The nonbreeding distribution of Western Sandpipers (Calidris mauri) was documented using 19 data sets from 13 sites along the Pacific and Atlantic coasts of the Americas. Western Sandpipers showed latitudinal segregation with regard to sex and age. Females wintered farther south than males. A “U” shaped pattern was found with respect to age, with juveniles occurring at higher proportions at both the northern and southern ends of the range. Distribution of sexes might be affected by differences in bill length and a latitudinal trend in depth distribution of prey. For age class distribution, two different life-history tactics of juveniles might exist that are related to the higher cost of feather wear for juveniles compared to adults. Most juveniles complete three long-distance migrations on one set of flight feathers whereas adults complete two. Juveniles may winter either far north, thereby reducing feather wear induced by ultraviolet light, migration, or both, or far south and spend the summer on the nonbreeding area.

We determined prevalence and altitudinal distribution of forest birds infected with avian pox at 16 locations on Hawaii, from sea level to tree line in mesic and xeric habitats, during 1977–1980. Isolates from lesions were cultured in the laboratory for positive identification of Poxvirus avium. Infected birds from the wild were brought into the laboratory to assess differences in the course of infection in native versus introduced species. We also documented distributions and activity cycles of potential avian pox vectors.>Native forest birds were (1) more susceptible to avian pox infection than were introduced species, (2) most likely to be infected during the wet season, and (3) found to have a higher prevalence in mesic when compared to xeric forests. Avian pox occurred in forest birds at all elevations, but highest levels were in the mid-elevational ranges (∼1,200 m) where vectors and native birds had the greatest overlap. Temporal and elevational differences in prevalence were apparent throughout the annual cycle. Avian pox probably did not reach epizootic proportions on Hawaii until after introduction of the mosquito and domestic birds in the early 1800s, and since then has had a negative effect on the population dynamics of native forest birds. Today, this introduced disease is an important factor that should be considered in future conservation efforts that are directed at the recovery of native forest birds in Hawaii.

Most recent research on character evolution attempts to identify either (1) homology or homoplasy (systematic use of the term character), or (2) the adaptive function or selective regime underlying the origin of a character (“adaptationist” use of the term character). There have been relatively few serious considerations or examples of neutral character evolution above the molecular level. Wing feather taxis in birds, the presence or absence of the fifth secondary feather, provides an intriguing possible example of nonadaptive character evolution. We examine the phylogenetic pattern of wing feather taxis among birds to (1) determine its polarity in modern birds (Neornithes), (2) hypothesize the frequency and taxonomic locations of changes in the taxic state, (3) test whether taxis is relatively labile or inert phylogenetically, and (4) allow preliminary consideration of whether adaptive or selectively neutral processes have produced those patterns. Minimum tree length necessary to explain the distribution of wing feather taxis was calculated at the family level using Sibley and Ahlquist's DNA–DNA hybridization tree (1990). Parsimony analysis indicates that the eutaxic condition (fifth secondary present) is ancestral in modern birds, and that diastataxy (fifth secondary absent) has originated independently at least 7 times and reversed to the eutaxic condition on at least 13 occasions within modern birds. Despite multiple independent origins and reversals, wing feather taxis is extremely conserved throughout the tree, such that one or the other state completely characterizes many large multiordinal or multifamilial clades. Lack of obvious correlations with morphological and ecological traits suggest that no single adaptive scenario will explain the evolution of wing feather taxis. Instead, the biological details and phylogenetic patterns make nonadaptive, or selectively neutral evolutionary processes, such as genetic drift, an equally if not more plausible explanation for the distribution of wing feather taxis.

Forest fragmentation has been implicated as a cause of population declines of several Neotropical migrant bird species. Fragmentation increases the amount of habitat edge, and reduced nesting success rates near forest edges are well documented in agricultural landscapes (“edge effects”). However, edge effects in predominantly forested landscapes, particularly those related to timber harvest, are poorly understood. This study examines nesting success of ground nesting birds in relation to clearcut edges in a forest-dominated landscape in north-central Minnesota. A total of 383 nests of seven species of ground nesting birds were found and monitored during 1992–1998. Ovenbird (Seiurus aurocapilus; n = 318) and Hermit Thrush (Catharus guttatus; n = 44) nests composed the majority of the sample. Predation accounted for 94% of all nest failures. Brown-headed Cowbird (Molothrus ater) parasitism was low (1.8% for all ground nests). Using proportional hazards regression, distance to nearest clearcut edge was the best predictor of nest failure. For all ground nests, nesting success was 0.18 at 0–100 m, 0.39 at 101–500 m, and 0.52 at 501–954 m from nearest clearcut edge. Source–sink modeling indicated that distances ≤100 m from clearcut edges were sink habitats for Ovenbirds (i.e. recruitment was lower than survival). These results provide strong evidence of a negative edge effect on ground nests, extending 100 m or more from clearcut edges in a forest-dominated area of north-central Minnesota.

