Thryothorus Research Articles

Nest-Site Limitation of Secondary Cavity-Nesting Birds in Even-Age Southern Pine Forests

I used a randomized, replicated, controlled study design to test the hypothesis that nest-site availability limits breeding densities of secondary cavity nesting species in even-age southern pine (Pinus spp.) forests. Breeding densities of secondary cavity nesters increased significantly on treated plots after nest boxes were introduced. Total number of nesting attempts also increased several fold post-treatment. These data indicate nest-site availability was a limiting factor for breeding densities of secondary cavity nesting species. The response of individual species to nest boxes ranged from moderately high (Great Crested Flycatcher [Myiarchus crinitus]), to low (Tufted Titmouse [Baeolophus bicolor], Eastern Bluebird [Sialia sialis]) to no response (Carolina Wren [Thryothorus ludovicianus]). At least three factors accounted for interspecific differences in this study: different levels of reliance on cavities excavated in snags, different body sizes, and differences in local population densities. The large number of unoccupied nest boxes (only 9% were used for nesting) suggests secondary cavity nesters were not limited solely by cavity availability but also by habitat quality. Prescribed burning appeared to facilitate discovery and use of nest boxes by birds in this study, consistent with the hypothesis that nest-site limitation is mitigated by habitat structure.

Birdsong Differs between Mercury-polluted and Reference Sites

Mercury is a toxic heavy metal that can cause obvious physiological and reproductive problems in animals. Very little is known, however, about its subtle behavioral effects. We examined whether birds that inhabited mercury-contaminated sites exhibited differences in singing behavior compared with birds at uncontaminated reference sites nearby. We recorded the songs of 3 oscines, the Carolina Wren (Thryothorus ludovicianus), House Wren (Troglodytes aedon), and Song Sparrow (Melospiza melodia), and 1 suboscine, the Eastern Phoebe (Sayornis phoebe). Spectrographic analysis revealed that songs of oscines living on contaminated sites contained a lower diversity of note types and were sung at lower tonal frequencies than songs of birds on reference sites. Additionally, both species of wren tended to sing shorter songs. By contrast, the songs of Eastern Phoebes did not differ between contaminated and reference sites, suggesting that mercury may affect singing behavior only in species that learn their songs. Such alterations in song could have important implications for the fitness of songbirds in polluted areas. Our results highlight the importance of considering behaviors in evaluations of contaminant effects.

Host Feeding Patterns of Potential Vectors of Eastern Equine Encephalitis Virus at an Epizootic Focus in Tennessee

In 2006, 2,817 blood-fed mosquitoes were collected from the site of a 2005 eastern equine encephalitis outbreak in Chester County, TN. Using a polymerase chain reaction-based assay, 264 vertebrate hosts were identified from seven mosquito species. Culex erraticus and Cx. nigripalpus fed on a diversity of mammalian, avian, and reptilian hosts, whereas Anopheles quadrimaculatus and An. punctipennis were predominantly mammalophagic. Overall, 27% of Cx. nigripalpus, 16% of Cx. erraticus, and 7% of An. quadrimaculatus blood meals were acquired from avian hosts. No avian-derived blood meals were identified from An. punctipennis. The house finch, Carolina wren, and mourning dove were the most commonly identified avian host species. By incorporating this study with flight range, vector competence, and virus field isolation data, we assessed certain aspects of the enzootic and epizootic vectorial capacity of the mosquito species present at this outbreak site.

Nest defense by Carolina Wrens

We examined nest defense behavior of the Carolina Wren (Thryothorus ludovicianus) in response to avian intruders at the nest. Freeze-dried mounts of a brood parasite (female Brown-headed Cowbird [Molothrus ater]), nest predator (Blue Jay [Cyanocitta cristata]), and a control (Swainson's Thrush [Catharus ustulatus]) were presented at wren nests to examine if wrens were able to distinguish among these threats. The primary nest defense behavior of Carolina Wrens was alarm calls. Wrens spent more time alarm calling to the Blue Jay model than the control and Brown-headed Cowbird models suggesting cowbirds were not recognized as threats to the nest. Wrens were less likely to respond during laying stage trials, probably due to a lack of nest visitations at this time. Intensity of alarm calls did not increase from the laying to nestling stage for any of the models presented. Carolina Wrens are both socially and genetically monogamous, and males should invest heavily in care of young due to their high confidence of paternity. Males and females did not differ in the amount of time spent alarm calling to any of the models, and defended their nests from intruders with equal intensity as predicted by the confidence of paternity hypothesis.

