Documentation of spatial genetic discordance among breeding populations of Arctic-nesting avian species is important, because anthropogenic change is altering environmental linkages at micro- and macrogeographic scales. We estimated levels of population subdivision within Pacific Common Eiders (Somateria mollissima v-nigrum) breeding on 12 barrier islands in the western Beaufort Sea, Alaska, using molecular markers and capture—mark—recapture (CMR) data. Common Eider populations were genetically structured on a microgeographic scale. Regional comparisons between populations breeding on island groups separated by 90 km (Mikkelsen Bay and Simpson Lagoon) revealed structuring at 14 microsatellite loci (F_ST = 0.004, P < 0.01), a nuclear intron (F_ST = 0.022, P = 0.02), and mitochondrial DNA (Φ_ST = 0.082, P < 0.05). The CMR data (n = 34) did not indicate female dispersal between island groups. Concordance between genetic and CMR data indicates that females breeding in the western Beaufort Sea are strongly philopatric to island groups rather than to a particular island. Despite the apparent high site fidelity of females, coalescence-based models of gene flow suggest that asymmetrical western dispersal occurs between island groups and is likely mediated by Mikkelsen Bay females stopping early on spring migration at Simpson Lagoon to breed. Alternatively, late-arriving females may be predisposed to nest in Simpson Lagoon because of the greater availability and wider distribution of nesting habitat. Our results indicate that genetic discontinuities, mediated by female philopatry, can exist at microgeographic scales along established migratory corridors.

We evaluated the influence of weather on reproduction of the Crested Caracara (Caracara cheriway) in an agricultural landscape in south-central Florida. We used a mixed logistic-regression modeling approach within an information-theoretic framework to examine the influence of total rainfall, rainfall frequency, and temperature on the number of breeding pairs, timing of breeding, nest success, and productivity of Crested Caracaras during 1994–2000. The best models indicated an influence of rainfall frequency and laying period on reproduction. More individuals nested and more pairs nested earlier during years with more frequent rainfall in late summer and early fall. Pairs that nested later in each breeding season had smaller clutches, lower nest success and productivity, and higher probability of nest failure. More frequent rainfall during early spring months that are usually characterized by water deficit (March–May), more frequent rainfall during the fall drawdown period (September–November), and a shorter winter dry period showed some association with higher probability of brood reduction and lower nest success. The proportion of nests that failed was higher in “wet” years, when total rainfall during the breeding season (September–April) was >10% above the 20-year average. Rainfall may influence reproduction in Crested Caracaras indirectly through food resources. As total rainfall increased during February–April, when most pairs are feeding nestlings or dependent fledglings, the proportion of drawdown-dependent species (those that become available as rainfall decreases and wetlands become isolated and shallow) in the diet of Crested Caracaras declined, which may indicate reduced availability of foraging habitat for this primarily terrestrial raptor.

The Rufous-naped Wren (Campylorhynchus rufinucha) is a sedentary, morphologically variable species distributed in the dry forests of Mesoamerica. It ranges from Colima, Mexico, south to Costa Rica along the Pacific slope, with a disjunct population in central Veracruz. Populations of two forms on the Pacific slope intergrade in Chiapas, Mexico, apparently as a result of secondary contact. We sequenced a mitochondrial DNA (mtDNA) gene to explore phylogeographic patterns and hybridization. We found three divergent lineages, two geographically spanning the Isthmus of Tehuantepec and a disjunct Veracruz population. Analyses of molecular variation and φ; statistics are consistent with genetically distinct populations. Morphological and behavioral evidence from other studies is consistent with the existence of these three independent evolutionary lineages. However, the geographic distribution of haplotypes suggests mtDNA introgression east of the isthmus. Our data suggest that this secondary contact could be explained by population expansions. We recommend recognizing three species, two of which hybridize in a narrow contact zone.

