In the Northern Hemisphere, several avian cavity excavators (e.g., woodpeckers) orient their cavities increasingly toward the equator as latitude increases (i.e. farther north), and it is proposed that they do so to take advantage of incident solar radiation at their nests. If latitude is a key driver of cavity orientations globally, this pattern should extend to the Southern Hemisphere. Here, we test the prediction that cavities are oriented increasingly northward at higher (i.e. colder) latitudes in the Southern Hemisphere and describe the preferred entrance direction(s) of 1,501 cavities excavated by 25 avian species (n = 22 Picidae, 2 Trogonidae, 1 Furnariidae) across 12 terrestrial ecoregions (15°S to 55°S) in South America. We used Bayesian projected normal mixed-effects models for circular data to examine the influence of latitude, and potential confounding factors, on cavity orientation. Also, a probability model-selection procedure was used to simultaneously examine multiple orientation hypotheses in each ecoregion to explore underlying cavity-orientation patterns. Contrary to predictions, and patterns from the Northern Hemisphere, birds did not orient their cavities more toward the equator with increasing latitude, suggesting that latitude may not be an important underlying selective force shaping excavation behavior in South America. Moreover, unimodal cavity-entrance orientations were not frequent among the ecoregions analyzed (only in 4 ecoregions), whereas bimodal (in 5 ecoregions) or uniform (in 3 ecoregions) orientations were also present, although many of these patterns were not very clear. Our results highlight the need to include data from under-studied biotas and regions to improve inferences at macroecological scales. Furthermore, we suggest a re-analysis of Northern Hemisphere cavity orientation patterns using a multi-model approach, and a more comprehensive assessment of the role of environmental factors as drivers of cavity orientation at different spatial scales in both hemispheres.

Studying processes acting on differentiated populations upon secondary contact, such as hybridization, is important to comprehensively understand how species are formed and maintained over time. However, avian speciation studies in the tropical Andes have largely focused on the role of topographic and ecological barriers promoting divergence in allopatry, seldom examining hybridization and introgression. We describe a hybrid zone involving 2 closely related Andean warblers (Parulidae), the Golden-fronted Redstart (Myioborus ornatus), and the Spectacled Redstart (Myioborus melanocephalus). Geographic ranges of these species abut near the Colombia-Ecuador border and many specimens from the region exhibit intermediate phenotypes, but a formal description of phenotypic variation in the contact zone was heretofore lacking. We collected specimens across a transect encompassing the area where ranges abut and areas where only “pure” parental phenotypes of M. ornatus chrysops and M. melanocephalus ruficoronatus occur. We described variation in plumage traits including patterns of head and ventral coloration and tail markings based on 321 specimens. To describe genetic variation in the contact zone and over a broader phylogeographic context, we used sequences of the mitochondrial ND2 gene for 219 individuals across the transect and the entire range of both species, including all subspecies, from Venezuela to Bolivia. We documented a hybrid zone ∼200 km wide based on head coloration, where intermediate plumage phenotypes are most common and “pure” forms do not overlap geographically, consistent with extensive hybridization. Across the range of the M. ornatus–M. melanocephalus complex, mitochondrial genetic structure was shallow, with genetic breaks only coinciding clearly with one topographic feature. Such a low genetic structure is striking given the high diversity in plumage phenotypes and the current taxonomy of the group. Our phenotypic data suggest that barriers to hybridization are not strong, and allow us to postulate hypotheses to be tested using forthcoming genomic data.

Co-infection with microparasites (e.g., bacteria) and macroparasites (e.g., helminths) is often the natural state for wild animals. Despite evidence that gut helminths can bias immune responses away from inflammatory processes, few field studies have examined the role that helminths, or their potential interactions with internal microbial communities, play in modulating immunity in free-living, wild birds. Here, we used anthelmintic drugs to treat wild Song Sparrows (Melospiza melodia) for helminth infections and measured markers of systemic inflammation (heterothermia and locomotor activity) in response to an immune challenge with lipopolysaccharide (LPS), a cell wall component of gram-negative bacteria. Using birds from a population that previously showed high helminth prevalence, we monitored skin temperature and activity remotely using automated radio telemetry. We also collected cloacal swabs to determine whether drug treatment was associated with changes in the cloacal microbiota, and whether cloacal microbial community structure was associated with the severity of birds' immune responses. Because helminths can reduce the severity of inflammatory immune responses in other species, we predicted that in comparison with untreated control birds, anthelmintic-treated birds would be more lethargic and display higher fevers when challenged with LPS. Consistent with these predictions, anthelmintic-treated birds expressed higher fevers in response to immune challenge. However, all LPS-challenged birds decreased locomotor activity to a similar degree, regardless of anthelmintic treatment. Although several individual indicator bacterial taxa were strongly associated with anthelmintic treatment, this treatment did not alter overall bacterial alpha- and beta- diversity. Similarly, we did not find evidence that bacterial community diversity influenced the severity of immune responses to LPS. These results suggest that under field conditions, natural helminth infection can reduce the severity of songbirds' thermoregulatory responses (fever) during an immune challenge, without major impacts on internal microbial communities or behavioral responses to infection.