Nesting success of birds often is influenced by habitat features surrounding nests. However, few studies have investigated habitat influences at multiple levels and spatial scales. We examined the relationship between Blue Jay (Cyanocitta cristata) nesting success and habitat at three levels (macrohabitat, microhabitat, and nest site) and at three different scales within the microhabitat level (3.1, 0.3, and 0.07 ha circular plots surrounding nests) in central Florida. We defined successful nests as those fledging at least one offspring. An eight-fold difference in probability of nesting success existed among seven distinct macrohabitats, with Mayfield success estimates ranging from 8 to 63%. However, no difference in number of fledglings per nest existed among macrohabitats when unsuccessful nests were excluded from the analysis. Macrohabitats in which nests were more often successful had more extensive slash pine (Pinus elliottii) forest. Logistic regression analyses indicated that slash pine canopy cover was significantly related to nesting success at all three microhabitat scales, although the tree species in which a nest was placed was unimportant to nesting success. Other microhabitat and nest-site features were unrelated to nesting success. Strong circumstantial evidence indicates predation was the major cause of nest failure, but relative abundance of one potential predator, the gray squirrel (Sciurus carolinensis), was not related to nesting success in macrohabitats or slash pine canopy cover within microhabitats. In this study, logistic regression models using habitat features measured at higher levels and larger scales had better predictive power, indicating that habitat may have influenced predation at scales larger than that of a typical nest patch.

The outlines of the phylogenetic relationships within the New World suboscine clade Tyrannoidea were investigated on the basis of nucleotide sequence data from two nuclear genes (c-myc and RAG-1) and one mitochondrial gene (cytochrome b), totaling over 2,400 bp. Representatives of the major tyrannoid lineages were sequenced, including Pachyramphus, Schiffornis, Tityra, and Oxyruncus. The data set with the three genes combined was analyzed under both the parsimony and maximum-likelihood criteria and under different character weighting schemes. The analyses resulted in similar topologies that differed only in poorly supported nodes. The three manakins (Pipra, Manacus, and Chiroxiphia) included in this study were found to be monophyletic, whereas Schiffornis—sometimes also considered to be a manakin—did not group with the manakins, but occurred with Pachyramphus and Tityra in the clade Tityrinae. The two clades Pipromorphinae and Tyranninae are also strongly supported in this analysis and appear as sister groups, thus supporting the monophyly of the tyrant flycatcher assemblage. Phytotoma was placed with the only cotingid species included in this analysis, whereas the position of Oxyruncus was unresolved.

Broods of geese spend time feeding according to availability and quality of food plants, subject to inherent foraging and digestive constraints. We studied behavioral patterns of broods of Emperor Geese (Chen canagica) on the Yukon–Kuskokwim Delta, Alaska, and examined how feeding and alert behavior varied in relation to habitat and goose density. During 1994–1996, time spent feeding by Emperor Goose goslings and adult females was positively related to multispecies goose densities near observation blinds, and not to just Emperor Goose density. Similarly, body mass of Emperor Goose goslings was more strongly related (negatively) to multispecies goose densities than intraspecific densities. A grazing experiment in 1995 indicated that most above ground primary production by Carex subspathacea, a preferred food plant, was consumed by grazing geese. Those results demonstrate that interspecific competition for food occurred, with greatest support for goslings whose behavioral repertoire is limited primarily to feeding, digesting, and resting. Although the more abundant Cackling Canada Geese (Branta canadensis minima) differed from Emperor Geese in their preferred use of habitats during brooding rearing (Schmutz 2001), the two species occurred in equal abundance in habitats preferred by Emperor Goose broods. Thus, Cackling Canada Geese were a numerically significant competitor with Emperor Geese. Comparing these results to an earlier study, time spent feeding by goslings, adult females, and adult males were greater during 1993–1996 than during 1985–1986. During the interval between those studies, densities of Cackling Canada Geese increased two to three times whereas Emperor Goose numbers remained approximately stable, which implies that interspecific competition affected foraging behavior over a long time period. These density-dependent changes in foraging behavior and body mass indicate that interspecific competition affects nutrient acquisition and gosling growth, which has a demonstrated effect (Schmutz 1993) on juvenile survival of Emperor Geese. Management of Emperor Geese should consider interspecific relations and densities of all goose species occurring on the Yukon–Kuskokwim Delta, Alaska.