Building Hierarchical Models of Avian Distributions for the State of Georgia

Abstract To predict the distributions of breeding birds in the state of Georgia, USA, we built hierarchical models consisting of 4 levels of nested mapping units of decreasing area: 90,000 ha, 3,600 ha, 144 ha, and 5.76 ha. We used the Partners in Flight database of point counts to generate presence and absence data at locations across the state of Georgia for 9 avian species: Acadian flycatcher (Empidonax virescens), brown‐headed nuthatch (Sitta pusilla), Carolina wren (Thryothorus ludovicianus), indigo bunting (Passerina cyanea), northern cardinal (Cardinalis cardinalis), prairie warbler (Dendroica discolor), yellow‐billed cuckoo (Coccyzus americanus), white‐eyed vireo (Vireo griseus), and wood thrush (Hylocichla mustelina). At each location, we estimated hierarchical‐level‐specific habitat measurements using the Georgia GAP Analysis18 class land cover and other Geographic Information System sources. We created candidate, species‐specific occupancy models based on previously reported relationships, and fit these using Markov chain Monte Carlo procedures implemented in OpenBugs. We then created a confidence model set for each species based on Akaike's Information Criterion. We found hierarchical habitat relationships for all species. Three‐fold cross‐validation estimates of model accuracy indicated an average overall correct classification rate of 60.5%. Comparisons with existing Georgia GAP Analysis models indicated that our models were more accurate overall. Our results provide guidance to wildlife scientists and managers seeking predict avian occurrence as a function of local and landscape‐level habitat attributes.

SEASONAL COMPONENTS OF AVIAN POPULATION CHANGE: JOINT ANALYSIS OF TWO LARGE-SCALE MONITORING PROGRAMS

We present a combined analysis of data from two large-scale surveys of bird populations. The North American Breeding Bird Survey is conducted each summer; the Christmas Bird Count is conducted in early winter. The temporal staggering of these surveys allows investigation of seasonal components of population change, which we illustrate with an examination of the effects of severe winters on the Carolina Wren (Thryothorus ludovicianus). Our analysis uses a hierarchical log-linear model with controls for survey-specific sampling covariates. Temporal change in population size is modeled seasonally, with covariates for winter severity. Overall, the winter-spring seasons are associated with 82% of the total population variation for Carolina Wrens, and an additional day of snow cover during winter-spring is associated with an incremental decline of 1.1% of the population.

Pair Roosting of Nesting Carolina Wrens (Thryothorus ludovicianus)

Abstract Carolina Wrens (Thryothorus ludovicianus), which maintain lifetime pair bonds and year-round territories, huddle in pair or communal roosts during the non-breeding season, particularly during cold winter nights. Pair roosting during the nesting season, however, is not known to occur. Here, we report huddled pair roosting by Carolina Wrens in Florida. The dates of pair roosting took place during nest construction through laying of the first egg (9–20 March 2004), and also on the date the fourth egg was laid in a clutch of five (24 March). The wrens roosted in a hanging flower basket located 2.4 m from their nest site. Although huddled pair roosting by wrens during periods of low ambient temperatures in the non-breeding season likely achieves thermal conservation, the benefits derived during the breeding season remain unclear. We discuss the possible thermoregulatory and pair-bond maintenance functions of pair roosting.

Carolina Wren Nest Successfully Parasitized by House Finch

We report the first observation of a House Finch (Carpodacus mexicanus) successfully parasitizing a Carolina Wren (Thryothorus ludovicianus) nest. On 24 May 2005, we found a Carolina Wren nest in south-central Oklahoma containing four Carolina Wren eggs and two House Finch eggs. The House Finch eggs hatched and nestlings grew rapidly. The Carolina Wren eggs hatched but the young did not survive. We observed a House Finch fledgling with the adult Carolina Wrens the day after fledging.

