Elaborate male and female plumage can be maintained by mutual sexual selection and function as a mate-choice or status signal in both sexes. Both male and female Turquoise-browed Motmot (Eumomota superciliosa) have long tails that terminate in widened blue-and-black rackets that appear to hang, unattached, below the body of the bird. I tested whether mutual sexual selection maintains the Turquoise-browed Motmot's elaborate tail plumage by testing the prediction that mating occurs in an assortative manner for tail plumage. I also tested whether assortative mating occurs for body size, a potential measure of dominance, and for phenotypic condition, a measure of individual quality. Assortative mating was measured (1) within all pairs in the study population, (2) within newly formed pairs, and (3) within experimentally induced pairs that formed after removal of females from stable pairs. Assortative mating was not found for tail plumage, body size, or phenotypic condition in any of these samples. Therefore, there was no support for the “mutual sexual selection” hypothesis. I discuss the hypothesis that the tail is sexually selected in males only, and that natural selection accounts for the evolutionary maintenance of the elaborate female tail.

Ausencia de Apareamiento Asociativo con Respecto a la Cola, el Tamaño Corporal o la Condición en Eumomota superciliosa

Conservation challenges enhance the need for quantitative information on dispersed bird populations in extensive landscapes, for techniques to monitor populations and assess environmental effects, and for conservation strategies at appropriate temporal and spatial scales. By estimating population sizes of shorebirds in the U.S. portion of the prairie pothole landscape in central North America, where most migrating shorebirds exhibit a highly dispersed spatial pattern, we determined that the region may play a vital role in the conservation of shorebirds. During northward and southward migration, 7.3 million shorebirds (95% CI: 4.3–10.3 million) and 3.9 million shorebirds (95% CI: 1.7–6.0 million) stopped to rest and refuel in the study area; inclusion of locally breeding species increases the estimates by 0.1 million and 0.07 million shorebirds, respectively. Seven species of calidridine sandpipers, including Semipalmated Sandpipers (Calidris pusilla), White-rumped Sandpipers (C. fuscicollis), and Stilt Sandpipers (C. himantopus), constituted 50% of northbound migrants in our study area. We present an approach to population estimation and monitoring, based on stratified random selection of townships as sample units, that is well suited to 11 migratory shorebird species. For extensive and dynamic wetland systems, we strongly caution against a monitoring program based solely on repeated counts of known stopover sites with historically high numbers of shorebirds. We recommend refinements in methodology to address sample-size requirements and potential sources of bias so that our approach may form the basis of a rigorous migration monitoring program in this and other prairie wetland regions.

Sobre la Determinación de la Importancia de Humedales Continentales Efímeros para las Aves Playeras Migratorias Norteamericanas

Avian responses to forest edges have received much attention in recent years, particularly because of the potential effects of deforestation on the quality of remaining forest patches. However, individual birds' responses to forest edges are more often inferred than observed, and most studies of space use emphasize territory placement, with little or no detail on within-territory movements. Thus, our understanding of the effects of edges on movements of forest birds remains limited. We recorded movements of 85 winter flocks of the little-known Boreal Chickadee (Poecile hudsonica), in a 66-km² boreal forest harvested for timber near Québec City, Québec. From January to March 2004 and 2005, we followed flocks on snowshoes and recorded their trajectory in real time using a handheld global positioning system (GPS) receiver. Boreal Chickadee flocks showed no response to forest edges when using mature forest stands. However, flocks mostly used edges of regenerating forest habitat (4–7 m high). Inside regenerating forest, flocks were significantly closer to both open edges (41 ± 6 m) and mature forest edges (11 ± 2 m) than would be expected from a lack of response to edges. Boreal Chickadee flocks did not avoid exposed edges of mature forest on the coldest or windiest days. On colder days, they were found disproportionately more often along edges between mature and regenerating stands, but generally avoided exposed edges of regenerating stands. Increasing edge densities resulting from clearcutting in boreal forest did not have a negative effect on use of remaining mature-forest patches, even under inclement weather. However, in regenerating stands resulting from timber harvest, Boreal Chickadee movements may be restricted during harsh weather.