We describe timing and location of the prebasic molt in Western Tanagers (Piranga ludoviciana), focusing on age class differences in premolt movements. Most adults migrate south to the American Southwest where they stop to molt before moving on to their wintering grounds. Molting adults are found in southern mountain regions (Sierra Madres and southern Rockies) and in the Mexican monsoon region, where late summer rains cause a substantial increase in productivity. In contrast, juvenile Western Tanagers move to nearby montane habitats to molt before migrating, a previously undocumented strategy in western passerines that show molt-related movements. By molting in nearby montane habitats, juveniles avoid the aerodynamic cost of migrating in their “fluffy”, aerodynamically inefficient juvenal plumage. Western Tanagers are the fifth species known to use the Mexican monsoon region during the prebasic molt, further affirming the importance of that area to the conservation of Neotropical migrants that breed in western North America.

Flush–pursuit foragers use exaggerated and animated foraging movements to flush potential prey that are then pursued and captured in flight. The Myioborus redstarts comprise 12 species of flush–pursuit warblers found in montane forests of the American tropics and subtropics. All members of the genus have contrasting black-and-white tail feathers that are exposed by spreading the tail during animated foraging displays. Plumage pattern and tail-spreading behavior were examined to see how they affected flush–pursuit foraging performance of the Slate-throated Redstart (Myioborus miniatus) in Monteverde, Costa Rica. Although flycatching was the most common foraging tactic used by Slate-throated Redstarts, flush–pursuit foraging increased in both frequency and intensity when birds were feeding nestlings. Flush–pursuit prey attacks occurred more frequently following hops in the spread-tail foraging posture than hops in more typical warbler-like posture, suggesting that tail-spreading behavior assists in startling and flushing potential prey. The hypothesis that the white tail feathers enhance flush–pursuit foraging was tested by means of a plumage-dyeing experiment. After locating nests containing young 5–7 days old, I captured the male and female at the nest and assigned one member of each pair to the experimental treatment group; its mate served as a control. For experimental birds, a permanent marker was used to blacken the white tips of the three outer retrices. For sham-darkened controls, the naturally black tips of the three inner retrices were also “blackened”. Experimental birds with darkened tail feathers were significantly less successful in flush–pursuit foraging, showed a significantly lower overall rate of prey attack, and fed their nestlings at a significantly lower rate than did their sham-darkened mates. For experimental birds, only 7.6% of hops in the spread-tail posture were followed by an attack on a prey item, compared to 20.9% of hops for controls. These experimental results, similar to those obtained recently by Jabłoński (1999) for the congeneric Painted Redstart (Myioborus pictus), indicate that white tail feathers are critically important in startling potential prey and raise questions about interspecific and intraspecific geographic variation in plumage pattern in the genus.

We quantified aspects of the breeding biology of Whiskered Auklets (Aethia pygmaea) at Buldir Island, western Aleutian Islands, Alaska, from 1993 to 1998 to provide a better understanding of this poorly known species' ecological relationship to other auklets (Aethia spp.). Whiskered Auklets nest in rock crevices in a range of habitats including talus slopes, cliffs, and beaches. No physical characteristic of nesting crevices predicted breeding success. Hatching dates varied among years; mean hatching dates averaged 20 June ±6.8 days (range 16–27 June). Chicks remained in the nest for 39 ± 2 days, with mean annual fledging from 27 to 29 July. Eggs were 44.1 ± 1.6 mm in length and 31.1 ± 1.61 mm in breadth, and varied significantly in size among years. Chicks weighed 17.4 ± 2.4 g (15.6% of adult mass) within two days of hatching and gained 3.8 g day⁻¹ in mass and 3.2 mm day⁻¹ in wing length during the linear growth period. Chicks fledged at 101.3 ± 11.3 g (90.5% adult mass) after a prefledging mass recession of 8.9% of peak mass, and a wing length of 102.2 ± 4.3 mm (93.5% adult wing length). Mass increase during the linear growth period and fledging mass varied among years, but wing growth and fledging wing length did not. Parents provisioned chicks after nightfall with copepods (Neocalanus spp.) and euphausiids (Thysanoessa spp.); the proportion of food types varied significantly among years. Eighty-nine percent of chicks that hatched survived to fledge; most of those that did not had died of starvation. Low-intensity monitoring underestimated breeding success but provided useful comparative information. Whiskered Auklets exhibited a suite of unique characteristics that make them an ecological outlier among Aethia auklets.