Animal culture changes over time through processes that include drift, immigration, selection, and innovation. Cultural change has been particularly well-studied for animal vocalizations, especially for the vocalizations of male animals in the temperate zone. Here we examine the cultural change in the vocalizations of tropical Rufous-and-white Wrens (Thryophilus rufalbus), quantifying temporal variation in song structure, song type diversity, and population-level distribution of song types in both males and females. We use data from 10 microsatellite loci to quantify patterns of immigration and neutral genetic differentiation over time, to investigate whether cultural diversity changes with rates of immigration. Based on 11 yr of data, we show that the spectro-temporal features of several widely-used persistent song types maintain a relatively high level of consistency for both males and females, whereas the distribution and frequency of particular song types change over time for both sexes. Males and females exhibit comparable levels of cultural diversity (i.e. the diversity of song types across the population), although females exhibit greater rates of cultural change over time. We found that female changes in cultural diversity increased when immigration is high, whereas male cultural diversity did not change with immigration. Our study is the first long-term study to explore cultural evolution for both male and female birds and suggests that cultural patterns exhibit notable differences between the sexes.

We explored adaptive factors affecting the preformative molt in the passerine family Cardinalidae, and concluded that the ancestor inhabited forest habitats and underwent a partial preformative molt that included wing coverts but not primaries. Later radiations within the family appeared to be characterized by transitions from forests toward more open habitats, and such transitions also correlated positively with increased preformative molt investment, plumage signaling, and flight. While previous studies had highlighted the role of time and energy constraints in the evolution of the preformative molt in passerines, we conclude that adaptation to the physical environment has had a greater influence than constraints on the evolution of this molt in Cardinalidae. Using molt data from 430 individuals from 41 of the 51 Cardinalidae species, we tested whether social and environmental factors (delayed plumage maturation and habitat openness), as well as that of physiological and time constraints (body mass, migration distance, and breeding latitude), have influenced the evolution of the preformative molt in this family. We predicted that these five factors could relate to the extent of the preformative molt in terms of energy investment, plumage signaling, and flight. We also examined whether or not the presence of an auxiliary preformative molt correlated to the extent of the preformative molt. We found a strong phylogenetic signal for the preformative molt in Cardinalidae due to shared ancestry, with habitat openness correlating significantly with increased molt extent. We also found a very strong relationship with the auxiliary preformative molt, indicating that both episodes form part of the early life molt strategy in this family. Evolutionary histories and the environments to which each taxonomic group has adapted can diverge across passerines, and we expect that future studies will further discover the nature and strength of the evolutionary drivers of preformative molt.

Eastern (Vireo gilvus gilvus) and western (V. g. swainsoni) forms of the Warbling Vireo have essentially allopatric breeding ranges across north-central North America, but come into contact in central Alberta, Canada. In 1986, Jon Barlow presented preliminary morphological and song evidence suggesting that the Warbling Vireo complex might comprise more than one valid species. However, to date, Barlow's suggestion is supported by only limited DNA evidence, demonstration of molt and migration differences between the taxa, and anecdotal accounts of differences in song, morphology, plumage, and ecology. We analyzed variation in both mitochondrial and nuclear DNA in birds from Alberta and surrounding areas to determine the levels of genetic differentiation and hybridization occurring in the contact zone, and whether the two taxa warrant recognition as separate biological species. Our analyses reveal that Warbling Vireos in Alberta and the surrounding areas are separated into two well-defined, genetically differentiated, and monophyletic clades corresponding to previously recognized taxonomic groups. The two taxa come into contact in a narrow (∼85 km) zone in Barrhead County, northwest of Edmonton, Alberta. They show evidence of limited hybridization. The distinct genetic differences are maintained in the contact zone, where individuals of the two taxa may occupy neighboring territories. Differences in spring arrival dates, molt schedules, and migration routes indicate that a migratory divide may play an important role in reproductive isolation. We suggest that the two taxa are distinct cryptic species: an eastern form, Vireo gilvus, and a western form, Vireo swainsoni.

Rapid high-intensity molt of flight feathers occurs in many bird species and can have several detrimental consequences, including reductions in flight capabilities, foraging performance, parental care, and plumage quality. Many migratory New World warblers (family Parulidae) are known to have intense remigial molt, and recent work has suggested that simultaneous replacement of the rectrices may be widespread in the family as well. However, the phylogenetic distribution of simultaneous rectrix molt, and high-intensity flight feather molt more generally, has not been systematically investigated in warblers. We addressed this issue by examining flight feather molt in 13 species, representing 7 different warbler genera, at Powdermill Avian Research Center in southwestern Pennsylvania, USA. All 13 species replaced their 12 rectrices simultaneously, with the onset of rectrix molt occurring in the early-middle stages of high-intensity primary molt. As expected, single-brooded early migrants molted earlier than double-brooded species whose nesting activities extend into late summer. However, our finding that late-molting species replaced their primaries more slowly and less intensively than early molting species was unexpected, as late-molting species are widely hypothesized to be under stronger migration-related time constraints. This surprising result appears to be at least partially explained by a positive association between the pace of molt and daylength; shorter late-summer days may mandate reduced daily food intake, lower molt intensity, and a slower pace of molt. In comparison to other passerines, flight feather molt in warblers of eastern North America is extraordinarily intense; at its peak, individuals are simultaneously replacing 50–67% of their 48 flight feathers (all 12 rectrices and 6–10 remiges on each wing) for 2–3 weeks or more. Because molt of this intensity is likely to present numerous challenges for flight, avoiding predators, foraging, and parental care, the period of flight feather molt for warblers constitutes a highly demanding phase of their annual cycle.

