration rate (146 km day^-1) was slower than that of most other songbirds tracked with geolocators from South America (>280 km day^-1). Red-eyed Vireos had an unexpectedly prolonged stopover (mean ± SD =18.6 ± 4.9 days) in Colombia soon after the onset of spring migration, and we suggest that this area may provide important fruit resources for fueling subsequent, more rapid, migration. The total duration of spring migration averaged 45.9 ± 4.6 days, but individuals covered the journey of ∼6,600 km in an average of only 13 days of flight. Males arrived at the breeding site over a 15-day period, and arrival date was significantly correlated with departure date from the wintering site in South America (r = 0.81, P = 0.002), which is surprising, considering the prolonged and variable durations of stopovers en route. Even more intriguing, fall arrival date in South America was significantly correlated with individual departure in spring, which suggests that some birds are on year-round early-versus-late schedules.

Descriptions of intra- and interspecific variation in migratory patterns of closely related species are rare yet valuable because they can help assess how differences in ecology and life-history strategies drive the evolution of migration. We report data on timing and location of migration routes and wintering areas, and on migratory speed and phenology, of Eastern Kingbirds (Tyrannus tyrannus) from Nebraska and Oklahoma and of Western Kingbirds (T. verticalis) and Scissor-tailed Flycatchers (T. forficatus) from Oklahoma. Eastern Kingbirds primarily departed the breeding site in September, migrating to the Amazon Basin (Bolivia and Brazil), >6,400 km from their breeding site, then used a second wintering site in northwestern South America (Colombia, Ecuador, and Peru) before returning to the breeding site in April. Western Kingbirds left Oklahoma in late July, migrating >1,400 km to northwestern Mexico, then to central Mexico and finally to Central America before returning to Oklahoma in April. Scissor-tailed Flycatchers departed Oklahoma mainly in mid-October, migrating to Central America (El Salvador, Guatemala, Honduras, and Nicaragua), ∼2,600 km from the breeding site, remaining there until early April before returning to Oklahoma. Timing of migration appears to be tightly linked to molt. Early departure of Western Kingbirds from the breeding site appears to be timed so that they molt in the Sonoran Desert region during the monsoon, whereas Scissor-tailed Flycatchers remain at their breeding site to complete molt in late summer, when insect prey are abundant. Eastern Kingbirds delay molt until reaching South America where, possibly, abundant fruit supports molt.

Some species of long-distance migrant birds are thought to follow spatiotemporal patterns of high food availability during the non-breeding season, a strategy termed “itinerancy,” instead of being sedentary in one specific site. We tracked the migration of a small Eurasian songbird, the Common Redstart (Phoenicurus phoenicurus), using archival light-level geolocators. The birds showed a distinct counterclockwise loop migration from northern Europe. Fall migration passed west of the Mediterranean Sea and along the northwest African coast before the birds made an abrupt change of direction at the southern edge of the Sahara toward the winter area farther inland in the West African Sahel. Spring migration was more direct: north to the Iberian Peninsula and back to northern Europe. The birds spent more time in only one winter site than they do during the breeding season in northern Europe, and they generally showed no signs of itinerancy—except for one bird that probably avoided an exceptional drought. Their arrival on the winter grounds was well timed with peak vegetation greenness, assumed to reflect food availability, but vegetation greenness declined rapidly during their stay, and resource availability was relatively low throughout most of the non-breeding season. Despite the highly seasonal conditions in the wintering area, itinerancy is apparently not an optimal strategy for the Common Redstart, possibly because of timing constraints. Alternatively, food availability may not be closely linked to vegetation greenness.