Réponse de Poecile hudsonica face aux bordures forestières en hiver: quelle est l'importance des conditions météorologiques?

We analyzed genetic variation in the Emerald Toucanet (Aulacorhynchus prasinus), a species complex that ranges primarily along the montane forests of southern and eastern Mexico south to Bolivia. Segments of three mitochondrial DNA genes (cytochrome b, ND2, and ND3) were sequenced for a total of 1,159 base pairs. Using maximum parsimony, maximum likelihood, and Bayesian analysis, we found a set of seven differentiated populations that correspond to clear geographic breaks throughout the highlands of the Neotropics. These genetically distinct populations also correspond with the geographic breaks found in previous analyses of morphological data. Molecular evidence suggests species treatment for four of the Central American clades and three South American clades.

Especiación en el Complejo de Aulacorhynchus prasinus

The annual migration of Pacific salmon (Oncorhynchus spp.) to freshwater streams and lakes provides an important nutrient subsidy to terrestrial systems in North America. We investigated the effects of salmon and other habitat variables on abundance of Winter Wren (Troglodytes troglodytes), Swainson's Thrush (Catharus ustulatus), Varied Thrush (Ixoreus naevius), Pacific-slope Flycatcher (Empidonax difficilis), Golden-crowned Kinglet (Regulus satrapa), and Chestnut-backed Chickadee (Poecile rufescens) on the central coast of British Columbia. In our comparisons of salmon-bearing and non-salmon-bearing reaches of two rivers that had waterfall barriers to salmon partway upstream, we found that position above or below the falls and proximity to the stream were the major predictors of songbird abundance. Each species, except for Chestnut-backed Chickadee, had higher densities below the falls at both rivers. Our results suggest that salmon-derived nutrients influence songbird density, and thus benefit multiple trophic levels within riparian ecosystems.

La présence du saumon augmente la densité de passereaux près des cours d'eau du nord-ouest du Pacifique

Many flowers associated with hummingbirds (Trochilidae) produce nectar rich in sucrose, a characteristic that reflects innate preferences or the metabolic capacities of their putative pollinators or both. However, studies on nectar sugar selectivity by hummingbirds in the field are lacking. Under natural field conditions, we studied the sugar preferences of the Green-backed Firecrown Hummingbird (Sephanoides sephaniodes), the southernmost hummingbird in the world and a key pollinator in the temperate forest of South America. We considered potential differences between males and females and the influence of environmental temperature on those preferences. To free-foraging Green-backed Firecrown Hummingbirds, we simultaneously offered four different sugar solutions with the same concentration (24% wt/wt): (1) sucrose, (2) glucose, (3) fructose, and (4) a 1:1:1 mixture of these three solutions, as well as a control of pure water. The experimental design involved three replicates per treatment assigned to three different “blocks” of five feeders each, during eight periods of seven days each. We scored the number of Green-backed Firecrown Hummingbirds feeding on each feeder, the number of consumptions made by each individual, and the amount of sugar (g) consumed per sampling period. Green-backed Firecrown Hummingbirds clearly discriminated against pure water, favoring sugar solutions; of the latter, they preferred sucrose over glucose and fructose. The mixed-sugar solution showed intermediate preference values. Neither sex nor temperature affected sugar preferences. Sugar preferences by the Green-backed Firecrown Hummingbird could influence the nectar composition of the flowers it pollinates through differential selection.