Sexual habitat segregation during the wintering period is a widespread phenomenon and has important implications for the ecology and conservation of migratory birds. We studied Black-and-white Warblers (Mniotilta varia) wintering in second-growth scrub and old-growth mangrove forest in Jamaica to quantify sexual habitat segregation and explore whether patterns of habitat occupation have consequences on physical condition. We used this information along with a body size analysis and simulated territorial intrusions to assess whether behavioral dominance or habitat specialization was responsible for habitat segregation. Based on standardized capture data, we found that females were more abundant than males in both scrub and mangrove forests. Foraging observations, however, suggested vertical segregation within each habitat, with females foraging primarily near the ground and males in the mid-canopy and canopy, indicating that our sex ratio estimates may be biased. Using 2 measures of body condition, we show that males were in better body condition than females, regardless of habitat. We found that males were on average slightly larger than females, and home range analysis and simulated territorial intrusions indicated that males were more territorial than females. We argue that the observed vertical sexual habitat segregation is likely caused by behavioral dominance rather than habitat specialization. Winter body condition is known to carry over to affect migration timing, reproductive success, and annual survival in other songbirds, and therefore sexual habitat segregation may have important implications for year-round population dynamics in Black-and-white Warblers.

The capacity for flight varies widely among bird species and influences their ecology, evolution, and conservation. Variation in vagility is influenced by behavioral responses to the nature of gaps between habitat elements as well as intrinsic characteristics of the species, particularly physiological traits influencing the physical capacity for sustained flight. Here, we briefly summarize the current state of knowledge revealing the wide variety of movement capacities of Neotropical birds. We then review current knowledge of avian muscle physiology and the role that muscle characteristics may play in influencing movement behavior. We argue that fundamental shifts in our understanding of avian muscle physiology and the influence of physiology on movement behavior remain to be elucidated, in part because knowledge from other vertebrates is being inappropriately applied to birds. In particular, critical evaluation of assumptions applied to birds from detailed studies of mammals is needed. Moving away from simple binary categorizations of avian flight muscles as “red vs. white” or “fast vs. slow” to characterize the cellular mechanisms and specific isoforms active at various life stages or seasons is also needed. An increasingly large number of avian species with a wide array of flight styles from hummingbirds to soaring raptors are appearing in GenBank, facilitating detailed physiological and evolutionary comparisons among species. Properly assessing the muscle physiological characteristics of Neotropical bird species with a wide array of movement capacities may improve our abilities to predict which species are most sensitive to landscape fragmentation and other factors that influence dispersal and migration.

The slow-paced life history of many Neotropical birds (e.g., high survival and low fecundity) is hypothesized to increase lifetime fitness through investments in self-maintenance over reproduction relative to their temperate counterparts. Molt is a key investment in self-maintenance and is readily shaped by environmental conditions. As such, variation in molt strategies may be a key mechanism underlying life-history trade-offs and adaptation to new environments. Here, we review molt strategies from a diversity of lowland Neotropical landbirds and examine how variation in molt strategies, characterized by differences in molt insertions, timing, extent, and duration contribute to life-history variation and adaptation to diverse ecological conditions. In addition to our synthesis, we present a case study to examine the relationship between home range size and duration of the definitive prebasic molt of a well-studied subset of Amazonian landbirds. Our results suggest a connection between prolonged molt duration and larger home range size of small-to-medium-sized Amazonian landbirds. Our aims were to identify key gaps in our knowledge of Neotropical bird molt, to stimulate further comparative studies into the evolution of molt strategies, and to highlight how variation in molt strategies may be a key mechanism underlying life-history variation across latitudes.

Army-ants (particularly swarm-raiding species Eciton burchellii and Labidus praedator) are keystone predators in Neotropical forests. Hundreds of associated species from diverse taxa depend on them for survival, the most conspicuous of which are the ant-following birds. These birds forage on cryptic arthropods and vertebrates as they attempt to escape raiding army-ants. Despite capturing the attention of tropical biologists for centuries, research on ant-following birds has been largely limited to natural history observations until more recently. Here, we argue that this complex system provides unique and underappreciated opportunities for testing hypotheses in 5 main areas of interest: (1) competitive interactions among attendant birds, (2) cognitive and behavioral adaptations for locating swarms, (3) evolution of ant-following behavior, (4) biogeography of ant-following behavior, and (5) conservation of ant-following birds. For each research area, we review the current state of knowledge and make suggestions for fruitful research avenues that we believe will help address important questions in the fields of ecology, evolution, and behavior.