Painted Buntings (Passerina ciris) that breed in Oklahoma and molt in Sinaloa, Mexico, demonstrate a clear bimodal pattern of stable isotope ratios in their flight feathers. Some birds had a C3 carbon signature in primary 1 (P1, the first feather replaced during wing molt) and a C4 carbon signature in primary 9 (P9, the last primary to molt), whereas other sympatric birds evinced a C4-based diet throughout feather molt. The bimodal pattern of stable isotope ratios in flight feathers suggests that some birds may initiate molt immediately upon arrival in northwestern Mexico (and carry a C3 signature with them from the breeding grounds) whereas others may delay molt (and grow feathers solely from C4 plants of Sinaloa). From 2010 to 2012, we used geologger tags to test whether differences in the timing and route of fall migration movements were related to stable isotope signatures in primary feathers. We analyzed stable isotopes of hydrogen and carbon in P1 and P9 from 25 individuals fitted with geologger tags in two consecutive years. Of these, 60% changed the diet (C3 vs. C4) that was used to grow P1 between years. We also observed variation among individuals in migration routes, wherein birds from the same breeding population differed greatly in their use of molting and wintering locations. However, we did not find a relationship between isotope signatures and the timing or route of fall migration. We speculate that the bimodal isotope signature we observed represents a carryover effect related to local landscapes (grassland or agriculture vs. shrubland) used during the late breeding season and early molting period, and that these effects diminish as molt progresses. If this is the case, there is the potential for breeding-season diet to directly affect plumage quality in this molt migrant.

Migratory connectivity for small migratory passerines has been quantified primarily with stable hydrogen isotopes in feathers (δ²H_F) because, until recently, we lacked the technology to track small organisms over long distances. Direct tracking of small passerines throughout the annual cycle is now possible with archival light-level geolocators. Our objective was to evaluate whether δ²H_F and geolocators produce similar breeding-origin assignments for the same individual birds sampled during the non-breeding season. We estimated breeding origin with geolocators and δ²H_F and validated those estimates using a population of Ovenbirds (Seiurus aurocapilla) from a known breeding location at Hubbard Brook Experimental Forest, New Hampshire. We also deployed geolocators on Ovenbirds in Jamaica and Florida during March 2010–2011. We performed stable hydrogen isotope analysis on feathers of birds whose geolocators we recovered (Jamaica: n = 9; Florida: n = 3). Probabilistic assignments of δ²H_F that accounted for regional variation in feather-isotope discrimination predicted breeding origins that agreed with kernel density estimates of origin derived from geolocators. By contrast, assignments of δ²H_F using the common assumption of a consistent feather-isotope discrimination across space predicted breeding origins that overlapped minimally with those from geolocators. Finally, Bayesian analyses that incorporated prior information of Ovenbird abundance across the breeding range yielded more accurate assignments for both site-independent and site-specific discrimination factors. Our findings suggest that creating more detailed feather isoscapes by increasing the number of validation locations and sampling underrepresented portions of species distributions could increase the accuracy of geographic assignments using δ²H_F.

We used light-level geolocators to describe the migratory geography of Swainson's Thrushes (Catharus ustulatus) from a breeding population in central coastal California. Between 11 June and 20 July 2010, we attached geolocator tags to 35 Swainson's Thrushes at three sites in Marin County, California, and recovered 12 in the following two breeding seasons. All 12 birds spent the winter in Mexico, north of the Isthmus of Tehuantepec. Eleven of the birds went to the Jalisco region in western Mexico, but two of those birds (18% of total) moved east in midwinter to either the Sierra Madre Oriental or Sierra Madre del Sur; the 12th bird went directly to one of the latter regions. The return rate over the next 2 years for tagged birds (34%) was not statistically lower than that for control birds (45%). We combined our results with the results of an earlier study that used geolocators on Swainson's Thrushes from British Columbia to quantify the strength of migratory connectivity using Mantel's correlation coefficient. Our estimate of the strength of migratory connectivity for Swainson's Thrushes (r_m = 0.72) was much stronger than that reported using band returns of Barn Swallows (Hirundo rustica) in the Eastern Hemisphere (r_m = 0.03). To our knowledge, this is the first quantitative measure of migratory connectivity derived from geolocators.

The migration rate of Nearctic—Neotropic songbirds is expected to be influenced by whether the route is around or across migration barriers such as the Gulf of Mexico. To examine factors that influence fall migration strategies, we used light-level geolocators to track the journeys of 91 Purple Martins (Progne subis) originating from breeding colonies across the eastern range of the species. We expected individuals that crossed the Gulf of Mexico to have slower migration rates, and more stopover days in Central America to refuel after the crossing, than birds that took routes around the gulf. Owing to expected variability in conditions experienced by individuals en route, we anticipated that departure date would be a poor predictor of arrival date within and among populations. Despite widely separated breeding origins, one-way journeys of >7,000 km, and high variability in departure dates and routes, individuals showed a strikingly similar fall migration strategy. Fall migration featured a rapid (450 km day^-1) initial migration covering ≥2,000 km, followed by prolonged stopovers and a slower rate of travel before the birds continued to South America. Contrary to predictions, route explained little of the variation in the overall migration rate or the rate to Central America. Stopover duration in Central America was unrelated to whether birds crossed or circumnavigated the Gulf of Mexico. As expected, breeding location (primarily longitude) was the strongest predictor of the routes that birds took at this barrier. Within-breeding-region departure date alone predicted much of the variation in arrival date at the first winter roost, but route was not a significant factor. Our results reveal a consistent range-wide pattern in fall migration strategy, with route and migration timing predicting little of the variation in rate or stopover duration.