Preferencia de Azúcares en Sephanoides sephaniodes: un Experimento de Campo

Extrapair fertilizations (EPFs) are an important component of many socially monogamous mating systems. However, information about the reproductive tactics used by male and female birds in extrapair mating systems is still rare. We used radiotelemetry to measure territory use, forays, and mate association in Wood Thrush (Hylocichla mustelina) when females were fertile and not fertile. We also conducted paternity analysis to determine the frequency of EPFs in our study population. Socially paired male and female Wood Thrush maintained a close physical association while on their own territory. Most of the off-territory forays observed during the fertile stage were conducted by social pairs. Lone males rarely engaged in off-territory forays during their mate's fertile period but made relatively frequent off-territory forays when their mates were not fertile. Male Wood Thrush may trade-off investment into paternity assurance and extrapair tactics by favoring a mate-guarding tactic when their within-pair paternity is most at risk. The overall rate of EPF (6%) exhibited by Wood Thrush is low compared with other synchronously breeding avian species. We suggest that extensive on- and off-territory mate association throughout the females' fertile period may limit female extrapair mating opportunities and also limit a male's ability to interact with any soliciting extrapair females.

Excursiones Hacia Afuera del Territorio y Sistema Genético de Apareamiento de Hylocichla mustelina

We examined the effects of mate and site fidelity on Western Sandpiper (Calidris mauri) nesting success in a breeding population on the Yukon-Kuskokwim River Delta, Alaska (1998–2005). We estimated return rates (percentage of banded birds that returned to the site in the subsequent season) and mate fidelity among 533 individually marked birds (296 females and 237 males) and monitored 430 nests with sufficient data for nest-survival analyses. Return rates were lower for females (40%) than for males (65%). Annual divorce rates (both members of a pair returned and each mated with a new individual) ranged between 10 and 29%, with 4–26% of pairs reuniting annually. Reuniting pairs initiated clutches earlier than newly formed pairs, and clutches that were initiated early in the season had higher nest-success rates than those initiated later in the season. However, when initiation date was controlled, mate fidelity did not explain significant variation in daily nest survival rate. When we controlled for seasonal variation, nests tended by individuals with prior breeding-site experience had higher daily survival rates than those tended by birds breeding at the site for the first time. This effect was greater for males than for females. We also observed annual and seasonal variation in nest survival, as well as variation associated with nest age. Future study of the proximate causes of temporal variation would add considerably to our understanding of temporal effects on individual behavior, fitness, and population dynamics.

Efectos de la Fidelidad de Pareja y de Sitio sobre la Supervivencia de Nidos en Calidris mauri

Populations of the Red-winged Blackbird (Agelaius phoeniceus) endemic to Cuba, which are distinct from continental forms in a number of important morphological and behavioral traits, have been recognized as a separate species, the Red-shouldered Blackbird (A. assimilis). We provide the first genetic evidence for species status of A. assimilis, by comparison of mitochondrial DNA sequence from this taxon with variation across the range of A. phoeniceus. The characteristics and phylogenetic placement of A. assimilis significantly complicate previously completed ancestral-state reconstructions for traits associated with mating-system evolution, including the mating system itself, as well as sexual dimorphism and male parental care. Although comparative data are nearly equally consistent with retention of the ancestral states of monogamy and associated characteristics in A. assimilis and with reversal from polygyny back to monogamy in this lineage, we currently favor the reversal scenario on the basis of biogeographic plausibility. Closer study of species-specific behavioral repertoires in this and related species, as well as studies of habitat characteristics and species interactions, should provide additional insights into the factors promoting the evolution of polygyny in this genus.

Estatus de Especie para Agelaius assimilis: Implicaciones para la Evolución Ecológica, Morfológica y del Comportamiento en Agelaius

Stable-isotope analysis of feathers has become a useful tool for examining migration patterns and annual connectivity of migratory songbird populations. We used this approach, combined with molecular sex-identification, to examine expected breeding destinations and migration ecology of Swainson's Thrush (Catharus ustulatus) at a spring stopover site on the Osa Peninsula of Costa Rica. The number of individuals passing through the site increased in late March and peaked in mid-April before declining toward late April. Plumage and expected feather-deuterium (δD_f) values for North America indicated that most individuals had probably bred or fledged in west-central regions of the boreal forest and western mountains, rather than farther north in the Yukon and Alaska or in eastern regions of Canada and the United States. Average δD_f values differed little between males and females, indicating similar breeding destinations, though females tended to show greater variability. The δD_f values of second-year and after-second-year birds differed only slightly, which suggests that most adults molted near the breeding grounds. Timing of migration through the site was not related to breeding latitude as inferred from feather δD. We also found little relation between the energetic reserves carried by individuals and their expected breeding destination, possibly because the reserves carried at this stage have little relation to the total reserves needed by northern breeders for the final stage of migration.

