Species richness and composition of Neotropical forest bird communities vary spatially at both large and small scales, but previous comparisons based on 100 ha plots have not replicated plots within a region. I sampled birds in two 100 ha plots in lowland forest of eastern Ecuador to better understand how species richness and composition vary over smaller spatial scales. Birds were sampled in February and April of 2002–2005 (only in February in 2005). Plots were approximately 1.5 km apart in predominantly terra firme forest. A total of 319 species (285 and 281 per plot) from 43 families were represented in ~16 000 detections per plot; number of species and detections per sample averaged approximately 185 and 2300, respectively. Numbers of species and detections per family were strikingly similar in the two plots, but numbers of detections of individual species often differed, likely in response to differences in habitat between the two plots. Species richness and composition were similar in many respects to comparable data from Ecuador, Peru, and French Guiana, but differed from those of Panama. Differences were most pronounced at the species level, less at the genus level, and least when comparisons were based on families. Differences among sites in South America were correlated with geographic distance at the species and genus levels, but not at the family level. Results illustrate the value of replicated plots within a region for understanding how species richness and composition can vary at small spatial scales, and highlight the importance of beta diversity for determining overall patterns of regional diversity.

Measurement of stable-hydrogen isotopes (δD) in feathers of migrating birds can provide information on where feathers were grown in North America, at least to an approximate band of latitude. This approach has greatly increased our ability to investigate aspects of avian migration and stopover ecology, since origins of unmarked individuals at migration stopover sites can be estimated for the first time. However, few studies have explored the power of combining isotope measurements with geographic information system (GIS) methods. We measured δD values in feathers of hatching-year (HY) Swainson's Thrushes (Catharus ustulatus; n  =  60), Wood Thrushes (Hylocichla mustelina; n  =  113), and Gray Catbirds (Dumetella carolinensis; n  =  158) at Ft. Morgan Peninsula, Alabama (30°10′N, 88°00′W), a migration stopover site along the Gulf coast. By applying an elevation-corrected hydrogen isotope basemap for birds in North America, we derived a GIS surface depicting expected feather δD values across the continent. We then used GIS to constrain the possible origins of the sampled populations by considering only values falling within the North American breeding ranges of the species. We depicted likely origins of migrating birds by the 50% and 75% tolerance limits of the data. Our GIS analysis indicated that our captured populations represented much-reduced regions of possible origin based on the North American breeding distributions. Gradients in abundance data from the North American Breeding Bird Survey (BBS) allowed us to further narrow possible origins within isotopic boundaries for Wood Thrushes and Gray Catbirds. This exercise provided a means by which priority regions and habitats could be assessed at large continental scales based on actual productivity. We suggest the combination of isotopic and GIS tools provides a powerful means to derive conservation priorities and to investigate key factors involved in the ecology of avian migration and stopover.

To better understand migratory connectivity between breeding and nonbreeding populations, we analyzed mercury (Hg) and stable isotope signatures of nitrogen (δ¹⁵N), carbon (δ¹³C), and sulfur (δ³⁴S) in Audouin's Gulls (Larus audouinii) breeding in two different colonies, the Ebro Delta (northeastern Spain) and the Chafarinas Islands (southwestern Mediterranean). Although abundant information is available on the biology and trophic ecology of this gull's breeding populations, little is known about migration patterns, distribution in winter, or conditions faced during the nonbreeding period. Analyses were carried out on first primary feathers, grown during the summer while gulls are on the breeding grounds, and mantle feathers, grown during the winter. Different isotopic signatures (δ¹⁵N, δ¹³C, and δ³⁴S) in summer (primary) feathers from each area agree with the observed differences in diet between the two colonies. In winter (mantle) feathers, isotopic signatures did not differ, consistent with a common wintering ground and common diet, although the lack of isotopic basemaps in marine systems precludes assignment to a geographical area of reference. Future research is needed to relate isotopic signatures and Hg values in mantle feathers to trophic ecology in wintering areas. Results for Hg indicate that the excretory role played by primary feathers precludes their use as indicators of trophic ecology.