Millions of migratory and resident passerines molt in southwestern North America each summer, when ecological productivity spikes in response to the North American Monsoon. Although a monsoon-region molt is a critical event in the annual cycle of many species, the benefits of molting in the monsoon region are unclear. I used museum specimens to compare molt dynamics of the Vermilion Flycatcher (Pyrocephalus rubinus) in relation to the timing and location of the monsoon rains, and between northern and southern monsoon latitudes. In the north and the south, populations did not vacate arid non-monsoon areas to molt in adjacent monsoon areas, unlike several other lowland western passerines with more northern distributions. Onset, rate, and intensity of molt were similar between populations in and out of the monsoon region at the same latitude. By contrast, molt started significantly later and was significantly faster at northern latitudes than in the south, suggesting a strong, latitude-based temporal influence on molt dynamics, which was correlated with later nesting in the north. Thus, for this insectivorous passerine, the monsoon region may simply provide adequate habitat for molting at a low latitude, rather than abundant resources important for molt, as has often been proposed to explain the evolution of molt-migration to the monsoon region by western migrants. Similarities in foraging ecology of Vermilion Flycatchers and other western passerines may make these results applicable to other species, helping to clarify the role of the monsoon region in the evolution of molt schedules in North American passerines.

The degree of asymmetry in feather replacement may be considered a reliable measure of developmental instability of avian molt. Although empirical evidence exists for the fitness consequences of molt asymmetry, the proximate cause of asymmetrical molt remains unknown. We tested for the possibility of a link between asymmetry in feather replacement and hemoglobin concentration of first-year Common Snipe (Gallinago gallinago) during the partial postjuvenile molt. Advancement and symmetry of molt were determined for 637 first-year Common Snipe captured at Jeziorsko reservoir, central Poland, during autumn migration. We found that the probability of asymmetrical molt did not vary with date of capture (W = 1.66, P = 0.20), but Common Snipe with lower hemoglobin concentrations had a higher probability of asymmetrical molt (W = 5.22, P = 0.022). Because Common Snipe molt during migration, high oxygen-carrying capacity of blood is presumably necessary to meet aerobic demands of the tissues during long-distance flights. We suggest that under these conditions, low hemoglobin concentrations may lead to an acute organismal hypoxia causing disruptions in developmental homeostasis, such as asymmetrical feather replacement.

Molt is a metabolically demanding process in the annual cycle of birds, particularly for species that undergo simultaneous remigial molt because nutritional and energetic costs occur during a short period. Birds that molt remiges simultaneously utilize many different body-mass and foraging strategies to meet the nutritional and energetic costs of remigial molt, and documentation of interspecific variation has contributed to understanding species-specific risks associated with molt. However, little is known about intraspecific variation in body-mass and foraging strategies among birds that molt remiges simultaneously. We documented body-mass dynamics and foraging effort of Barrow's Goldeneyes (Bucephala islandica) during simultaneous remigial molt at two important postbreeding sites, including a large, hypereuthrophic lake and a small, mesotrophic lake in Alberta, to determine whether strategies for meeting nutritional costs of remigial molt varied across sites, years, and cohorts. Average body mass of all age and sex cohorts on both lakes increased during remigial molt in both 2009 and 2010. Birds were heavier on the smaller lake, and heavier in 2010 than in 2009, and adult males were heavier than subadult males. Radiomarked adult males exhibited similar foraging effort on each lake in each year (approximately 120–140 min day^-1); however, birds foraged primarily diurnally on the large lake and nocturnally on the small lake. We conclude that Barrow's Goldeneyes exhibit considerable intraspecific variation in body-mass and foraging dynamics during remigial molt across sites, years, and cohorts, which suggests that these components of molt strategy are plastic and responsive to local environmental conditions.