Destinos de Cría y Patrones de Migración de Primavera de Catharus ustulatus en un Sitio de Escala en Costa Rica

We modeled daily nest-survival rates (DSR) of a reintroduced population of Northern Aplomado Falcons (Falco femoralis septentrionalis; hereafter “falcons”) in southern Texas. Although falcons have nested in the region since 1995, the factors influencing reproductive success are not understood. Nest sites were classified into nest-type groups using a cluster analysis based on nest substrate and nest builder. Using an information-theoretic approach, we compared relative effects of nest age, study area, nest-type group, and physical nest-site characteristics. Our results were similar to previous studies of passerine nesting ecology, in that DSR varied throughout the nesting cycle: it was constant during incubation, declined at hatching and the early nestling phase, and increased through the remainder of the nestling phase. Nest type and nest height were important physical nest-site covariates, overhead cover was less important, and study area and nest initiation date were not influential. We note that although nest success was well correlated with simple nest-site covariates, many falcons chose poor-quality nest sites, which raises the question of whether nest-site selection by falcons is optimal in this region.

Las Características de los Sitios de Nidificación Afectan las Tasas de Supervivencia Diaria de los Nidos de Falco femoralis septentrionalis

Breeding seabirds have been well studied but seabird ecology during the nonbreeding season is poorly understood because many species disperse far from breeding colonies to molt at sea. We characterized the timing of prebasic molt and postbreeding dispersal, described postbreeding dispersal movements, and estimated changes in body mass during molt for Marbled Murrelets (Brachyramphus marmoratus; Alcidae) in central California, 1999–2004. According to mark–recapture and at-sea surveys, 248–315 of 496–637 individuals (43–50%) used Año Nuevo Bay, located immediately adjacent to nesting areas, for their prebasic molt in August–October. Long-distance dispersal (≥ 100 km) from Año Nuevo Bay by radiomarked Marbled Murrelets was low during breeding (9–13%, n = 46), but was greater for individuals radiomarked at the end of the breeding season (69–90%, n = 20). The mean dispersal dates were 18 May and 21 October for the breeding and postbreeding samples, respectively, and postbreeding dispersal occurred an average of two weeks after molt completion. Mean dispersal distances were 184 km and 256 km in the breeding and postbreeding periods, respectively. Of 12 long-distance dispersers, all moved south except one. Marbled Murrelets gained mass during molt (n = 184), except during a moderate El Niño event in 2002 when mass remained constant. However, birds did not take longer to molt in 2002, which suggests that individuals allocated more energy reserves to molt processes in that year. Apparently, sufficient prey resources were available in Año Nuevo Bay for both basic metabolic requirements and the demands of molt, even when water was moderately warm.

Efectos de la Muda Rápida de las Plumas de Vuelo Sobre la Dispersión Post-Reproductiva en un Ave Marina que se Zambulle para Perseguir Presas