Determining the sex of a bird on the basis of its morphological characteristics is difficult in >50% of avian species; in nestlings, this rate is even higher. Given that male birds have two identical sex chromosomes (ZZ), whereas females are heterogametic (ZW), the development of a DNA-based sexing method has been widely investigated in several species. Griffiths et al. (1998) reported a simple DNA test using intronic size variation; however, this method cannot differentiate birds that have small or nonexistent size variations. We report a simple and novel DNA test for avian sex determination using a novel primer (p0) specific for the unique sequence of the chromohelicase-DNA-binding (CHD)-W gene. Use of a multiplex polymerase chain reaction (PCR) and simple agarose-gel electrophoresis with p0 and with the primers P2 and P8 described by Griffiths et al. (1998) revealed that the P0–P2 combination could amplify the partial CHD-W gene alone. Consequently, we found that p0 could be used to identify only the CHD-W gene. We anticipate that this multiplex PCR will be useful for universal avian sexing, regardless of the intronic size variation.

Philopatry has long been assumed to structure populations of waterfowl and other species of birds genetically, especially via maternally transmitted mitochondrial DNA (mtDNA), yet other migratory behaviors and nesting ecology (use of ground vs. cavity sites) may also contribute to population genetic structure. We investigated the effects of migration and nesting ecology on the population genetic structure of two Holarctic waterfowl, the Red-breasted Merganser (Mergus serrator) and Common Merganser (M. merganser), using mtDNA control-region sequence data. Red-breasted Mergansers (a ground-nesting species) exhibited lower levels of population differentiation across their North American range, possibly as a result of post-Pleistocene range expansion and population growth. By contrast, Common Mergansers (a cavity-nesting species) breeding in western and eastern North America were strongly differentiated, as were continental groups in North America and Europe. Our hypothesis that population differentiation of breeding female Common Mergansers results from limited migration during non-breeding periods was not supported, in that equally heterogeneous mtDNA lineages were observed in males and females on several wintering areas. The interspecific differences in mtDNA patterns for these two closely related species may have resulted from factors related to nesting ecology (ground vs. cavity nesting) and responses to historical climate change.

Although critical to habitat and population management, the proximate cues that birds use to establish territories are largely unknown. Understanding these cues is important for birds, such as many grassland birds, that exhibit high annual variability in population density and make new habitat-selection decisions annually. Identifying the actual cues used is difficult in the field, but the factors associated with the arrival densities of birds can help uncover variables that are involved in or correlated with cues used for selection. During the summers of 2002–2004, we investigated how weather and local vegetation factors were related to arrival densities of Grasshopper Sparrows (Ammodramus savannarum) and Baird's Sparrows (A. bairdii) at three locations across North Dakota and Saskatchewan. Spring densities of Grasshopper Sparrows were positively correlated with concurrent May precipitation, whereas densities of Baird's Sparrows were negatively correlated with the previous winter's snowfall. We used a model-selection approach to evaluate the vegetation characteristics associated with arrival densities of birds. Grasshopper Sparrow densities showed a strong negative relationship to woody cover, and Baird's Sparrow densities showed a negative relationship to vegetation height and vegetation density near the ground. Our results provide a first detailed look at habitat and weather associations immediately after arrival in spring and an important first step in uncovering factors that may be involved in habitat selection in two grassland species.

Climatic changes induced by the El Niño—Southern Oscillation (ENSO) commonly influence biological systems; however, climatic variability and multitrophic interactions within tropical latitudes remain poorly understood. We examined relationships between migrant condition and ENSO during spring migration in Costa Rica. Our study is based on correlating an ENSO index with body-condition metrics of six Nearctic—Neotropic migrant bird species during stopover in Costa Rica over a 10-year period. This study is the first to report correlations between El Niño and changes in condition of multiple migrant species. The condition of one insectivore improved, whereas two frugivores experienced reduced condition during El Niño. Our detected correlations suggest that the ENSO cycle may influence changes in migrant mass; such interactions may ultimately affect demographics of certain frugivorous and insectivorous migrant species. Potential future management considerations are discussed in light of global climate change and our findings.