Developing an understanding of how differences in the dietary ecology and physiology of different species can influence the incorporation of hydrogen and oxygen from resources into consumer tissues is an important factor to consider when designing, or interpreting data from, ecological studies using stable isotope analyses of these elements. Here, we present the results of an experiment designed to examine the relationship between the δ²H and δ¹⁸O of drinking water and the δ²H and δ¹⁸O values of body water, blood plasma, red blood cells, intestine, liver, muscle, and feathers of Japanese Quail (Cortunix japonica). Because Japanese Quail have high drinking-water requirements compared with many other bird species, we hypothesized that the relative contribution of drinking water to tissue δ²H and δ¹⁸O values would be higher than that of bird species with lower drinking-water requirements. Our results demonstrate that the contribution of drinking water to the δ²H and δ¹⁸O values of tissues is generally higher in Japanese Quail than in other birds with lower drinking-water rates. However, we failed to find significant relationships between drinking-water isotope values and tissue isotope values in many of the tissues that we examined. We suspect that this lack of significant relationships is the result of variation in tissue isotope values caused by differences in drinking-water consumption rates among individual birds. Given these results, we recommend that researchers use caution when interpreting data from ecological investigations using δ²H and δ¹⁸O analyses.

Populations of nocturnal burrow-nesting seabirds are notoriously difficult to measure because of their cryptic behavior at remote breeding sites. However, there is an urgent need to identify factors that influence recovery of these populations, because of the increasing number of introduced-predator eradication projects whose ultimate goal is to facilitate seabird and, thus, ecosystem recovery. We asked whether the relative status of nocturnal burrow-nesting seabirds across the Aleutian Islands, Alaskainferred from levels of vocal activity collected with automated acoustic recording devices-can be explained in terms of ecological factors such as time since eradication, island size, and distance to source population. We deployed a total of 19 acoustic recorders on six islands during 2008–2010. Overall nocturnal call activity (mean number of calls night^-1 ± SE) was high (493 ±287) where predators were never introduced, low (0.3 ± 0.1) where introduced rats were present, and intermediate (29 ± 18) where introduced predators have been eradicated. Using an information-theoretic approach, we found support for multiple factors as an explanation for the call activity of Leach's Storm-Petrels (Oceanodroma leucorhoa), Fork-tailed Storm-Petrels (O. furcata), and Ancient Murrelets (Synthliboramphus antiquus). Specifically, we conclude that although recovery of nocturnal burrow-nesting seabird populations in the Aleutians is not straightforward, the presence of nearby “predator refugia” may maximize the probability of seabird recovery and can be used when prioritizing islands for eradication programs.

Climate cycling during the Quaternary played a critical role in the diversification of avian lineages in North America, greatly influencing the genetic characteristics of contemporary populations. To test the hypothesis that North American Northern Goshawks (Accipitergentilis) were historically isolated within multiple Late Pleistocene refugia, we assessed diversity and population genetic structure as well as migration rates and signatures of historical demography using mitochondrial control-region data. On the basis of sampling from 24 locales, we found that Northern Goshawks were genetically structured across a large portion of their North American range. Long-term population stability, combined with strong genetic differentiation, suggests that Northern Goshawks were historically isolated within at least three refugial populations representing two regions: the Pacific (CascadesSierra-Vancouver Island) and the Southwest (Colorado Plateau and Jemez Mountains). By contrast, populations experiencing significant growth were located in the Southeast Alaska-British Columbia, Arizona Sky Islands, Rocky Mountains, Great Lakes, and Appalachian bioregions. In the case of Southeast Alaska-British Columbia, Arizona Sky Islands, and Rocky Mountains, Northern Goshawks likely colonized these regions from surrounding refugia. The near fixation for several endemic haplotypes in the Arizona Sky Island Northern Goshawks (A. g apache) suggests long-term isolation subsequent to colonization. Likewise, Great Lakes and Appalachian Northern Goshawks differed significantly in haplotype frequencies from most Western Northern Goshawks, which suggests that they, too, experienced long-term isolation prior to a more recent recolonization of eastern U.S. forests.