Asynchronous hatching creates a size hierarchy among siblings and a survival disadvantage for last-hatched nestlings. Female birds can influence this disadvantage by differentially investing maternal resources, such as carotenoids, across the laying sequence. We studied intraclutch variation in carotenoid concentrations in Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and predicted that yolk concentrations would decrease across the laying sequence, because nestling mortality is inversely related to hatching order in this species. We quantified intraclutch variation in the concentrations of total and individual identifiable carotenoids (β-carotene, β-cryptoxanthin, lutein, and zeaxanthin) in Yellow-headed Blackbird eggs collected from five breeding colonies, and correlated these concentrations with egg mass, yolk mass, and yolk water content. Carotenoid concentrations were not related significantly to any of the egg metrics measured. The concentration of total identifiable carotenoids increased across the laying sequence in Yellow-headed Blackbird eggs, which is the opposite of what has been found in most other passerines. Concentrations of the two most abundant carotenoids, β-carotene and β-cryptoxanthin, and zeaxanthin, all increased across the laying sequence, whereas the concentration of less-abundant lutein decreased. If yolk carotenoid concentrations contribute to increased survival of older over younger Yellow-headed Blackbird nestlings, concentrations of specific carotenoids, such as lutein, may be important. The differential change in concentrations of the various carotenoids across Yellow-headed Blackbird clutches may be attributable to the availability of these compounds in the diet combined with differences in absorption and antioxidant function.

Variation des concentrations en caroténoïde vitellin dans les œufs de Xanthocephalus xanthocephalus selon la séquence de ponte

We examined the demographic response of Middle Spotted Woodpeckers (Dendrocopos medius) to habitat fragmentation in an 880-km² study area in the Cantabrian Mountains (northwest Spain), 2000–2005. We used a set of reproductive parameters to examine 26–72 nests in 10–14 habitat patches. Fifty-nine of 72 nests (81.9%) were successful (i.e., at least one fledgling was produced). Average clutch size was 5.1. Seventy of 94 hatchlings (74.4%) survived to fledging. Mean number of fledglings for successful nests was 3.1, average fledgling mass was 50.6 g, and mean fledging date was 21 June. Generalized linear mixed models showed no significant correlations between patch sizes and any of the reproductive parameters, which suggests that habitat fragmentation did not increase nest predation and parasitism pressures or reduce food in small habitat patches during the breeding season. In 33 habitat patches inspected (4.2 years on average), 190 of 228 territorial males (83.8%) were paired. Pairing success varied strongly across years (77.1–97.4%) and was lower in smaller and more isolated patches. Low pairing success in isolated patches may be associated with disruption in connectivity between habitat patches. On the other hand, the presence of unpaired males in small patches with low numbers of territories suggests that females may use the abundance of conspecifics as an indicator of habitat quality when deciding to mate.

Respuestas Demográficas de Dendrocopos medius a la Fragmentación del Hábitat

Four Great Knots (Calidris tenuirostris) were kept for six years in a constant-temperature indoor aviary. For two of those six years, they were kept under photoperiodic conditions that mimicked natural changes in daylength for wild birds, followed by four years under a constant photoperiod (light:dark cycle 12:12 h). Under cyclical “natural” photoperiods, three of the four birds maintained cycles of body mass and contour and flight-feather molt somewhat comparable to that of free-living birds, though the multiple mass peaks characteristic of northward migration were replaced by a single period of high body mass; the mass peaks for southward migration appeared to be absent. Contour-feather molts between nonbreeding and breeding plumages were delayed, and the period of wing molt was longer than in free-living birds. Under constant photoperiods, clear circannual phenotype cycles were maintained. The length of the period with elevated body mass tripled but was partly compensated by a shortening of the duration of wing molt (which never coincided with high body masses). Nevertheless, total cycle lengths were >13 months. Perhaps most interestingly, under constant photoperiod, there was evidence that two components of what is normally considered an integrated phenotypic event, the prebasic molts of contour and wing feathers, were desynchronized. This suggests that the underlying organizational structure of traits is modular to some extent. Such modularity would increase the flexibility and versatility of the cyclic phenotype in evolutionary contexts.

