Understanding the mechanisms causing temporal variability in demographic parameters is essential to understanding fluctuations in populations. As part of a long-term demographic study, we evaluated influence of climate on Mexican Spotted Owl (Strix occidentalis lucida) annual survival and reproduction in two study areas, one in Arizona and one in New Mexico. Spotted Owl survival in New Mexico and reproductive output in both study areas were positively related to total amounts of precipitation from the previous year, previous winter, or monsoon season. For both study areas, temporal process variation in reproductive output (CV[R] = 51.2 and 75.2% for Arizona and New Mexico, respectively) was greater than that for survival (CV[ϕ] = 12.9 and 7.1% for Arizona and New Mexico, respectively). Precipitation from the previous year explained 73% of σ̂²_temporal reproductive output for Arizona owls and precipitation from the previous monsoon explained 42% of σ̂²_temporal in reproductive output for New Mexico owls. Precipitation from the previous monsoon season explained 53% of σ̂²_temporal in Arizona owl survival and precipitation from the previous winter explained 56% of σ̂²_temporal in New Mexico owl survival. The two populations of Spotted Owls we studied appeared to have the same life-history strategy hypothesized for a population of Northern Spotted Owls (S. o. caurina), although the Mexican subspecies apparently responded quite differently to climatic variation.

We present a robust, fully resolved phylogeny for the oropendolas that will serve as a basis for comparative studies in this group. We sequenced 2,011 base pairs (bp) of the mitochondrial cytochrome-b and ND2 genes from 22 individuals to reconstruct relationships between recognized species and subspecies and to assess variation within polytypic taxa. A single phylogenetic tree was produced despite the use of a wide range of weighting schemes and phylogenetic reconstruction methods. Our data provide strong evidence that oropendolas are polyphyletic, with two distinct groups within a larger clade of oropendolas and caciques. We confirm the monophyly of recognized species, but indicate that some within-species relationships do not conform to recognized subspecies limits. Our findings thus demonstrate the importance of including multiple exemplars from each taxon of interest. The two genes provided complimentary and equally effective phylogenetic information for comparisons within the oropendolas, but exhibited lower resolution in comparisons above the species level.

Vocal dialects have presented a puzzle for evolutionary biologists for nearly 50 years. However, there is still a dearth of detailed description of variation in species with dialects, especially regarding acoustic characteristics that discriminate among dialects. In this study, I show that dialects occur in the “flight whistle” song type of the Bronzed Cowbird (Molothrus aeneus). Dialects in this species occur at a very large scale, spanning hundreds of kilometers. A discriminant function analysis revealed that the initial element of the song shows greater differences among dialects than a major element in the final portion of the song. This quantitative analysis of the dialect system provides the basis for understanding how Bronzed Cowbirds recognize songs from different dialects.

The recording in 1992 of an unknown antbird loudsong initiated a journey of discovery that led to the resolution of the conflict between John Zimmer (1932) and Melvin Carriker (1934) regarding the taxonomic standing of the population of Myrmeciza hemimelaena, a widespread southern Amazonian antbird, that occurs in lower Andean elevations in San Martín, Peru. Zimmer had described a new subspecies, M. h. castanea, from the Moyobamba Valley, but Carriker, on the basis of specimens he collected at the same elevation in an adjoining valley, could not find morphological differences between his specimens and the widespread nominate form of M. hemimelaena. Both authors were correct. Two cryptic sister taxa coexist in the foothills of San Martín. Diagnostic vocal and morphological characters and syntopy confirm their status as distinct species. Myrmeciza hemimelaena castanea Zimmer is revived and raised to species status. Concurrently, analysis of the vocalizations and morphology of the lowland population north of the Río Marañón in Peru, the loudsong recording of which initiated the project, revealed that this population was closely related to M. castanea of which it is described as a subspecies, M. c. centunculorum, under the biological species concept. The discovery that there are two cryptic species in the Myrmeciza hemimelaena complex exemplifies the continuing need to pursue field knowledge and analysis of avian species limits in the Neotropics required for the development of conservation strategies as well as phylogenetic understanding.