Fluctuating asymmetry (FA), small random deviation from bilateral symmetry, often increases with stress during development. Common Terns (Sterna hirundo) typically lay two to three eggs that hatch asynchronously. I predicted that C-chicks (last of three) should have greater FA than A- and B-chicks at hatching and that FA should be higher in chicks from smaller clutches, because of differences in parental quality. Tarsus length of newly hatched chicks was measured across three years, and middle toe length was measured in one year. Sample sizes exceeded 100 chicks in two of three years. Variation in tarsus FA with hatching order and clutch size was statistically significant in one year (P < 0.01) and nearly so in another (P < 0.10). No significant differences were present for toe FA. A-chicks from three-egg clutches appeared to have the lowest tarsus FA among categories of chicks in both years, and in one year they were significantly more symmetrical than B- and C-chicks from three-egg clutches. As predicted, A-chicks from three-egg clutches were also more symmetrical than A-chicks from two-egg clutches and singletons. However, C-chicks did not differ significantly from B-chicks in tarsus FA. Fluctuating asymmetry also varied with hatching date, but no clear pattern emerged. Fluctuating asymmetry was not associated with trait size or body mass, although there was significant variation in body mass and toe size among groups, C-chicks being relatively small and A-chicks and singletons relatively large.

We recorded and analyzed the songs of 23 male House Finches (Carpodacus mexicanus) at six sites in Sioux County in far northwestern Iowa. We compiled a catalogue of all the syllables detected in the population and examined the extent of syllable sharing and song sharing among individuals both within sites and between sites. Unlike in a population of House Finches studied in Colorado, we found no significant difference between the level of syllable sharing within sites and between sites and no relationship between the extent of syllable sharing and distance between birds. Over 67% of the House Finch syllables recorded previously in eastern Iowa were found in Sioux County, and all House Finches in Sioux County shared two major song themes that were similar to themes found in eastern Iowa. Our findings suggest that the House Finches in Sioux County originated from eastern populations of the species, corroborating the colonization patterns that have been inferred from Breeding Bird Surveys and Christmas Bird Counts.

Food-supplementation studies on birds during egg laying have generally manipulated macronutrients and energy. Such studies have shown variable effects on egg size in American Coots (Fulica americana). We determined the relative influences of local environment, food quantity, and food quality on egg size in American Coots by supplementing food and carotenoids at three sites in Saskatchewan. Eggs were collected and analyzed for carotenoid content and stable isotopes (δ¹⁵N and δ¹³C) in the yolk to determine whether variation in the type of food eaten contributes to egg size. We also report on the isotopic analysis of American Coot tissues and their eggs to assess evidence of endogenous versus exogenous protein and lipid allocations to reproduction. We provide isotopic evidence that American Coots used endogenous lipid, but not protein, reserves for egg formation and that egg size is more dependent on exogenous sources of nutrients. Laying sequence was the only variable across nesting locales that had a significant influence on the size of eggs. There was an effect of provisioning food at only one site; however, this response was driven by a component of diet quality other than carotenoids. Yolk δ¹³C values were positively correlated with egg volume, which further suggests the importance of diet quality in determining egg size in American Coots.

Many shorebird species have widespread geographic distributions comprising several continents. Because shorebirds are excellent flyers and can migrate large distances, it is often unclear whether reproductive barriers between subspecies and populations from different continents exist. Kentish-Snowy Plovers (Charadrius alexandrinus) are cosmopolitan shorebirds. Whether the American and Eurasian subspecies-Snowy Plover and Kentish Plover, respectively—constitute a single species is the subject of a longstanding debate. We examined the divergence between American and Eurasian populations to reassess the current taxonomy by comparing genetic and phenotypic characters of the American subspecies C. a. nivosus and the Eurasian subspecies C. a. alexandrinus from seven populations. Genetic analyses revealed that American and Eurasian populations have strongly diverged, the Kentish Plover being more closely related to the White-fronted Plover (C. marginatus) than to the Snowy Plover. These results were consistent across all assessed nuclear markers (26 microsatellites and a partial CHD sequence) and two mitochondrial markers (ND3 and ATPase 6/8). Within subspecies, populations sampled across large geographic distances were not genetically differentiated (all F_st ≤ 0.01 and all Φ_st ≤ 0.06), which suggests panmixia. Snowy Plovers differed morphologically from Kentish Plovers, having significantly shorter tarsi and wings. Chick plumage and calls also may serve as diagnostic characters to distinguish Snowy and Kentish plovers, although more data are needed to quantify these differences. Our combined results suggest that the taxonomic status of C. alexandrinus needs to be revised, and we propose that Kentish Plover and Snowy Plover be recognized as separate species: C. alexandrinus and C. nivosus, respectively.