Of the world's five flamingo species, the rarest and least known are the Puna Flamingo (Phoenicoparrus jamesi) and the Andean Flamingo (P. andinus). These two species coexist with the more common Chilean Flamingo (Phoenicopterus chilensis) throughout much of their range. We conducted four simultaneous surveys from 1997 to 2001 (two in summer and two in winter) to estimate the distribution and abundance of all three species in Argentina, Bolivia, Chile, and Peru, at a regional scale. Of 224 wetlands surveyed, 179 had flamingos; 63% of these were in the high Andes (above 4000 m), 25% were in the puna (3000 to 4000 m), and the remainder were in lowlands (below 3000 m). Maximum counts were 64 000 Puna Flamingos (summer 1998), 34 000 Andean Flamingos (summer 1997), and 83 000 Chilean Flamingos (winter 1998). In summer, Puna Flamingos congregated at wetlands in the high Andes, with 50% of the population in just three lakes: Colorada, Grande, and Vilama. Andean Flamingos were more uniformly distributed across a broader elevational range (2500 m), and Chilean Flamingos showed a heterogeneous distribution pattern. In winter, all species moved to lower latitudes within the high Andes and to lower altitudes on the central plains of Argentina. The most important nesting wetlands were Colorada, in Bolivia, for the Puna Flamingo, Surire and Atacama, in Chile, for the Andean Flamingo, and Surire for the Chilean Flamingo. We recommend continued monitoring through simultaneous summer surveys, and a conservation strategy that considers the large spatial and temporal scales at which these species operate, including their seasonal migrations.

Many studies investigating edge effects on forest-nesting birds have focused on nest success. Fewer have examined edge effects on other components of fitness. Nestling growth rates have been positively correlated with food availability, which may differ at edges compared to the forest interior. However, previous work has not examined growth as a function of distance to edge and edge type. We investigated whether edge type, distance to edge, hatching date, brood size, brood parasitism, year, and site influenced growth rates of wings, tarsi, and mass of nestling Wood Thrushes (Hylocichla mustelina) using regression tree analyses. Simultaneously, we examined edge effects on daily nest survival. We conducted our study in southwestern Michigan in 2002 and 2003. We located 175 nests and measured nestlings from 61 nests. Contrary to expectations, tarsal and mass growth rates were more rapid near edges than in the forest interior and tarsal growth rates were more rapid near gradual edge types than near abrupt edge types. Wing chord growth rates were more rapid in the forest interior than at edges. Mass and tarsal growth rates were more rapid later in the breeding season, when brood sizes were smaller. We found no effect of edge type or distance to edge on nest survival. The influence of edge on nestling growth rate but not nest survival suggests nestling growth rates may indicate edge-interior and edge type differences in habitat quality, even when high regional fragmentation levels overwhelm potential edge-interior differences in nest survival.

Reproduction is physiologically stressful for many animals. Female birds often lose body mass after their young hatch, which has been commonly attributed either to an energetic deficit incurred while breeding (the energetic stress hypothesis) or to an adaptive reduction in wing-loading to save energy during flight (the wing-loading hypothesis). We tested these two hypotheses for an open-cup nesting passerine, the Black-throated Blue Warbler (Dendroica caerulescens), using a food-supplementation experiment. We found that females provided with extra food lost less body mass between the incubation and nestling periods than did control females and that mass loss varied seasonally, with females in both treatments that nested earlier in the season losing more mass. We conclude that greater mass loss in control females supports the energetic stress hypothesis in Black-throated Blue Warblers.