Comparative analyses constitute an important complement to studies of adaptive behavior. Previous studies of avian mating systems considered the role of paternal care and habitat type on the evolution of polygyny. We extended those studies and included in our analyses the role of habitat quality, as characterized by food supply. Species in the monophyletic lineage of acrocephaline warblers (Acrocephalus, Chloropeta, Hippolais) are widely distributed, inhabit a variety of different habitats, and show a variety of breeding systems. We present a phylogenetic analysis of parental care and mating system characteristics in relation to ecological traits in 17 species. On the basis of a molecular phylogeny, we reconstructed patterns of changes from social monogamy to polygyny, and in paternal brood care. Specifically, we analyze the coevolution of brood care participation of males and social system, and how it relates to habitat quality. Furthermore, we assessed the phylogenetic inertia of mating systems. We found support for the hypothesis that change to highly productive habitats was associated with a greater emancipation of males from brood care, and with polygyny and promiscuity. Poor habitats, on the other hand, were associated with monogamy and the occurrence of helpers. In contrast to some morphological characters, mating systems appear to be phylogenetically labile.

Most obligate cavity-nesting birds are considered to be nest-site limited, either by time or energy to excavate or to acquire suitable holes for nesting. We examined rates of nest-cavity reuse for a rich community of cavity-nesting birds in mixed forests in interior British Columbia. Using a sample of 402 cavity-reuse cases over five years, we measured cavity reuse for 20 cavity-nesting bird and mammal species (three guilds), and examined the relationship between nest-cavity reuse and features of cavities, nest trees, and forest stands. Eight percent of used cavities were destroyed between years. Reuse rates were 17% for the cavities of weak excavators such as nuthatches and chickadees, 28% for formerly active woodpecker nests, and 48% for cavities previously used by secondary cavity nesting birds, but there was considerable species variation within all guilds. Nest cavities in aspen that were deep with large entrances had the highest reuse. At the forest stand level, cavities in trees close to edges and in sites with more edge habitat had greater reuse. Reused cavities tended to be occupied in sequential years rather than being inactive for a year. With increasing amounts of managed landscapes, availability of suitable cavities for forest nesting vertebrates is decreasing. Reuse of existing cavities might help mitigate the problem of nest-site limitation.

The waterfowl family Anatidae is one of very few avian taxa that possesses an intromittent organ. This paper examines the adaptive significance of the intromittent organ in waterfowl by determining the relationships between intromittent organ morphology and the intensity of sperm competition (as reflected by frequency of forced extrapair copulations [FEPCs]). Intromittent organ morphological characteristics, including length and circumference (adjusted for body size), number of ridges and knobs (per unit area), ridge or knob height, ridge or knob span, and area covered by ridges and knobs, were measured from scaled drawings of museum specimens of 54 waterfowl species (33 genera), 27 of which were ranked by frequency of FEPC (1 = monogamous, 2 = rare FEPC, 3 = frequent FEPC, and 4 = polygynous or promiscuous). Testes sizes were also investigated in relation to FEPCs, where testes mass (adjusted for body size) from 44 species (24 genera) were obtained (29 species with mating strategies). The size of the testes, the length of the intromittent organ, the height of the intromittent organ ridges and knobs, and the area covered by ridges and knobs increased significantly with the frequency of FEPC, and those relationships exist after correcting for common ancestry constraints. These results are consistent with the hypothesis that waterfowl intromittent organs are involved in sperm competition. Further research into the actual mechanism by which the intromittent organ is involved would be worthwhile.