Combining Breeding Bird Survey and Distance Sampling to Estimate Density of Migrant and Breeding Birds

Abstract We combined Breeding Bird Survey point count protocol and distance sampling to survey spring migrant and breeding birds in Vicksburg National Military Park on 33 days between March and June of 2003 and 2004. For 26 of 106 detected species, we used program DISTANCE to estimate detection probabilities and densities from 660 3-min point counts in which detections were recorded within four distance annuli. For most species, estimates of detection probability, and thereby density estimates, were improved through incorporation of the proportion of forest cover at point count locations as a covariate. Our results suggest Breeding Bird Surveys would benefit from the use of distance sampling and a quantitative characterization of habitat at point count locations. During spring migration, we estimated that the most common migrant species accounted for a population of 5000–9000 birds in Vicksburg National Military Park (636 ha). Species with average populations of >300 individuals during migration were: Blue-gray Gnatcatcher (Polioptila caerulea), Cedar Waxwing (Bombycilla cedrorum), White-eyed Vireo (Vireo griseus), Indigo Bunting (Passerina cyanea), and Ruby-crowned Kinglet (Regulus calendula). Of 56 species that bred in Vicksburg National Military Park, we estimated that the most common 18 species accounted for >8150 individuals. The six most abundant breeding species, Blue-gray Gnatcatcher, White-eyed Vireo, Summer Tanager (Piranga rubra), Northern Cardinal (Cardinalis cardinalis), Carolina Wren (Thryothorus ludovicianus), and Brown-headed Cowbird (Molothrus ater), accounted for >5800 individuals.

Sexual size dimorphism and assortative mating in Carolina Wrens

Sexual size dimorphism (SSD) may be due to sexual and natural selection, but identifying specific mechanisms that generate such dimorphism in a species is difficult. I examined SSD in Carolina Wrens (Thryothorus ludovicianus) by examining (1) the degree of SSD in the population and between pairs using five morphometrics, (2) assortative mating patterns based on size and age, and (3) relationships between size and longevity. Analysis revealed that males were significantly larger than females in all body measurements. For example, mass, bill, and wing measurements yielded a canonical variable that permitted separation of the sexes and linear classification functions correctly determined the sex of 95% (238/250) of all wrens measured. No evidence was found to suggest that SSD was related to resource partitioning. However, assortative mating trends based on morphometrics (e.g., wing length), positive associations between longevity and morphometrics (e.g., wing length in females and body size in males), and intense male-male contests for territorial resources year-round provide evidence that sexual selection may contribute to SSD in Carolina Wrens.

Molecular data delineate four genera of “ Thryothorus” wrens

Wrens of the genus Thryothorus comprise over a third of the species diversity in the family Troglodytidae. In addition to this species diversity, these wrens vary in a number of behavioral characteristics, in particular in the presence and structure of vocal duets, which makes them an interesting target for comparative evolutionary ecological and behavioral study. However, no phylogenetic hypothesis for this group—which would provide a sound basis for comparative analysis—is currently available. While previous molecular phylogenetic work established conclusively that the type of this genus, Thryothorus ludovicianus (Latham), was not part of a monophyletic group with other Thryothorus, the exact limits of the genus could not be established due to limited taxon sampling. Here, we present molecular data from all but four currently recognized species of Thryothorus. These data confirm that Thryothorus is paraphyletic, and that the type T. ludovicianus does not form a monophyletic group with any other member of the genus. Based on analyses of our data, we resurrect two previously recognized wren genera, Pheugopedius and Thryophilus, and erect a new genus— Cantorchilus—to house the remaining ex- Thryothorus species. Our hypothesis of relationships will provide a firm basis for future behavioral and morphological analyses of these species.

Seasonal Variation in Response to Neighbors and Strangers by a Territorial Songbird

AbstractReduced aggression toward territorial neighbors, termed the ‘dear‐enemy’ effect, is thought to arise because territorial animals benefit by avoiding contests with neighbors with whom they have already established relationships. The dear‐enemy effect has been described in many taxa, but few studies have considered whether or not neighbors’ relationships are affected by changes in the social environment. In this study, I tested whether Carolina wrens, Thryothorus ludovicianus, behaviorally discriminate between neighbors and strangers in two different social environments: in spring when territories have been established for several months, and in fall when an influx of new birds claiming territories might de‐stabilize wren neighborhoods. Comparisons of responses of territorial males to playbacks of songs from neighbors and strangers showed that Carolina wrens show the dear‐enemy effect in spring, but not in fall in this design. The apparent lack of a differential response to neighbors and strangers in fall might be due to a reduction in aggression toward strangers. This study provides evidence that seasonal changes in the dear‐enemy effect coincide with seasonal changes in the social environment.