Prey capture in owls and hawks is largely dependent on the biomechanics of the hindlimbs, and both limb size and grip forces potentially determine the size of prey that can be captured and the extent of possible resource partitioning among sympatric species. Morphological study of six species of sympatric raptors—the owls Otus asio, Strix varia, and Bubo virginianus; and the hawks commonly considered their diurnal “ecological equivalents,” Falco sparverius, Buteo lineatus, and Buteo jamaicensis—revealed that, in both groups, talon closure is effected by two discrete mechanisms that function together in a potentially additive or alternative fashion. Grip force measurements obtained from live owls and hawks using “hydraulic” perches showed that grip force increases exponentially with body size and that owls produce greater forces than hawks. That finding is consistent with the distinctive osteology and myology of their hindlimbs and with their hunting behavior. These data provide some understanding of the different demands of diurnal and nocturnal hunting as well as the mechanism of coexistence for those six species in eastern woodlands.

Using temperature loggers and radio telemetry, foraging and brooding behaviors of female and male Thick-billed Murres (Uria lomvia) were measured during chick rearing at the Gannet Islands, Labrador, to quantify parental roles during this crucial stage of reproduction. We recorded 2,725 foraging dives by 10 females and 7 males during 1,416 bird-hours of monitoring. Our study birds made between 21 and 107 dives per day between 0327 and 2220 hours, with 54% of females' dives and <1% of males' dives occurring in conditions of low illumination between sunset and sunrise. Males foraged mostly at midday and, overall, made longer and presumably deeper dives (mean duration 116.2 ± 37.6 s) than females (mean duration 84.1 ± 46.0 s). The maximum dive duration recorded was 240 s. Final dives of foraging trips were inferred to include dives during which chick meals (∼95% by number daubed shanny [Lumpenus maculatus]) were obtained. Those dives (mean duration 134.0 ± 36.1 s) and dives in final diving bouts prior to return to the breeding site (mean duration 139.1 ± 40.2 s) averaged longer than other dives, and longer in males than in females, but there was no difference between the sexes in their daily frequency. The modal travel time from the last dive in a feeding trip to arrival at the breeding site was 8 min (no difference between sexes), which represents a maximum travel distance at 65 km h⁻¹ of 8.7 km, suggesting that foraging occurred close to the colony. Females spent 9 h day⁻¹ (38% of day, mostly diurnal brooding) at breeding sites whereas males spent 16 h (67% of day, mostly nocturnal brooding). Taken together, these data indicated drastic differences in patterns of foraging and brooding behavior between male and female Thick-billed Murres, but suggest that individual and intercolony variation were also significant.

Buffleheads (Bucephala albeola) and the North American population of Barrow's Goldeneyes (Bucephala islandica) typically breed in boreal and montane regions where food is less available relative to most habitats used by temperate nesting ducks. We investigated diets and digestive organ morphology of sympatrically breeding female Buffleheads and Barrow's Goldeneyes in central British Columbia. Because those congeners exhibit interspecific aggression in defense of breeding territories, we predicted that competition for food could be a principal factor in evolution of that behavior. We also hypothesized that breeding Buffleheads would show greater variation in digestive organ morphology than Barrow's Goldeneyes because of their smaller body size and consequently greater reliance on their diet to meet nutritional requirements for egg production. Both species fed predominantly on aquatic insects during all reproductive periods, particularly larvae of damselflies (Zygoptera), midges (Chironomidae), and phantom midges (Chaoborinae). Plant foods, primarily seeds of submergent and emergent vegetation, generally represented <15% of diets in both species. Diets of breeding female Buffleheads and Barrow's Goldeneyes were similar throughout reproduction, hence competition for food has likely been a factor in evolution of territorial aggression between those species. The digestive tract morphology of Buffleheads was more variable than that of Barrow's Goldeneyes suggesting that the former relied more on dietary nutrients during reproduction than did their larger congener.