Use of point-count surveys is a popular method for collecting data on abundance and distribution of birds. However, analyses of such data often ignore potential differences in detection probability. We adapted a removal model to directly estimate detection probability during point-count surveys. The model assumes that singing frequency is a major factor influencing probability of detection when birds are surveyed using point counts. This may be appropriate for surveys in which most detections are by sound. The model requires counts to be divided into several time intervals. Point counts are often conducted for 10 min, where the number of birds recorded is divided into those first observed in the first 3 min, the subsequent 2 min, and the last 5 min. We developed a maximum-likelihood estimator for the detectability of birds recorded during counts divided into those intervals. This technique can easily be adapted to point counts divided into intervals of any length. We applied this method to unlimited-radius counts conducted in Great Smoky Mountains National Park. We used model selection criteria to identify whether detection probabilities varied among species, throughout the morning, throughout the season, and among different observers. We found differences in detection probability among species. Species that sing frequently such as Winter Wren (Troglodytes troglodytes) and Acadian Flycatcher (Empidonax virescens) had high detection probabilities (∼90%) and species that call infrequently such as Pileated Woodpecker (Dryocopus pileatus) had low detection probability (36%). We also found detection probabilities varied with the time of day for some species (e.g. thrushes) and between observers for other species. We used the same approach to estimate detection probability and density for a subset of the observations with limited-radius point counts.

We explored aspects of renal function in American Kestrels (Falco sparverius), birds that habitually eat high-protein meals. In particular, we hypothesized that, like granivorous birds switched to high-protein food, kestrels would have high urine flows with high proportions of waste N as urate. In nestlings in the field, 80–93% of urinary N was in the form of urate, though concentrations of ammonia and urea were higher in older nestlings. Feeding resulted in reduced osmotic and ion concentrations in urine, but concentrations and proportions of nitrogenous wastes were unaffected. In adult kestrels in the laboratory, urine flow rate in fed animals, ∼1 mL h^–1, was similar to that previously measured in avian granivores. Urine flow declined with fasting, accomplished by both a reduction in glomerular filtration rate (significant after 24 h fast) and a rise in tubular water reabsorption (significant after 48 h). During the course of a 48 h fast, proportion of urinary N excreted as urate fell and that as ammonia rose. Both filtration and secretion of urate dropped during fasting, so that tubular secretion remained responsible for ∼90% of urinary urate. Composition of fluid voided from the cloaca differed little from that of the ureteral urine.

Plasma lipid metabolites may be useful indicators of mass changes in migratory birds. To test utility of plasma metabolites in field studies, we examined effects of several extrinsic (bleed time, time of day, location) and intrinsic (body mass, sex, age, migratory state) factors on plasma concentrations of triglycerides (TRIG), glycerol (GLYC), and B-OH-butyrate (BUTY) in free-living Western Sandpipers (Calidris mauri). TRIG and GLYC decreased rapidly following capture (2–20 min), whereas BUTY did not change. GLYC and BUTY were negatively correlated to body mass. TRIG was positively correlated to body mass in migrant females, but not consistently in migrant males, or in females captured on the wintering grounds. Taking into account other sources of variation, the two measures of lipid utilization (GLYC and BUTY) varied little through the year. TRIG showed the greatest potential for use in field studies. TRIG was lowest during winter, when birds were leanest, and highest during spring and fall migration, when sandpipers were gaining mass rapidly at stopovers. TRIG differed between sandpipers refuelling a two stopover sites separated by 35 km, demonstrating that populations of birds can have characteristic lipid metabolite profiles that may reflect local differences in fattening rate.