An interspecific comparison using immunofluorescence reveals that synapse density in the avian song system is related to sex but not to male song repertoire size

Immunofluorescent labeling of synaptic vesicle protein 2 (SV2) and confocal microscopy were employed to assess the role of synapse density in the functioning of the avian song system. Synapse density in premotor nuclei HVC and RA was measured, in both sexes of two species characterized by male-only singing behavior: the zebra finch Taeniopygia guttata, which sings a single, stereotyped song, and the Carolina wren Thryothorus ludovicianus, which sings a large repertoire of different songs. Multiple levels of analyses demonstrate overall similarity of synapse density between nuclei HVC and RA, suggesting that synapse density is regulated uniformly across these regions within individuals. Male zebra finches and male Carolina wrens have equivalent synapse densities, suggesting a common pattern of masculinized development despite dramatic behavioral differences. Female Carolina wrens have synaptic density similar to that of males of both species, while female zebra finches exhibit greater synaptic densities in both regions than do male zebra finches or both sexes of wrens. Prior reports implicate testosterone as a regulator of synapse density in this system; sex differences in circulating or neural testosterone may contribute to the sexual dimorphism of synapse density observed here. Interspecific comparison of song system synapse density in nonsinging females suggests that synapse density in female songbirds may be a particularly labile trait.

32 YEARS OF CHANGES IN PASSERINE NUMBERS DURING SPRING AND FALL MIGRATIONS IN COASTAL MASSACHUSETTS

Using standardized mist-net captures collected over a 32-year period (1970–2001), we examined changes in the capture rates of passerines recorded in coastal Massachusetts during fall (78 species) and spring (72 species) migration. Capture rates of 45 species of fall migrants (58%) declined significantly between early (1970–1985) and late (1986–2001) years of the study; 36 species of spring migrants (50%) showed significant declines. Only Carolina Wren (Thryothorus ludovicianus), Tufted Titmouse (Baeolophus bicolor), Northern Cardinal (Cardinalis cardinalis), and Orchard Oriole (Icterus spurius) showed significant increases during spring migration; fall sampling indicated that Carolina Wren, Tufted Titmouse, Black-throated Blue Warbler (Dendroica caerulescens), and Northern Cardinal had significantly higher capture rates. Of 37 species included in the migration monitoring data but not reliably represented by Breeding Bird Survey (BBS) data in any of the northeastern physiographic strata, 23 (62%) showed significant declines at Manomet during at least one of the two migration periods. There were significant correlations in percent changes in migrant capture rates between fall and spring. BBS trends reported from the southern New England and northern New England physiographic strata were correlated with changes in migrant capture rates. However, there were also inconsistencies between results obtained by the two monitoring approaches, suggesting that factors in addition to actual changes in breeding populations may be reflected in the migration capture data.

Differential responses of yellow-breasted chats, Icteria virens, to male and female conspecific model presentations

A detailed understanding of the behavioural interactions between males and females is crucial for elucidating the selection pressures shaping mating system evolution; however, these interactions are often difficult to observe, particularly in free-living populations. We simulated extraterritorial intrusions by presenting conspecific models on to territories of a Neotropical migratory passerine, the yellow-breasted chat, Icteria virens. Simulated intrusions elicited different responses based on the sex of the focal individual and the sex and species of the model. Behavioural responses of the focal individuals were largely directed towards conspecific models, whereas simulated heterospecific intrusions, using a Carolina wren, Thryothorus ludovicianus, model, elicited minimal response. Male song was not related to any particular model presentation. Males directed mating behaviours, including courtship displays and copulations, exclusively towards the female conspecific model. Males behaved aggressively towards the conspecific male model. In contrast, females never showed any courtship behaviour towards the male model and showed significant aggression towards the conspecific female model. The results of this study reveal that territorial aggression in yellow-breasted chats is strongly intrasexual. Additionally, simulated female extraterritorial intrusions, but not male intrusions, resulted in extrapair courtship and copulation. This pattern is likely to be typical of many socially monogamous species and points to the behavioural mechanisms underlying both monogamy and extrapair mating systems.