Ritmo del Ciclo Anual Endógeno del Peso Corporal, Muda y Plumaje en Calidris tenuirostris

Birds are known to modify their foraging behavior in relation to food availability. Once understood, these relationships can be used to draw inferences about relative food availability and habitat quality. We measured foraging behavior of Surf Scoters (Melanitta perspicillata) and White-winged Scoters (M. fusca) feeding on clams during winter in the Strait of Georgia, British Columbia, to evaluate the relative quality of the foraging landscape for wintering scoters. Because clam biomass does not increase appreciably during winter through growth or recruitment, scoters are faced with a depleting and potentially exhaustible food supply. Along with this temporal variation, clam densities vary widely by site. We considered the influence of variation in clam density on scoter foraging behavior, along with other factors known to affect foraging in other sea duck species, such as season, sex, age, and environmental attributes. Clam-capture success (clams captured per dive) and foraging effort (minutes underwater per hour) of Surf Scoters were not related to variation in clam density. Clam-capture success of White-winged Scoters was unrelated to clam density; however, their foraging effort was negatively related to clam density, though varying by only 4 min across the range of observed clam densities. For both species, foraging behavior was generally more strongly related to other factors, especially seasonal and age effects. These results suggest that (1) observed variation in clam density was relatively minor from the perspective of foraging scoters and (2) our study site constituted high-quality winter habitat in which scoters were not constrained by food availability.

Comportamiento de Forrajeo de Melanitta perspicillata y M. fusca con Relación a la Densidad de Almejas: Inferencias sobre la Disponibilidad de Alimento y la Calidad del Hábitat

Approximately 55% of the world's population of Cassin's Auklets (Ptychoramphus aleuticus) nests at Triangle Island, British Columbia. To improve our understanding of the biology of these birds during the breeding season, we tracked 112 radiomarked individuals over three years, 1999–2001. We flew high-altitude surveys to describe the at-sea distributions of the marked birds during the chick-rearing period. Using radiotelemetry point locations, we determined that the location of marine use areas, distance from colony, and water depth varied significantly across years. In 1999 and 2000, radiomarked birds were, on average, ∼50 km southwest of Triangle Island in waters approximately 1400–1800 m deep. However, in 2001, radiomarked birds were, on average, ∼80 km northwest of Triangle Island in waters ∼725 m deep. Intra-annually (i.e., between surveys spanning days or weeks), there were no such large-scale directional shifts in marine use area. The size of marine use areas (quantified using kernel home range [KHR] analyses) varied across the three years, from approximately 650 to 1,400 km² (50% KHR) and from approximately 3,200 to 8,200 km² (95% KHR).

Variation des distributions en mer de Ptychoramphus aleuticus se reproduisant sur l'île Triangle, en Colombie-Britannique

We developed a spatially explicit population-viability-analysis (PVA) model to evaluate different management options for the endangered Florida Grasshopper Sparrow (Ammodramus savannarum floridanus). We also conducted a sensitivity analysis to determine which parameters had the greatest effect on population viability. The model was most sensitive to environmental variation and least sensitive to carrying capacity, initial abundance, and level of correlation in demographic rates between subpopulations. The model that represented present conditions predicted a 22% chance that the population will fall below the extinction threshold of 60 male Florida Grasshopper Sparrows within 50 years. Reintroduction to large prairie sites and prairie restoration reduced the chance of falling below the extinction threshold and increased the number of subpopulations occupied. Loss of dry prairie located on private land adjacent to existing Florida Grasshopper Sparrow subpopulations increased the probability of falling below the extinction threshold to 66%. We caution that the strength of PVAs is not in predicting absolute values of viability or number of individuals but, rather, in evaluatuating the relative effects of different management options. We found that increasing the amount of prairie habitat, especially core habitat, had a strong positive effect on viability. If the Florida Grasshopper Sparrow reaches recovery goals developed by the U.S. Fish and Wildlife Service (10 subpopulations with ≥50 males per site), we estimate that the metapopulation has a 99% probability of remaining above the extinction threshold. Our analyses indicate that the Florida Grasshopper Sparrow is currently vulnerable to extinction, but various management practices can increase population viability.