Researchers commonly collect blood samples from wild birds, and most workers assume that blood sampling has no adverse effect on the birds' survival. Few studies, however, have done controlled comparisons among bled and non-bled individuals and estimated survival using modern statistical methodology. We used a data set on Cliff Swallows (Petrochelidon pyrrhonota) that included 2,945 bled and 7,822 non-bled birds captured at the same times and sites in southwestern Nebraska from 1986 to 2006 to estimate annual survival and recapture probabilities of each group. Blood was collected with brachial venipuncture in amounts varying from 0.3% to 1.2% of the birds' body mass. Apparent survival of bled birds was lower than that of non-bled birds: bled birds experienced a 21–33% reduction in average survivorship, depending on amount of blood taken and whether the individuals were resident at a fumigated (parasite-free) or non-fumigated colony at the time of sampling. The percent reduction in annual survival was higher for individuals at non-fumigated colonies. All effects of blood sampling applied only in the year after sampling, and there were no effects in later years. Our results suggest that brachial blood sampling is not a benign technique. Researchers following the 1%-of-body-mass guideline may be collecting too much blood from wild birds, especially when research requires repeated samples over short periods

The fusion of two or more thoracic vertebrae, independent of the synsacrum, is more widespread in the Passeriformes than has previously been reported. The bone thus formed is known as a “notarium.” I surveyed oscine passerine skeletons and found a notarium with fully fused vertebrae in Chabert's Vanga (Leptopterus chabert), certain woodswallows (Artamidae) and shrikes (Laniidae), the Willie Wagtail (Rhipiduridae, Rhipidura leucophrys), the Phainopepla (Bombycillidae, Phainopepla nitens), the penduline tits (Remizidae) including the Verdin (Auriparus flaviceps), various larks (Alaudidae), the Common Starling (Sturnus vulgaris), the sickle-billed thrashers (Toxostoma spp.), and the crossbills (Loxia spp.). Mapping of character evolution on a supertree suggests that a fully fused notarium has evolved independently at least 12 times in the oscine passerines and that notaria with less extensive fusion of the vertebrae (only the spinous processes fused, for example) are even more widespread phylogenetically. Phenotypic expression of a notarium is fixed in some species and higher taxonomic groups but varies within the species in others. Ontogenetically, the fully fused notarium forms when the bird is immature. The evolutionary development of notaria probably depends on mutations that alter expression patterns of transcription genes (Pax and Hox genes are likely candidates) that control embryological differentiation of the vertebrae. Among the systematic implications of this study are additional support for placement of the Verdin in the Remizidae and for the monophyly of a group of western thrashers (Toxostoma spp.) with strongly decurved bills.

The “spring condition” hypothesis (SCH) states that nutrition during spring migration affects survival, reproductive success, and, ultimately, population size of migratory birds. The North American population of Lesser Scaup (Aythya affinis) has experienced a marked decline, apparently because of poor recruitment. An important prediction of the SCH is that female Lesser Scaup have low lipid reserves during spring migration. We previously reported that lipid reserves and body mass of females collected on migratory stopover areas in northwestern Minnesota in springs 2000–2001 were lower than those on the same areas in the 1980s and markedly lower than those collected at Pool 19 of the Mississippi River in 2000–2001, an important preceding stopover area. However, it was unclear whether these findings represented a site-specific result or a landscape-scale phenomenon. Accordingly, we examined lipid and body mass of 641 female Lesser Scaup migrating across seven eco-physiographic regions of Iowa, Minnesota, and North Dakota during springs 2003–2005. We found that lipids and body mass of females throughout the Upper Midwest were similar to or less than the low values documented in northwestern Minnesota in springs 2000–2001 and markedly lower than those of females at Pool 19 in springs 2000–2001. Accordingly, our results are consistent with a prediction of the SCH, because lipid and body mass of females are low throughout this large landscape, lower than at an important preceding stopover area, and lower than all historical values. Finally, our results suggest the potential for cross-seasonal influences of nutrition on recruitment and that a stronger management focus on spring migration habitats may be necessary for conservation and recovery of declining migratory birds, especially Lesser Scaup.