One potential determinant of colony size in birds is the local availability of food near a nesting site. Insectivorous Cliff Swallows (Petrochelidon pyrrhonota) in southwestern Nebraska nest in colonies ranging from 2 to over 3,000 nests, but they feed on so many kinds of insects that direct sampling of food resources is impractical. Instead, we investigated the degree to which swallow colony size was correlated with the extent of different habitat types, land use diversity, and plant species diversity in the colony's foraging range, and used those parameters as indices of potential variation among sites in food availability. Amount of flowing and standing water in the foraging range was a significant predictor of mean colony size across years at a site, with larger colonies associated with more water. The same result held for most years when analyzed separately. The extent of flowing water in the foraging range also was a significant predictor of the frequency with which a site was occupied across years. In addition, univariate tests suggested that the amount of cultivated cropland in the foraging range varied inversely with colony size. Land use diversity, as measured by Simpson's index, increased significantly with colony size, and all of the sites with perennially very large colonies (mean colony size >1,000 nests) were associated with foraging ranges of relatively high land use diversity. Repeatability of colony size across years differed significantly from zero across all sites, but repeatabilities were significantly lower (colony sizes less similar between years) for sites situated in low-diversity habitats and for sites used less often. There was no strong effect of plant species diversity within the foraging range on either colony size at a site or likelihood of site use. We conclude that land use diversity per se (and possibly the extent of water near a site) might influence insect distribution and constrain formation of the larger colonies to certain sites. These findings emphasize that colony choice in Cliff Swallows is complex, reflecting both the socially mediated costs and benefits of group size that vary among individuals and the effects of habitat heterogeneity that may influence food availability at some sites.

Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) in Manitoba breed in dense colonies in cattail marshes. Their reproductive success is affected mainly by predation. The most important predator on blackbird nests is the Marsh Wren (Cistothorus palustris), which breaks blackbird eggs and kills small nestlings. We examined whether colonial nesting in Yellow-headed Blackbirds may represent an adaptation to reduce Marsh Wren predation. Marsh Wren predation may be reduced by (1) mutual nest defense by adult blackbirds, (2) predator satiation or dilution, or (3) selfish-herd effects. We tested these hypotheses using experimental nests and found that their safety increased with decreasing distance to the nearest blackbird nest and with increasing density of simultaneously active blackbird nests located nearby. Safety also was higher for nests placed inside a blackbird colony rather than outside. These findings support the nest-defense hypothesis. We also found that Marsh Wrens are capable of destroying a whole blackbird colony in a few days, and that colony size is not correlated with nest safety. These results suggest that the satiation or dilution benefits are negligible. Finally, we found that central nests are safer than peripheral nests in a blackbird colony, but not in an artificial colony, providing weak support for the selfish-herd hypothesis. We conclude that nest predation is reduced mainly by mutual nest defense of adult birds and may represent an important selective force favoring colonial nesting in this species.

We studied prey selection of Sandwich Terns (Sterna sandvicensis) in the presence of kleptoparasitising Black-headed Gulls (Larus ridibundus) on Griend, Dutch Wadden Sea, from 1992 to 1998. Most often, gulls robbed larger sandeels rather than herring from terns. Provided that one parent attends the chick, energy intake would be too low for the chicks' survival if exclusively fed on sandeel. By provisioning an increasing proportion of herring, parent terns could theoretically overcome a potential food shortage when the chicks grow older. However, the proportion of herring did not increase with age of chicks in most years. Instead, parents increased their foraging effort with the growth of their offspring. In years with high proportions of food loss, parents left their chicks unattended at the nest more often. In that way, parents were largely able to counterbalance food shortage of offspring. In 1992 and 1996, when diet composition was unfavorable and food losses to gulls were high, parents left their chicks unattended at the nest for ∼80% of the daylight period. It is argued that the present working level of parents might be higher than in the 1960s.

European Starlings (Sturnus vulgaris) exhibit marked sex differences in behavior during spring. Song activity, nest-box occupation, the carrying of green nesting materials into a nest hole (typical mate-attraction behaviors), and aggression occur much more frequently in males who also have higher testosterone (T)-levels than females. Here, we examined whether male-like concentrations of T would activate these behaviors in female starlings. We treated females with implants that were either empty (C-females) or packed with T (T-females) during late December. Although elevated T-levels significantly increased song activity in females, the proportion of time spent singing was still much lower than in males. T-treatment in females also failed to induce singing behavior in the nest box, a typical male mate-attraction behavior. Those results suggest both activational and organizational effects of T on singing. Nest-box occupation, carrying of green nesting material into a nest hole, and aggressive behavior were not activated by elevated T-levels, indicating that sex differences in these behaviors are probably based on early organizational effects of steroid hormones. We also evaluated effects of increased T-concentrations on some physiological, morphological, and immunological parameters. T-treatment strongly suppressed tail feather regrowth after experimental plucking, and also delayed onset of molt of wing feathers and slowed its progress. T-implantation caused color of the bill to change from black to yellow, but did not affect body mass. Immune function was determined by using two indirect measures: blood composition (haematocrit and buffy coat values) and indications of infections. Although haematocrit and buffy coat values did not differ between C- and T-females, T-females were significantly more infected with Staphylococcus aureus than C-females at the end of the experiment.