Sexual dimorphism, mating system, and parental care are known for only a few species of the large passerine family Furnariidae. We conducted a study of sexual dimorphism in morphology, coloration, and parental roles during incubation and chick-rearing in the Thorn-tailed Rayadito (Aphrastura spinicauda), a characteristic resident ovenbird of the southern temperate rainforests of Chile and Argentina. Through molecular sexing, morphological measurements, and spectrophotometric analysis of body plumage and rectrices of reproductive adults captured on Chiloé Island (southern Chile), we determined that males were between 2% and 10% larger than females in mass, tarsus length, and wing length, while no difference was found for the length of the bill or the two longest central rectrices and their characteristic spines, or in plumage coloration. Heavy males were paired with heavy females and light males with light females. Males and females participated equally in all reproductive activities during the incubation and nestling phases, except removal of nestling feces, in which females were twice as active as males. In a study of habitat use on Navarino Island (extreme southern Chile) we found that the extended graduated tail, with rectrices that end in spines, which gives the species its name, was not used as a support while foraging and could be related to another function such as sexual or social signaling. The absence of sexual dimorphism in plumage and parental roles in rayaditos may be related to the use of the long, graduated tail as a signal of quality by both sexes, although this hypothesis requires confirmation through future mate choice studies.

The breeding ecology of south temperate bird species is less widely known than that of north temperate species, yet because they comprise a large portion of the world's avian diversity, knowledge of their breeding ecology can contribute to a more comprehensive understanding of the geographic diversity of avian reproductive traits and life history strategies. We provide the first detailed examination of the reproductive strategies of 18 forest passerines of subtropical, northwestern Argentina. Mean clutch sizes were smaller and egg mass was greater than for north temperate birds, but differed among species and nest types, with cavity-nesters having larger clutches than species with open-cup and enclosed nests. Across all species, the average breeding season duration was 50 days; thus, the common perception that southern species have smaller clutch sizes because of longer breeding seasons is not supported in this community. Daily nest predation rates were influenced by nest type, cavity nests suffering the least from predation, as found in north temperate systems. Only females incubated eggs in all but one species, whereas both parents fed and cared for nestlings in all species. Mean nest attentiveness was low compared to north temperate passerines. Mean hourly nestling feeding rates differed among species and were negatively related to nest predation risk. In short, coexisting species in this subtropical forest varied in their life history strategies, in part correlated with variation in nest predation risk, but also differing from north temperate species.

Acorn Woodpeckers (Melanerpes formicivorus) at Hastings Reservation in central coastal California exhibit a bimodal peak in annual breeding activity. One peak occurs in spring during which the majority of breeding takes place, while a second is centered in late August as the new acorn crop matures. These latter nests are mostly initiated in late summer but often do not fledge until at least late September and are thus referred to here as ‘fall’ nests. Fall nests occur in about one-third of all years, taking place when the acorn crop is large and summer temperatures are relatively high. Fledglings from fall nests constitute 4.3% of the population's total productivity and survive and recruit to the population at levels comparable to spring fledglings. Fall nesting is less likely in groups in which either the male or female breeding adults have undergone a change from the prior year, but groups are otherwise indistinguishable. Ecologically, fall nesting is closely tied to the acorn crop and thus to breeding success in the following, rather than the prior, spring. Among North American terrestrial birds in general, fall breeding has been reported in 16% of all species and is significantly more common among residents and colonially nesting species, in which the frequency exceeds 25%. Furthermore, fall nesting is likely to have been underreported in the literature. Thus, this phenomenon is at least an irregular part of the breeding biology of a substantial fraction of North American birds and should be considered a possibility in population studies of temperate-zone species. This is especially true given that fall nesting is likely to increase as global warming takes place.