We collected 328 freshly laid Great Tit (Parus major) eggs from 38 clutches in 1999 to determine the relationship of whole egg weight, wet yolk weight, wet albumen weight, dry shell weight, and the occurrence of laying gaps with mean ambient temperature in the three days preceding laying, while controlling for laying date and position in the laying sequence. We also reanalyzed existing data on whole egg weight in 1978 and 1979 in relation to temperature, controlling for the same other variables. Egg weight was correlated with temperature in 1978 and 1979, but not in 1999. None of the egg components was related to temperature preceding laying, apart from a nonsignificant tendency for albumen weights to be lower at higher temperature. Egg weight increased with laying date in 1978 and 1979 after controlling for other variables. In 1999, albumen and shell weight increased seasonally, but shell weight only did so to the extent expected on the basis of the seasonal increase in whole egg weight. Yolk weight did not increase significantly seasonally, but neither did yolk weight relative to whole egg weight decrease significantly. Whole egg weight and the weights of all components varied between clutches. Most weights also varied through the laying sequence. The probability of a laying gap occurring varied between clutches, and increased with position in the laying sequence and decreased with temperature and laying date. Eggs preceding laying gaps were lighter and contained less albumen but had higher shell weights than other eggs. Eggs following laying gaps also had heavier shells relative to the size of the egg than other eggs.

A new intergeneric hybrid manakin is characterized using morphological characters and mitochondrial and nuclear DNA sequences. Many morphological traits suggest the bird is intermediate between Ilicura militaris and Chiroxiphia caudata. Sequences of the maternally inherited mitochondrial ND2 gene demonstrate the female parent to be Chiroxiphia caudata, and the nuclear beta-fibrinogen intron 5 sequences can not eliminate Chiroxiphia and Ilicura as parents. Therefore, DNA sequence data lead to the conclusion that Ilicura is the male parent of this unusual bird. This study is important because it highlights the significance of using multiple character systems in diagnosing unusual bird specimens.

We measured the daily energy expenditure of free-living Common Loon (Gavia immer) chicks using doubly labeled water (DLW). Average body mass of chicks during the DLW measures were 425, 1,052, and 1,963 g for 10 day-old (n = 5), 21 day-old (n = 6), and 35 day-old (n = 6) chicks, respectively, and their mean daily energy expenditures (DEE) were 686 kJ day⁻¹, 768 kJ day⁻¹, and 1,935 kJ day⁻¹, respectively. Variation in DEE was not due solely to variation in body mass, but age was also a significant factor independent of body mass. Energy deposited in new tissue was calculated from age-dependent tissue energy contents and measured gains in body mass, which were 51, 54, and 33 g day⁻¹ from the youngest to oldest chicks. Metabolizable energy (the sum of DEE and tissue energy) was used to estimate feeding rates of loon chicks and their exposure to mercury in the fish they consume. We calculated that loon chicks in Wisconsin consumed between 162 and 383 g wet mass of fish per day (depending on age), corresponding to intakes of mercury of 16–192 μg day⁻¹.

We used miniature infrared video cameras to monitor Wood Thrush (Hylocichla mustelina) nests during 1998–2000. We documented nest predators and examined whether evidence at nests can be used to predict predator identities and nest fates. Fifty-six nests were monitored; 26 failed, with 3 abandoned and 23 depredated. We predicted predator class (avian, mammalian, snake) prior to review of video footage and were incorrect 57% of the time. Birds and mammals were underrepresented whereas snakes were over-represented in our predictions. We documented ≥9 nest-predator species, with the southern flying squirrel (Glaucomys volans) taking the most nests (n = 8). During 2000, we predicted fate (fledge or fail) of 27 nests; 23 were classified correctly. Traditional methods of monitoring nests appear to be effective for classifying success or failure of nests, but ineffective at classifying nest predators.