Countersinging as a signal of aggression in a territorial songbird

Neighbouring territorial songbirds often interact through countersinging, where birds sing in response to the singing of neighbours such that their song bouts are temporally related. Complex forms of countersinging such as song type matching or song overlapping appear to be correlated with aggressiveness and readiness to escalate confrontations. Less attention has been paid to the importance of simpler forms of countersinging, where matched song types are not used and where individual songs do not temporally overlap. I examined countersinging behaviour in male Carolina wrens, Thryothorus ludovicianus, which countersing regularly. Why they countersing and how countersinging is perceived by neighbours is unknown. By comparing singing behaviour before and after simulated intrusions, I determined that subjects countersing with their neighbours more readily when highly aroused. Comparing responses to countersinging and noncountersinging playbacks showed that countersinging elicited more aggressive responses than did noncountersinging. Carolina wrens appear to exchange aggressive signals regularly through countersinging. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Use of Northern Cardinal Nest by Fledgling Carolina Wrens

We report on the use of a Northern Cardinal (Cardinalis cardinalis) nest by fledgling Carolina Wrens (Thryothorus ludovicianus) during a period of inclement weather. We found four fledgling wrens in a newly lined cardinal nest on the morning of 22 May 2002 during a heavy frost. The use of the cardinal nest by the wrens may have been precipitated by the unexpected frost. This represents a unique method of protection from inclement weather taken by young birds.

Host associations and seasonal occurrence of Haemaphysalis leporispalustris, Ixodes brunneus, I. cookei, I. dentatus, and I. texanus (Acari: Ixodidae) in Southeastern Missouri.

Hemaphysalis leporispalustris (Packard), Ixodes brunneus Koch, Ixodes cookei Packard, Ixodes dentatus Marx, and Ixodes texanus Banks were collected during a 3-yr study of pathogen-tick-host interactions in southeastern Missouri. H. leporispalustris was collected from the eastern cottontail rabbit, Northern bobwhite, and Carolina wren, and it was active all year. I. brunneus was collected by drag and from passerine birds during December, March, and April. I. cookei was collected from raccoons and mink during April, June, September, October, and November. I. dentatus was collected from the cottontail rabbit and Carolina wren throughout the year. I. texanus was collected from the eastern gray squirrel, Virginia opossum, and raccoon throughout the year.

Conditional strategies in territorial defense: do Carolina wrens play tit-for-tat?

Neighboring territorial animals are viewed primarily as intense rivals, but there are also opportunities for cooperation among competitors. Many animals respond less aggressively toward neighbors than to strangers. This phenomenon, termed the dear enemy effect, should be stable only when the reduced aggression is reciprocal. Territory owners should use conditional strategies in territorial defense, showing reduced aggression toward neighbors who cooperate by respecting territorial boundaries but increasing aggression toward invading neighbors. In this study I examined the response of territory owners to playbacks of neighbors at shared boundaries before and after intrusions by that neighbor or by strangers. Results showed that territory owners did not increase their aggression toward defecting neighbors but did increase their aggression toward neighbors after a simulated intrusion by a stranger. This surprising result might reflect the long-term relationship between neighboring Carolina wrens and the threat posed by rare intruding strangers. Copyright 2002.

Importance of Coarse Woody Debris to Avian Communities in Loblolly Pine Forests

Abstract: To investigate the importance of standing (snags) and down coarse woody debris ( DCWD) to bird communities in loblolly pine ( Pinus taeda) forests, we compared breeding (1997–1999) and nonbreeding (1997–1998, 1998–1999) responses of birds among two coarse woody debris (CWD) removal and control treatments. In each of four blocks, we established four experimental units: (1) DCWD removed, (2) snags and DCWD removed, and (3) and (4) unmodified control plots. We quantified vegetation layers to determine their effects on the experimental outcome. Total breeding bird abundance, abundance of resident species, breeding bird diversity, breeding species richness, and abundance of Great Crested Flycatchers ( Myiarchus crinitus) were reduced by the removal of DCWD and snags. Total woodpecker and Carolina Wren ( Thryothorus ludovicianus) breeding territories were reduced by snag removal. Weak excavating and secondary cavity‐nesting species, Neotropical migrants, and Eastern Towhees ( Pipilio erythrophthalmus) had fewer breeding territories on plots where DCWD was removed. Red‐headed Woodpeckers ( Melanerpes erythrocephalus) and midstory and canopy‐nesting species were at lowest densities on plots where all CWD had been removed. The CWD removal had no effect on the nonbreeding bird community. Most breeding and nonbreeding species used habitats with sparse midstory and well‐developed understory, whereas sparse canopy cover and dense midstory were important to some nonbreeding species. Snag and DCWD retention, and practices that maintain a dense understory and sparse midstory and canopy, will create favorable breeding habitat for many bird species of loblolly pine forests.