As late as the 1940s the Semipalmated Sandpiper (Calidris pusilla) was the most abundant sandpiper breeding at Churchill, Manitoba. By the 1960s it had undergone a sharp decline, and by the mid-1990s the local population consisted of 11 pairs in a single colony. Nesting was last documented in 2001. Declines had also become evident at several other breeding sites along the Hudson Bay coast of Manitoba and Ontario, as well as in the number of migrants detected on the Atlantic coast of Canada and the northern United States. Information on the biology of the Churchill population in 1993–2004 largely agreed with that gathered at La Pérouse Bay, Manitoba, in the 1980s: reproductive success was good and new birds continued to join the colony; however, the number of breeding attempts by individuals was low and decreasing. As there is no evidence that the decline was related to local factors (e.g., altered habitats, climate change), it is probably attributable to mortality in the nonbreeding season, which leaves fewer birds available to return north. Whether causality can be fully resolved is problematic. Monitoring migrants can reveal population trends and studies on the breeding grounds can help frame hypotheses, but both approaches are time-consuming and provide only partial answers. In such cases, restoration of declining species may be best served by fostering habitat conservation throughout a species' range.

Although mesic tundra is a habitat commonly used by arctic-nesting geese, their feeding ecology in this habitat is little known compared to wetlands. Our objectives were to determine the diet and food selection of Greater Snow Goose (Chen caerulescens atlantica) goslings in relation to the nutritional quality of plants growing in mesic tundra habitats on Bylot Island, Nunavut, Canada. We used two different but complementary approaches: examination of esophageal contents of sacrificed wild goslings, and direct observation of the feeding activity of captive, human-imprinted goslings. The latter method was innovative and provided a reliable description of the diet, with results comparable to those obtained from wild goslings. Although mesic habitats have a more diverse floristic composition than wetlands and sparse graminoid cover, Gramineae were preferentially selected and dominated the diet (~50%). The rest of the diet consisted mainly of members of the Juncaceae, Polygonaceae, and Leguminosae families. The diet of very young goslings was diverse, but as they aged and gained efficiency, they concentrated on a few taxa. Goslings ate mostly leaves (~80%), but flowers (~20%) were also important. Food selection was influenced by nitrogen and total phenolic compounds content of plants, but the ratio of phenolic compounds to nitrogen in plant organs was most determinative of food choice. Neutral detergent fiber content of plants did not influence plant selection. Both plant nutritional quality and availability determined gosling diet across different mesic habitats and growing goslings appeared to maximize their intake of metabolizable proteins.

Nest predation is the most important factor limiting reproductive success of Willow Ptarmigan (Lagopus lagopus), as it is for many bird species. Using program MARK, we examined patterns of daily nest survival in two widely separated populations of Willow Ptarmigan in Manitoba and British Columbia, Canada, by examining variation among years, over the course of the breeding season or in relation to the age of the nest, or age and condition of the female. At La Pérouse Bay, Manitoba, daily nest survival increased linearly throughout the season, with moderate annual variation. Nests also had higher daily survival during laying and late in incubation, and lower survival early in incubation. At Chilkat Pass, British Columbia, daily nest survival varied strongly by year and nests had higher survival early and late in the season, but survival was not affected by the age of the nest. At both sites, but especially at Chilkat Pass, periods of lower nest survival tended to coincide with peak breeding periods, suggesting predators may adjust their search effort based on the likelihood of locating nests. Neither female age nor condition were included in the best models. Thus, in both ptarmigan populations, nest survival patterns appeared to be influenced more by ecological factors than by individual characteristics of breeding females. The extent of annual variation and seasonal patterns may have differed between sites due to the different predator and alternative prey communities.

Introduced feral populations offer a unique opportunity to study the effects of social interaction and founder effects on the development of geographic variation in learned vocalizations. Introduced populations of Monk Parakeets (Myiopsitta monachus) have been growing in number since the 1970s, with a mixture of isolated and potentially interacting populations. We surveyed diversity in contact calls of Monk Parakeet populations in Connecticut, Texas, Florida, and Louisiana. Contact call structure differed significantly among the isolated populations in each state. Contact call structure also differed significantly among potentially interacting nest colonies in coastal Connecticut, and these differences did not follow a geographic gradient. Limited dispersal distances, founder effects, and social learning preferences may play a role in call structure differences.