In December 1999, an exceptional windstorm traveled across northern France (Côte d'Or). The severity of the stand damages ranged from <1 to >80%, and over 10% of the overall surface of several homogeneous woodlands (from ∼1,000 to 8,000 ha) was affected by the windstorm. Data were available on Northern Goshawk (Accipiter gentilis) density, productivity, and nesting stand preferences in that area before the disturbance. The species showed tolerance to the natural disturbance induced by the windstorm: we found no difference in density, nesting stand choice, or productivity between the six breeding seasons before the windthrow and in the one immediately after. Breeding pairs moved away (50–200 m) from their traditional nest stand only when its original structure was altered by >30% in the 50 m surrounding the nest tree. We hypothesize that the Northern Goshawk tolerance to windstorm damage may be the result of an adaptive response, which they have developed under both climate- and human-originated stresses.

Increased experience with age may allow long-lived seabirds to forage more efficiently and ultimately to invest more into reproduction. In many seabirds, egg size increases with female age, but declines with laying date. Declines in laying date might be due either to quality of individual females or females may adjust their egg size in response to poorer conditions typically found late in the season. We investigated the influence of female age, date of laying, and annual variation on egg size in Yellow-eyed Penguins (Megadyptes antipodes). Further, we tested whether there was an effect of individual females or female age on date of laying. Whereas female age influenced egg volume, mass, and width, laying date had no effect on egg size or mass in Yellow-eyed Penguins. Laying dates differed significantly among females but were not affected by their ages. These results indicate that in Yellow-eyed Penguins, some individual females lay consistently early or late, regardless of age, but that egg size generally increases with age.

Long considered to utter only simple calls, hummingbirds are now known to produce songs distinct from calls. Some species have complex songs, but their acoustic characteristics, structure and organization, function, and the evolution of vocal repertoires are not completely known. Here, we describe the aerial display and most common vocalizations of Amethyst-throated Hummingbird (Lampornis amethystinus) under different behavioral contexts, during its breeding season in western Mexico. We identified four types of vocalizations: (1) a territorial call, produced mainly by males often in response to another neighbor's chattering, when leaving his territory, and during chases; (2) feeding call, given mostly while foraging; (3) display call, a series of descendent frequency whistle notes, given while performing aerial displays; and (4) song, composed of an introductory phrase and a mixture of notes arranged into four to nine complex phrases, covering a wide frequency range. Our observations of horizontal aerial displays are the first for Amethyst-throated Hummingbirds. Comparisons with L. amethystinus congeners indicated similarities in song complexity. Our results supported the hypothesis that the evolution of diverse acoustic signals may be favored, independently of habitat characteristics, by the complex foraging interactions that hummingbirds experience every day as they need to communicate intra- and interspecifically.

Environmental variability affects life-history decisions such as allocation of resources to reproduction. However, in the case of small passerine birds, pairs that acquire a territory do not usually skip a breeding season. This study presents breeding-success data from two contrasting years for a population of Bridled Titmice (Baeolophus wollweberi). During an El Niño year (1998), 20 pairs attempted to breed, and 85% were successful. Survival of juveniles from 10 banded broods through nutritional independence was 100%, excluding one depredated nest. Breeding success was much lower in the following La Niña drought year. Fifty-seven pairs established territories in the study area. Twenty-seven never progressed to nest building. Thirteen pairs were successful; survival of young from banding to nutritional independence was 41.4%. An average of 0.47 fledglings per breeding pair was produced in 1999, compared to 4.40 the previous El Niño year. This study examines possible adaptive significance of forgoing breeding in a poor year.

Northern Saw-whet Owls (Aegolius acadicus) exhibit large fluctuations in annual number of individuals migrating in eastern United States. Underlying large differences in the magnitude of the migration may be important density-dependent effects on body condition and stopover patterns. We investigated such effects using data from saw-whet owls that were trapped and banded during autumn migration at the southern tip of the Delmarva Peninsula (near Cape Charles, Virginia) from 1994 to 2000. Irruptive migration events occurred in 1995 and 1999 when 1,002 and 700 owls were captured, respectively. Capture totals ranged from 22 to 105 owls during five nonirruptive years. Irruptive migration years were dominated by immature owls (82% of captures) and were characterized by low recapture rates (13%) and shorter minimum stopover lengths (median = 5 nights) for all owls. Body masses and body-condition index scores were significantly lower for both immature and adult age classes during years when owl density was very high. During such years, a smaller percentage of owls elected to stopover and, among those that did, individuals in good condition departed sooner than individuals in poor condition. Conversely, during nonirruptive years, adults constituted 67% of captures, recapture rates were nearly 3× higher (35%), and stopover lengths were twice as long (median = 10 nights). Recapture rates were significantly higher for adults (43%) than for immature owls (23%) and initial body condition of adults was positively correlated with their length of stopover. During light migration years, conditions may be conducive for extended residency, particularly for adult owls in good condition. Our results suggest that density-dependence is an important mechanism driving migration and stopover patterns of saw-whet owls.