We evaluated spatiotemporal variation in clutch sizes of Northern Pintails (pintails; Anas acuta) nesting in California (1985 to 1996), North Dakota (1982 to 1985), Saskatchewan (1982 to 1985) and Alaska (1991 to 1993) to determine whether seasonal declines in clutch size varied in ways that were consistent with a controlling influence of increasing day length. Pintails began nesting in mid-March in California, mid-April in North Dakota and Saskatchewan, and mid-May in Alaska. Observed durations of nesting were 70 ± 2.6 days (SE) in California, 60 ± 6.3 days in North Dakota, 66 ± 1.3 days in Saskatchewan, and 42 ± 0.7 days in Alaska. Annual differences were the principal source of variation in mean clutch sizes (σ̂_Y² = 0.15, SE = 0.049), which varied little among study locations (σ̂_A² = 0.002, SE = 0.013). Predicted rates of seasonal decline in clutch sizes increased with latitude early in the nesting season, but declined as the nesting season progressed, except in California. Rates of decline in clutch sizes thus were not directly related to rates of increase in day length. Predicted declines in numbers of eggs per clutch over the nesting season were similar for all four locations (range, 3.05–3.12) despite wide variation in durations of nesting. Evidence suggests that reduced nutrient availability during nesting contributes to a higher rate of decline in clutch sizes in Alaska than in temperate regions. Pintails that nest early lay large initial clutches, but thereafter clutch sizes decline rapidly and breeding terminates early. This reproductive strategy is adaptive because young that hatch earliest exhibit the highest survival rates; however, the conversion of grassland to cropland on the primary prairie breeding grounds has reduced hatching rates of clutches laid early in the nesting season. Under these conditions, the limited capacity to renest in late spring on their prairie breeding grounds probably has contributed to Pintail population declines.

We used capture–recapture methods to estimate adult survival rates for adult female Black Brant (Branta bernicla nigricans; hereafter “brant”) from three colonies in Alaska, two on the Yukon-Kuskokwim Delta, and one on Alaska's Arctic coast. Costs of migration and reproductive effort varied among those colonies, enabling us to examine variation in survival in relation to variation in these other variables. We used the Barker model in program MARK to estimate true annual survival for brant from the three colonies. Models allowing for spatial variation in survival were among the most parsimonious models but were indistinguishable from a model with no spatial variation. Point estimates of annual survival were slightly higher for brant from the Arctic (0.90 ± 0.036) than for brant from either Tutakoke River (0.85 ± 0.004) or Kokechik Bay (0.86 ± 0.011). Thus, our survival estimates do not support a hypothesis that the cost of longer migrations or harvest experienced by brant from the Arctic reduced their annual survival relative to brant from the Yukon-Kuskokwim Delta. Spatial variation in survival provides weak support for life-history theory because brant from the region with lower reproductive investment had slightly higher survival.

In double-brooded species, the sex that contributes less to breeding success may desert offspring before the end of the rearing period to start a new breeding attempt with another partner. I evaluated that prediction in the Barn Owl (Tyto alba), a species in which the male feeds the brood on average twice as often as the female. Among birds that produced a second brood, 46% of females and 4% of males deserted their offspring before completion of parental duties to remate with another partner at a distance of 1 to 10 km. Offspring desertion did not appear to incur a reproductive cost to the deserter, because deserting females at the first nest produced a similar number of offspring as nondeserting ones. In most cases, the new mate of deserting females had not been previously captured in the study area, and hence had probably not previously bred that season. Although the second clutch of deserting females was larger and produced two weeks earlier than that of nondeserting females, their nesting success did not differ.