Better knowledge of bird response to savanna and woodland restoration is needed to inform management of these communities. We related temporal and habitat variables to breeding demography and densities of the Eastern Wood-Pewee (Contopus virens) across a gradient of savanna, woodland, and forest. We determined nest success, clutch size, young fledged, and breeding densities and evaluated support for relationships with year, nest stage, date, nest height, tree cover, and percent forest in a 10-km radius. One hundred and twenty-eight of 310 nests (41.3%) fledged young. The most supported nest-survival model included nest stage and percent forest in the landscape. Daily nest survival was greater in the incubation than in the nestling stage and increased substantially with decreasing forest in the landscape. Four nests (1.3%) were parasitized by Brown-headed Cowbirds (Molothrus ater). Eastern WoodPewee density increased 83% over a range of 10–170% tree cover (percent tree stocking). Increased nest success with decreasing forest in the landscape indicates that Eastern Wood-Pewees are not highly susceptible to forest-fragmentation effects in the Missouri Ozarks, probably because they were not very susceptible to brood parasitism. The absence of any strong relationships between habitat measures and nest success, clutch size, or young fledged is in contrast to the large increase in density over the range of tree cover and is further evidence that variation in bird density does not always correspond to similar patterns in productivity.

In numerous avian species, males and females display ornaments that could act as signals in interactions with conspecifics. Much of the research on these ornaments has focused on mutual mate choice and intrasexual competition between males. However, few studies have examined female and male ornamentation simultaneously in the context of intrasexual competition and the acquisition of breeding resources. I examined whether plumage coloration of male and female Prothonotary Warblers (Protonotaria citrea) was related to the quality of their nest site by placing nest boxes in preferred locations over deep water and at less preferred sites over land. Age and body size were not related to the ability of males or females to obtain a nest site over deep water. Males that were brighter yellow and more pure yellow obtained boxes over deep water, and older females that were more pure yellow did as well. However, in an analysis that included young and old females, I found no relationship between plumage coloration and nest-box quality. These results indicate that coloration could be a signal of competitive ability, particularly in males.

The propensity to replace a clutch is a complex component of avian reproduction and poorly understood. We experimentally removed clutches from an Arctic-breeding shorebird, the Dunlin (Calidris alpina arcticola), during early and late stages of incubation to investigate replacement clutch rates, renesting interval, and mate and site fidelity between nesting attempts. In contrast to other Arctic studies, we documented renesting by radiotracking individuals to find replacement clutches. We also examined clutch size and mean egg volume to document changes in individual females' investment in initial and replacement clutches. Finally, we examined the influence of adult body mass, clutch volume, dates of clutch initiation and nest loss, and year on the propensity to renest. We found high (82–95%) and moderate (35–50%) rates of renesting for early and late incubation treatments. Renesting intervals averaged 4.7–6.8 days and were not different for clutches removed early or late in incubation. Most pairs remained together for renesting attempts. Larger females were more likely to replace a clutch; female body mass was the most important parameter predicting propensity to renest. Clutches lost later in the season were less likely to be replaced. We present evidence that renesting is more common in Arctic-breeding shorebirds than was previously thought, and suggest that renesting is constrained by energetic and temporal factors as well as mate availability. Obtaining rates of renesting in species breeding at different latitudes will help determine when this behavior is likely to occur; such information is necessary for demographic models that include individual and population-level fecundity estimates.

Intensive agriculture, as is typical of corn and soybean production, may be responsible for declines in the abundance and diversity of farmland birds. In Quebec, the transition to intensive crops is evidenced by marked increases of corn and soybean fields. From 2008 to 2010, we used satellite telemetry to study use of corn (Zea mays) and soybean (Glycine max) fields, other farmlands, wetlands, urban areas, and other habitats by 10 female Peregrine Falcons (Falco peregrinus) of the anatum—tundrius complex, a taxon of “special concern” in Canada. We monitored females during the nesting season, from hatching of eggs to independence of young, but before the young dispersed away from the nest site. Adult females were less likely to use corn and soybean fields than the “other farmlands” and “other habitats” categories during the nestling stage and the first month after young fledged. Once young fledged, other farmlands and urban areas were more likely to be used than the “other habitats” category when females were hunting in the areas that were farthest from the nest. The expansion of corn and soybean fields in the Quebec agricultural landscape has occurred to the detriment of other crops and may contribute to the decline in quality of hunting habitat of Peregrine Falcons and other avian top predators.