In autumn, >99% of North American Eared Grebes (Podiceps nigricollis) stage at Mono Lake, California, and Great Salt Lake, Utah, creating an exceptional opportunity to measure population trends in a common and otherwise widely distributed species. Aerial photocounts showed that numbers at both lakes fluctuated in parallel, from a total of ∼3.56 million in 1997, then crashing to 1.60 million in 1998. The decline was coincident with a major El Niño, during which hundreds of thousands starved in the Gulf of California. The population rebounded to 3.27 million by 2000, illustrating the species' potentially high survivorship throughout the year. Catastrophic events are evidently the major factors affecting proximate changes in population size. Factors that set the ultimate limit to the population can be clarified through long-term studies at staging areas.

Measurements of corticosterone levels from blood samples of birds provide accurate snapshots of systemic hormone concentrations. However, those birds must be caught and handled, which may be unfeasible especially when multiple sampling is required. Furthermore, handling causes stress and may therefore interfere with hormone measurements. Therefore a non-invasive technique was developed to measure metabolites of corticosterone in excreta of European Stonechats (Saxicola torquata rubicola) using a corticosterone enzyme-immunoassay. High-performance liquid chromatography of excreta of a female and a male stonechat injected with tritiated corticosterone showed that corticosterone is excreted in the form of numerous metabolites and that the corticosterone enzyme-immunoassay cross-reacted with most of those metabolites. Injection of adrenocorticotrophic hormone in one female and seven male stonechats led to a significant increase in the levels of excreted corticosteroid metabolites within 1 h 20 min after administration of adrenocorticotrophic hormone. These results suggest that the corticosterone enzyme-immunoassay used in this study provides a quantitative measure of excreted corticosteroid metabolite levels in European Stonechats and has the potential to replace plasma measurements of these hormones.

The doubly labeled water method was used to measure daily energy expenditure of adult Ovenbirds (Seiurus aurocapillus) feeding nestlings in large and small forests in northern New England. Carbon dioxide production for all birds averaged 7.67 ± 1.29 mL g⁻¹ hr⁻¹ (n = 27), equivalent to 90.1 ± 15.0 kJ day⁻¹. Carbon dioxide production did not differ between males and females within the same pair measured over the same period. Ratio of daily energy expenditure to basal metabolic rate (BMR) for Ovenbirds (3.4) was closer to the mean value derived for aerial foragers (3.8) than that for ground foragers (2.3) in a sample of passerines feeding nestlings. High daily energy expenditure for Ovenbirds may be related to their relatively brief breeding season in northern climates and their use of cool, closed-canopy forests. Although daily energy expenditure for Ovenbirds was high when compared to other ground-foraging passerines, most individuals were not working near the hypothetical maximum of 4× BMR (Drent and Daan 1980). Daily energy expenditure for Ovenbirds feeding young was greater in large forests than small forests, although the difference was not statistically significant. We speculate that time used for foraging (and hence energy expenditure) may be lower in small forests as a result of increased prey density, or alternatively, greater risk of nest predation in forest fragments leads to greater vigilance at the nest site and less time available for foraging. Further studies of avian energetics in large and small forests, including detailed time–activity budgets, may reveal hidden costs of forest fragmentation.

Precise quantification of the oval of a bird egg can provide a powerful tool for the analysis of egg shape for various biological problems. A new approach to the geometry of a bird egg oval is presented here using a simple algebraic equation to fit all normal bird egg shapes. Only two parameters are needed in the equation for complete shape description of an egg oval to quantify the equation's capacity for curve fitting all species and shapes of bird egg ovals. The equation was fitted to egg silhouettes from a sample of 250 different bird egg species containing one egg per species. Standard regression analysis was used to fit the equation to each egg profile. The 99% CI for the curve fit acceptance rate was calculated to determine the equation's statistical significance for all species of bird eggs. Compared to the power series multiequation models (Preston 1968, Todd and Smart 1984), the equation used here is the simplest analytic description of a bird egg oval.