John T. Zimmer described Xiphorhynchus necopinus (Zimmer's Woodcreeper) as a sibling species of the more widespread Xiphorhynchus picus (Straight-billed Woodcreeper). Despite his initial suspicion that a preexisting name (kienerii) could be applied to the new species he described, Zimmer discarded that possibility without directly examining the type of X. p. kienerii housed at the Museum National D'Histoire Naturelle in Paris, France. We studied plumage and measured wing chord and tail lengths of the type specimen of X. p. kienerii, comparing it with series of specimens of X. necopinus and X. picus. We show that X. necopinus and the type of X. p. kienerii are morphologically and morphometrically indistinguishable, and thus necopinus is synonymized with kienerii, the name that has priority. Because Zimmer mistakenly applied the name kienerii to a western Amazonian population of X. picus, most specimens in museums identified as X. p. kienerii after Zimmer's review (Zimmer 1934) are likely to refer to X. p. peruvianus.

Among songbirds, does reduced fidelity to a breeding site lead to vocal improvisation? Data for Cistothorus wrens suggest it does, because North American Sedge Wrens (C. platensis) have low breeding-site fidelity and improvise their large song repertoires, but sedentary or site-faithful populations of this and other Cistothorus species in the Neotropics and North America all imitate. We attempted to falsify this hypothesis by studying extreme south-temperate zone populations of Sedge Wrens in the Falkland Islands. We banded and recorded males on Kidney Island and Carcass Island, and then compared song matching among males both within and between islands. Birds on those islands were highly site-faithful from one breeding season to the next. Song repertoires were large, up to 400 in one bird, and songs of birds within an island were more similar to each other than to songs on the other island, showing that these birds do imitate. These results further support the idea that site fidelity promotes imitation of neighbors, and continue to highlight the unique correlation between reduced site-fidelity and song improvisation in the North American Sedge Wren.

We used radio telemetry to examine extraterritorial movements of female Hooded Warblers (Wilsonia citrina) breeding in small, isolated woodlots. Only 44% of the nine females tracked left their woodlots (n = 6 forays in total). Compared to males followed in the same fragments (Norris and Stutchbury 2001), females spent significantly less time off territory and traveled shorter distances across nonforested gaps. Unlike males in fragmented habitat and females in continuous forest, we found no evidence that females were actively seeking extrapair copulations. We conclude females are relatively more restricted to isolated fragments during the breeding season and suggest that the lack of extrapair copulation opportunity could contribute to female avoidance of fragmented habitat, resulting in a high number of unmated males. This study emphasizes the importance of examining the potential effects of fragmentation in both sexes.

We used data on breeding bird communities of the oases of southern Tunisia to investigate variation in species richness and composition among local communities in relation to spatial configuration of the oasis system and to assess significance of the relationship between oasis size and local richness. Oases could be grouped into three regional systems, namely littoral, saharan, and mountainous oases. We found that at the scale of the entire oasis system, species richness and species composition exhibited significant spatial autocorrelation. That result was consistent with our prediction that the particular spatial organization of oases may have led to regional differences in availability of potential colonizers and that higher species exchange within oasis zones than among oasis zones may have played an important role in shaping local communities. With regard to the species–area relationship, we thus found that a model accounting for spatial covariance was more parsimonious than a standard regression model not incorporating information on the spatial location of oases. Overall, oasis size was a good predictor of species richness, but results were sensitive to spatial scale at which the relationship was examined. Aside from oceanic islands and forest patches, oases may constitute interesting systems to study how regional processes affect local diversity.