Análisis de Viabilidad Poblacional de Ammodramus savannarum floridanus: Evaluación de Metas de

Kittlitz's Murrelet (Brachyramphus brevirostris) is a rare seabird whose populations are concentrated in glaciated areas of Alaska. Declines in some parts of its range have led to increased concern over population viability. The remote and cryptic nesting habits of Kittlitz's Murrelets, in contrast to colonial seabirds, preclude monitoring their populations at nest sites and necessitate use of at-sea surveys to count birds. We compared surveys for seabirds in Glacier Bay, Alaska, during 1991, 1999, and 2000, to identify trends in the local Kittlitz's Murrelet population. The surveys conducted in 1999–2000 covered much of the same habitat as those conducted in 1991 but differed in aspects of survey design (i.e., start and stop points, navigation methods, and amount of offshore sampling). We developed a technique using a geographic information system to extract and recompile data from the 1999–2000 surveys that allowed spatially “matched” comparisons with the 1991 survey transects. This comparison of using “matched” transects indicated that the Kittlitz's Murrelet population in Glacier Bay had declined by 83% between 1991 and 1999–2000. Our analytical approach may be useful in similar situations in which current and historical surveys are not spatially uniform, particularly where there is a strong spatial component to the species distribution.

Utilización de Sistemas de Información Geográfica para Comparar Censos No-uniformes de Aves Marinas: Detección de la Disminución de Brachyramphus brevirostris en la Bahía Glacier, Alaska

Studies on the effects of forest fragmentation on the reproductive ecology of forest songbirds have focused almost exclusively on the egg and nestling stages. Little is known about survival or habitat use of fledglings in their first weeks out of the nest. We radiotracked adult Hooded Warblers (Wilsonia citrina) attending fledglings from large (>150-ha) and small (<30-ha) forest fragments during 2002 and 2003. Mark–recapture models were used to assess the effects of forest-fragment size and age of the attending adult on the survival of young. We also examined the structure of nesting- versus postfledging-habitat used by fledglings. Only 19% of fledgling Hooded Warblers survived the 28-day fledgling period prior to independence, and fledglings' daily survival probability was lowest in the first four days after they left the nest (1–2 days postfledging: 0.72; 3–4 days postfledging: 0.69). Fledgling survival did not differ between large and small forest fragments. Large fragments were more likely to contain older adults, and age of parent had a positive influence on fledgling survival, independent of fragment size. As in many other studies, overall productivity was higher within larger fragments; this was largely attributable to differences in number fledged rather than fledgling survival. Habitat used by fledglings was more structurally complex than habitat at nest sites, which indicates that fledglings may seek habitat with high cover to avoid predation. Fledgling survival was much lower than estimates used in demographic models, which suggests that for many species, these models may need re-evaluation.

Supervivencia de Volantones de Wilsonia citrina en Fragmentos de Bosque Pequeños y Grandes

Niche overlap and resource partitioning have seldom been investigated in the marine habitats of seabirds but are obvious determinants of community and population status. We investigated interspecific differences in densities and spatial aggregations of seabirds during summer (June–August) off southwest Vancouver Island, British Columbia. Two 300-m-wide parallel transects were sampled along 66.6 km of shoreline in 1993–1996, centered 200 m and 600 m from shore. Analysis focused on the threatened Marbled Murrelet (Brachyramphus marmoratus) and three other fish-eating alcids. Densities of Marbled Murrelets were among the highest reported at sea in the species' range; they were concentrated close to shore (>75% within 0.6 km and virtually all within 2.0 km of shore), usually in water <20 m deep, and showed strong fidelity to certain sectors along the coast from year to year. Common Murres (Uria aalge) and Rhinoceros Auklets (Cerorhinca monocerata) were not as closely associated with nearshore habitat as Marbled Murrelets, and their longshore distribution also differed from that of the latter. Neighbor-K analysis showed that Marbled Murrelets were usually segregated from Common Murres and, to a lesser extent, from Rhinoceros Auklets, but the latter two species were usually aggregated together. Marbled Murrelets also left the area after breeding, whereas numbers of Common Murres and Rhinoceros Auklets increased from June through August. Pigeon Guillemots (Cepphus columba) showed variable distributions and no consistent associations with other alcids. Marine distributions of Marbled Murrelets were markedly different from those of other fish-eating alcids, and we discuss this in relation to possible interference competition from the larger Common Murres and Rhinoceros Auklets.