The Margarornis assemblage includes four genera: Margarornis, Premnoplex, Premnornis, and Roraimia, all thought to be closely related. Differences in vocalizations and habitat use between Premnoplex brunnescens (Spotted Barbtail) and P. tatei (White-throated Barbtail) are consistent with their full species status. In light of the weak anatomical (hindlimb musculature) evidence supporting the inclusion of Premnornis in the Margarornis assemblage, the convergence-prone nature of characters associated with climbing habits, and the differences in their nests and foraging behavior, I propose that Premnornis is not a member of the Margarornis assemblage and that Premnoplex is closely related only to Margarornis. These results are supported by recent molecular phylogenetic analyses. Although Roraimia differs in plumage, tail shape, and song from other members of the Margarornis assemblage, it must be provisionally included in the assemblage until evidence clarifies its phylogenetic position.

A recent taxonomic revision of the Malagasy Scops-Owl (Otus rutilus) recognized two distinct endemic species on the island based on plumage, vocal, and morphological characters: O. rutilus (sensu stricto) from eastern humid forest formations and O. madagascariensis from western dry forest areas. An evaluation of these characters calls into question their validity for taxonomic studies, as they may be ecologically linked. To independently assess the two-species hypothesis, we used sequence data from 1449 base pairs (bp) of mitochondrial DNA (mtDNA) from 34 scops-owls obtained across the range of these two putative species. Nineteen haplotypes were detected, four of which were shared by more than one individual. Maximum sequence divergence was 0.6% (mean  =  0.24%). While the most common haplotype was shared by 10 individuals originating from different eastern and western localities, 12 haplotypes were exclusive to O. rutilus and five to O. madagascariensis. An analysis of molecular variance showed significant partitioning of the genetic variability between O. rutilus and O. madagascariensis. The estimate of the divergence time between populations associated with the names O. rutilus and O. madagascariensis was 8070 years BP. Based on haplotype frequencies and sequence divergence, we conclude that there are two populations of Otus on Madagascar that started to diverge in recent geological time following an ecological parapatric model, perhaps associated with Quaternary climatic shifts. Using these results, it is inappropriate to recognize two species of Otus on Madagascar.

I examined 963 study skins of Lesser Goldfinches (Carduelis psaltria) from the United States to central Mexico, for color, measurements, molt, and plumage wear. These specimens had been previously assigned to as many as three subspecies. Females did not vary geographically in color. Green-backed, green-eared males predominated in the Pacific coastal United States and northern Mexico. Black-backed, black-eared males predominated in the interior, east of 106° West longitude. This west to east color difference varied clinally, and may be a simple genetic polymorphism, so subspecific distinctions cannot be justified by coloration alone. Measurements of bill, tail, and wing, but not tarsus, varied slightly but significantly from smallest along the Pacific coast to largest in south-central Mexico. Since measurements varied clinally, and overlapped extensively, subspecific separation based on size is not practical. Pacific coastal birds subjected to winter rainfall maxima had a complete postbreeding molt, with a sparse prebreeding body molt only in females, and a postjuvenal molt that replaced significantly more flight feathers in males. Interior birds subjected to summer precipitation maxima had a nearly complete or complete prebreeding molt and a usually less complete postbreeding molt. The postjuvenal and postbreeding molts of interior birds were very similar and did not differ between the sexes. Although these sexual and geographic variations of molt are striking, they may reflect phenotypic plasticity in response to environmental contingencies, rather than genetic differences. These variations in one intergrading population support doubts that the Humphrey-Parkes terminology for molts and plumages can reflect phylogenetic homologies among species.

An analysis of the elevational distributions of Southeast Asian birds over a 28-year period provides evidence for a potential upward shift for 94 common resident species. These species might have shifted their lower, upper, or both lower and upper boundaries toward a higher elevation in response to climate warming. These upward shifts occurred regardless of habitat specificity, further implicating climate warming, in addition to habitat loss, as a potentially important factor affecting the already imperiled biotas of Southeast Asia.