Differential migrants are species in which one population class migrates farther than another. White-throated sparrows (Zonotrichia albicollis), which migrate into the southern United States each autumn in large numbers, have been proposed as a candidate differential migrant (Cristol et al. 1999). Using Bird Banding Lab data, we tested for latitudinal variation in the sex ratio of White-throated Sparrows in the Atlantic and central flyways. We found that the proportion of females increases with decreasing latitude in at least the Atlantic flyway, strongly suggesting that females migrate farther than males. Sex ratios calculated from those banding data were compared to those of museum specimens collected at the same latitudes, as well as single-season population samples at three latitudes in the heart of the winter range. Broad agreement was found using the three independent methods of sampling the sex ratio, so we conclude that White-throated Sparrows are differential migrants.

We analyzed data on 535 Ural Owl (Strix uralensis) breeding attempts and consecutive survival of both adults and offspring from 1987–1998 in relation to the regional abundance of the Ural Owl's main prey, voles, which show a cycle of low, increase, and peak phases in their population numbers. Vole abundance varied up to 49×, crashing during spring–summer every three years. The breeding population tracked abundance of voles in the previous autumn with respect to percentage of pairs breeding and their reproductive output (laying date, clutch size), largely irrespective of phase. Survival depended on vole density in the preceding autumn, but was generally highest in the increase phase. There was thus a paradoxical situation in the peak phases, when vole populations crashed; the owls produced large clutches, but those survived poorly. Some adaptive and nonadaptive scenarios of the Ural Owl's life history are discussed.

Accurate measurements of food consumption by young birds are of great value in a variety of ecological studies, but traditional measurement methods are problematic and prone to high error. In animals that obtain their water primarily through food, it is possible to estimate food consumption from measurements of water influx rate, using radioactively labeled water (³H₂O), and diet water content. We evaluated that method by comparing actual food consumption with simultaneous estimates of food consumption based on the labeled-water method in captive-reared nestling Tricolored Herons (Egretta tricolor). There was good agreement. The average error in the isotope method was −2% (SD = 8%), and that difference was not statistically significant. An errors analysis indicated that the accuracy of this method is sensitive to determinations of dietary water content and animal body water content, and to estimates of rates of metabolic water production and unidirectional water vapor input.

We experimentally parasitized Chestnut-collared Longspur (Calcarius ornatus) nests with mimetic and nonmimetic Brown-headed Cowbird (Molothrus ater) eggs to determine whether the low frequency of parasitism reported for that grassland host is due to egg rejection behavior. Longspurs accepted all 23 mimetic eggs (real and wooden) and rejected 9 of 22 blue nonmimetic eggs. Our results suggest longspurs accept cowbird eggs even though they are physically capable of ejecting them from their nest. Thus, the low parasitism frequency reported for that species is not a result of egg rejection behavior, but may be due to antiparasite strategies that reduce their chances of being parasitized (e.g. nest defense behavior).

We examined primary sex-ratios of two brood parasitic species, Brown-headed Cowbird (Molothrus ater) and the Common Cuckoo (Cuculus canorus), to determine whether there was any evidence of primary sex-ratio manipulation as has been demonstrated in other species of birds. Despite good reasons for why female brood parasites should manipulate the primary sex-ratio of their young, we found a lack of evidence for a bias in the sex-ratio of eggs produced at the population level, with respect to the host species parasitized or time of breeding season, or in terms of the sex ratio of eggs produced by individual females. Thus, this study provides another example in birds of little evidence for sex-ratio variation in relation to environmental factors.

Recent studies reveal that asymmetries of bilateral characteristics may reflect poor condition, serving as an honest indicator of mate quality. We asked whether bilateral asymmetry might also influence behavior during aggressive intrasexual contests. In the Common Chaffinch (Fringilla coelebs), bilateral, white wing patches (epaulettes) are used during aggressive interactions. We recorded responses of males to model intruders placed on territories of focal males, to determine whether their response to asymmetry varied with wing-patch size. When epaulettes were large, models with asymmetric patches elicited more rapid, directed aggression from the territory owners than did models with symmetric patches. We observed no such difference when model intruders had small epaulettes. If asymmetry of epaulettes in chaffinch males indicates poor condition, our results suggest higher costs of aggressive signaling for individuals in poor than good condition.