Coexistence et répartitions spatiales de Brachyramphus marmoratus et autres alcidés au sud-ouest de l'île de Vancouver, en Colombie Britannique

Parents inluence the phenotype of their offspring by determining the environment in which early development occurs. The many factors that affect growth in avian brood parasites provide an excellent context in which to examine how ecological variables and sex differences influence plasticity of early development. We used a model-selection approach to determine the most important variable(s) for explaining patterns in growth rate of the Brown-headed Cowbird (Molothrus ater). Using published growth-rate estimates across various host species, we found that the age-adjusted size of Brown-headed Cowbird chicks increases with increasing hatching synchrony between host and parasite chicks. We also quantified Brown-headed Cowbird growth rates in nests of Song Sparrows (Melospiza melodia) and Yellow Warblers (Dendroica petechia), two common host species at Mono Lake, California, to examine the role of variation in hatching synchrony in broods within host species. Statistical models to explain variation in Brown-headed Cowbird chicks' growth rates were constructed from ecological variables (host species, brood size, multiple parasitism, hatching synchrony between parasite and host chicks) and chick sex. The best model included only sex and there was a 99% chance that this was the best model, given the data set and models compared. Male Brown-headed Cowbird gained an average of 0.7 g day^—1 more than females and weighed 13% more at the same age. The only significant ecological variable, host-parasite hatching synchrony, was found to be sex-dependent, with males more likely than females to hatch earlier than their nest mates. We discuss the possible mechanisms underlying this sex effect and the importance of determining sex when studying nestling growth and competition.

Correlatos Ecológicos y Diferencias de Sexo en el Desarrollo Temprano de un Parásito de Nidada Generalista

We used satellite telemetry to study year-round movements of two cohorts of juvenile Golden Eagles (Aquila chrysaetos) from Denali National Park and Preserve, Alaska. Radiotagged Golden Eagles started autumn migration between 15 September and 5 October and arrived on their winter areas 31 to 86 days later. Cumulative tracking distances during autumn migration ranged from 818 to 4,815 km. Peak tracking velocities during autumn migration reached 261 km day⁻¹ in 1997 and 472 km day⁻¹ in 1999. Golden Eagles wintered from southern Yukon Territory to southern New Mexico, and most spent the winter within 75 km of the location where they terminated their autumn migration. Spring migration occurred from late March through mid-June. Eagles showed little fidelity to their autumn migration paths as they migrated northwest in spring through western Canada and into Alaska. Duration of spring migration ranged from 24 to 54 days, and cumulative tracking distance during spring migration ranged from 2,032 to 4,491 km. Peak tracking velocities during spring migration reached 284 km day⁻¹ in 1998 and 330 km day⁻¹ in 2000. In contrast to juvenile Golden Eagles raised at temperate latitudes in North America, juveniles raised in Denali traveled thousands of kilometers across western North American during their first year of independence. Our results suggest that conservation strategies for migratory Golden Eagles from Denali, and perhaps from other areas in northern North America, require a continental approach.

Movimientos de Individuos de Aquila chrysaetos desde el Interior de Alaska durante su Primer Año de Independencia

I compared the effects on plasmacorticosterone levels of twomethods of collecting blood samples during standardized capture-and-handling stress protocols. In one method, individual nestling American Kestrels (Falco sparverius) were bled three times: when initially removed from the nest, and 15 and 30 min later. In the other method, siblings removed from a nest were bled once each, either at the time of removal, or 15 or 30 min later. I found no difference between the two groups in plasma corticosterone levels at the first sampling time, but 15 and 30 min after capture the singly bled birds had significantly higher plasma corticosterone levels than the multiply bled birds. The results suggest that data from multiply bled birds underestimate actual circulating hormone levels. The underlying mechanism for this phenomenon is unknown, but it may involve hemodilution.

Réponse au stress chez les poussins: une comparaison de deux techniques d'échantillonnage