Phenotypic diversity is essential to the evolutionary process. We used a partial cross-foster design to investigate sources of variation in a component of immunity and three morphological traits in nestling House Sparrows (Passer domesticus). We found substantial differences in the causes of variation across traits. Most traits were influenced by date in the season; nestlings hatching later in the year were less developed, were smaller, and exhibited a lower immune response at age 10 days than nestlings hatched early in the season. Variation in rearing environment had significant effects on condition, growth, and immune response, but this could not be attributed to variation in provisioning rate. The number of offspring reared together had a negative effect on condition, but no effect on immune response. Only immune response showed significant origin-related variation. Interestingly, offspring sex had several complex effects. Males showed greater origin-related and less rearing-related variance in immune response than females. Males also were in better body condition but showed significantly more sensitivity to variation in rearing environment than females. Females were more advanced in feather development than males. These results differ from most studies of heritability in adult birds and suggest that selection may contribute to differences in heritability by changing other nongenetic components of phenotypic variation. This has direct and indirect consequences for evolution.

Breeding populations of grassland birds are declining across North America. Alteration of wintering-ground habitat in Mexico may be a significant causal component in these declines, considering the rapid changes in land use occurring there. We investigated the influence of precipitation, habitat structure, and plant community on the abundance and distribution of grassland passerines in the state of Chihuahua, Mexico. We conducted bird and vegetation surveys during 1998–2007 in 65 plots at two study areas 75 km apart, Sueco and Tinaja Verde. Grassland guild species were the dominant component of the wintering bird community at both study areas. Passerine density at Sueco during winter was directly related to cumulative rainfall from the preceding December to September, a factor known to be strongly correlated with grass production. Herbaceous and woody vegetation structure and habitat type influenced occurrence of migratory grassland birds at the plot level. Nine of 11 study species occurred in a greater proportion of plots at Sueco than at Tinaja Verde, even though grasslands at the latter area were apparently in better condition. Whereas we may have overlooked key factors differentiating habitat quantity in the two study areas, some of the evidence suggests that Sueco is on a migratory route and Tinaja Verde is not. The future of desert grasslands in Chihuahua is in peril because of the ongoing wide-scale conversion to agriculture there and climate-change predictions of lower rainfall in coming decades.

In altricial birds with biparental care, the pattern of food distribution within a brood is a consequence of conflicts between family members. To optimize their investment, parents may feed nestlings according to at least two different types of cues: begging behavior of nestlings and nestling characteristics such as size or sex. We investigated the begging displays of nestlings and the feeding response of parents in 12 broods of the Green-backed Tit (Parus monticolus) by video recording. Male and female parents provisioned nestlings at similar rates and with similar prey loads and often used highly predictable, consistent feeding locations across trips and on different days. Furthermore, both sexes chose separate locations in 7 of the 12 broods. Nestlings that begged more intensely and those that occupied positions closer to the provisioning parent had a higher probability of receiving food from both parents. Begging intensity and positioning did not differ with respect to nestling size. Moreover, no significant sexual dimorphism was detected among the nestlings. When controlling statistically for nestling behavior, we found that male and female parents differed in their distribution of food to different-sized nestlings: males preferentially provided food for the largest and medium-sized nestlings, whereas females did not prefer nestlings of a particular size. These results suggest that the food-distribution pattern in Green-backed Tits is determined not only by nestling behavior but also by parentally biased favoritism toward nestling size.