The relationship between nest entrance orientation and latitude among ground-nesting passerines was reviewed using published information. Data were collated for seven North American and European species. Pooling within-species comparisons, there was a clear trend from a preference for north-facing nests at lower latitudes to eastward- or southward-facing nests farther north. Orientations differed significantly in eight of 12 cases for which statistical comparison was possible, means differing in the expected direction in six of these cases. These results highlight how the influence of solar radiation on nest microclimate typically delineates preferred nest orientation in these species, i.e., at lower latitudes, the need for shade results in a preference for northward orientations; at mid latitudes, eastward orientations predominate, reflecting a probable balance between the benefits of warmth in the early morning and shade in the afternoon; while at high latitudes, nests may be oriented southward to gain warmth throughout the day.

We measured metabolic heat production (H ˙_m) of perching and foraging Black-capped Chickadees (Poecile atricapillus) to determine if the heat produced during foraging activity, or exercise thermogenesis, could replace thermoregulatory heat production requirements. H ˙_m and activity of chickadees in winter were measured at ambient temperatures (T_a) ranging from −11.5° to 15.5°C. Mean activity amplitude recorded with an activity detector was significantly higher in foraging birds than perching birds. H ˙_m did not vary significantly between perching and foraging birds, indicating that heat produced during foraging does substitute for heat produced by shivering for thermoregulation. Evaporative water loss and dry thermal conductance did not vary significantly between perching and foraging chickadees. These results suggest that heat produced from locomotor muscles during foraging activity substitutes for thermoregulatory requirements in glean-and-hang foraging species, such as chickadees, as well as in ground-foraging birds.

We measured the fractionation of stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ¹⁵N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ¹³C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ¹⁵N and 0.1‰ to 2.5‰ for δ¹³C. Studies that did not remove lipids from prey samples showed higher δ¹⁵N and δ¹³C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ¹⁵N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ¹³C fractionation, (2) high interstudy and interspecific variation in δ¹³C makes species-specific studies essential, and (3) use of muscle tissue rather than whole bodies of fish will minimize unexplained variation in δ¹⁵N fractionation.

Recent mitochondrial DNA (mtDNA) and morphological studies suggest that the Curve-billed Thrasher (Toxostoma curvirostre) consists of three historical groups, corresponding to the subspecies curvirostre and palmeri and a new taxon distributed in Oaxaca and Puebla. Previous mtDNA analysis did not include the subspecies T. c. insularum found on Tiburon Island (Sonora, Mexico). We collected new samples to address the distinctiveness of the three groups, to explore possible contact between the curvirostre and southern clades, to clarify the status of T. c. insularum, and to explore population histories of the major clades. Our mtDNA results confirm the existence of three major haplotype clades, which represent either two or three species depending on the species concept used. We interpret the occurrence of two mismatched haplotypes in Puebla and Oaxaca as recent gene flow. Haplotypes from Tiburon Island were intermixed within the palmeri clade, thereby failing to support the subspecies T. c. insularum. In contrast, principal components analysis of morphological data showed that insularum was distinct and, given the lack of mtDNA distinctiveness, suggests rapid phenotypic evolution on the island. The molecular data suggest that the widespread curvirostre clade has undergone recent population growth, whereas the other two clades appear to have been more stable.

Estudios recientes de ADN mitocondrial (ADNmt) y de morfología sugieren que Toxostoma curvirostre consiste de tres grupos históricos que corresponden con los grupos de las subespecies palmeri, curvirostre y un nuevo taxón distribuido en Oaxaca y el sur de Puebla. Análisis previos de ADNmt no incluyeron a T. c. insularum, subespecie endémica de la Isla Tiburón (Sonora, México). Colectamos ejemplares clave para evaluar el reconocimiento de los tres grupos, para explorar el posible contacto entre curvirostre y el clado del sur, para esclarecer el estatus de T. c. insularum y para explorar las historias poblacionales de estos grupos. Nuestros resultados confirman la existencia de tres grandes clados haplotípicos, los cuales representan dos o tres especies dependiendo del concepto de especie utilizado. Interpretamos como un flujo genético reciente la ocurrencia de haplotipos intercambiados en Puebla y Oaxaca. Los haplotipos de la Isla Tiburón se mezclaron con los del clado palmeri, fallando en dar soporte a su estatus como un distinto taxón. En contraste, los análisis de componentes principales de los datos morfológicos, mostraron que insularum es distinto, y dada la carencia de reconocimiento a través de ADNmt, sugiere una rápida evolución fenotípica en la Isla. Los datos moleculares sugieren que el clado curvirostre, ampliamente distribuido, ha experimentado un crecimiento poblacional reciente, mientras que los otros dos clados aparentan una mayor estabilidad.

The reuse of old nests by open-cup nesting passerines is a seemingly rare but potentially adaptive behavior if, as a consequence, females begin to breed earlier, lay larger clutches, or fledge more young. We report an unusually high rate of nest reuse (~10% of 341 nests) for Eastern Kingbirds (Tyrannus tyrannus) breeding at Malheur National Wildlife Refuge, Oregon. We found no difference in availability of nesting habitat or food abundance in territories in which nests were and were not reused. We also found no support for the hypotheses that kingbirds benefited from nest reuse by breeding earlier, laying larger clutches, or fledging more young, and, contrary to expectations, females that reused nests laid significantly smaller eggs than females who built their own nests. Nest reuse was independent of age: a roughly equal number of females for which we had multiple years of data both reused nests and built new nests, but at different points in their lives. Competition for nest sites seems high at Malheur National Wildlife Refuge because many open-cup nesting species utilize similar nest sites in the limited zone of riparian vegetation. A shortage of high-quality nest sites, coupled with interspecific competition, may underlie the high frequency of nest reuse in this kingbird population.

In this first study of vocal individuality in the Charadriidae, we describe vocal variation in the endangered Piping Plover (Charadrius melodus). We studied a long-distance advertisement call given by breeding males during aerial displays, because functionally similar vocalizations of scolopacids are known to be individualistic. We analyzed recordings of 10 breeding males on Prince Edward Island, Canada, made in 1998 and 1999. Calls varied substantially among males: variance among males (in one-way ANOVAs) averaged 61% of the total variance across variables, similar to two scolopacid species. Males were distinguishable from one another even on the basis of single calls. However, discrimination ability (classification success) improved substantially when multiple-call averages per male were used; this approach more realistically represents how plovers experience one another's calls. Calls of two males changed little over days to weeks. In future work, individual identification can be improved by using averages from multiple calls. To improve techniques for application to the Piping Plover, we recommend research on marked individuals of common related species, which share many acoustic homologies.

We studied the intraspecific territorial defense behavior of wintering Willow Flycatchers (Empidonax traillii) in Costa Rica using a randomized playback experiment that exposed male and female birds to recordings of Willow Flycatcher songs and calls, Lesser Ground Cuckoo (Morococcyx erythropygius) vocalizations, and random noise. Flycatchers of both sexes responded most strongly to simulated conspecific territory intrusion, and the agonistic behaviors that we observed were similar to those seen during natural intraspecific encounters in winter. Both males and females engaged in song and aggressive behaviors in defense of territories, and there was no significant difference between the sexes in scored agonistic responses. The similarity between the sexes in intraspecific territorial defense behaviors and aggressiveness may account for both sexes of flycatchers using the same habitats at our study sites in Costa Rica, and wintering females defending territories against males. The Willow Flycatcher, a sexually monomorphic species, differs in this way from a number of sexually dimorphic passerines, in which behaviorally dominant males occur in more optimal winter habitats.