Sialia Sialis Research Articles

Cavity Depth, Not Experience, Determines Nest Height In Eastern Bluebirds

The functional consequences of nest size variation in cavity nesting birds have been thoroughly investigated in recent years. However, it is unknown whether recent nest-building experience influences subsequent nest architecture. By providing Eastern Bluebirds (Sialia sialis) with either deep or shallow nest boxes for their initial nest of the breeding season, we tested whether the height of their subsequent nest in the same breeding season reflected the height of their initial nest or whether they built nests to match their current nest cavity regardless of their previous nest height. We found that second nests built in deep boxes were significantly taller than first nests built in shallow boxes by the same females. This difference could be because of nest date, cavity depth, or experience. We tested each possible driver and found that cavity depth better explains variation in nest height than either nest date or prior experience. We conclude that, for this multi-brooded secondary cavity nesting passerine, the roles of learning and experience are secondary to the importance of behavioral plasticity.

Effect of Prenatal and Natal Administration of Testosterone on Production of Structurally Based Plumage Coloration

Testosterone has been implicated as a developmental mechanism involved in the organization and expression of sexually dimorphic traits, such as plumage coloration, in birds. Although research findings relating testosterone levels to plumage expression is equivocal, few studies have investigated how testosterone may influence the expression of structurally based plumage coloration. Here, we use experimental and correlational evidence to test the hypothesis that testosterone influences the development and maintenance of structurally based plumage coloration in a wild-breeding population of eastern bluebirds (Sialia sialis). First, we experimentally manipulated yolk testosterone and measured the effect on the development of plumage coloration of nestlings. Second, we implanted juvenile bluebirds with testosterone and measured the effect on nestling growth, body condition, and plumage coloration of nestlings. Third, we measured covariation between circulating testosterone and plumage coloration of breeding males. Yolk testosterone injections had no significant effect on nestling plumage coloration. Testosterone implantation, however, caused a reduction in plumage brightness, elevated corticosterone, and slower growth in nestlings. Finally, in breeding adult males we found no significant relationship between structural coloration and testosterone; however, males with higher testosterone levels exhibited duller chestnut (melanin-based) plumage. Our observations lead us to reject the hypothesis that testosterone increases structural plumage coloration in male eastern bluebirds.

Evidence that fathers, but not mothers, respond to mate and offspring coloration by favouring high-quality offspring

Parents have limited resources and, when simultaneously rearing multiple offspring, are expected to favour particularly valuable offspring. Because offspring characteristics like sex and ornamentation influence future reproductive value, these traits may influence parental favouritism. Parents in high-quality nest environments should prefer sons because sons show greater variability in reproductive potential than daughters. Moreover, when ornamental plumage coloration is condition dependent, parents should favour brighter over duller sons. We tested parental favouritism for fledgling-aged eastern bluebirds, Sialia sialis, a species in which fledglings display condition-dependent UV-blue coloration on their wings and tails. We simultaneously threatened two offspring with mock predators and recorded which offspring the parents defended. In accordance with the predicted value of offspring, fathers protected brighter over duller sons and protected sons over daughters when mated to high-quality mates. Mothers did not discriminate between sons and daughters or between brighter and duller sons. These results suggest that ornamental plumage coloration in juvenile bluebirds functions as a signal in father–offspring interactions. Paternal and maternal responses of eastern bluebirds to offspring characteristics may differ because of differences in parental allocation strategies or because males assess offspring quality based on variation in plumage coloration, a trait that also functions in competitive interactions among adult males.

Where Are the Bluebirds of the Bluegrass? Eastern Bluebird Decline in Central Kentucky

Eastern bluebird (Sialia sialis) populations are increasing across most of the species' range; however, we document a decline in a bluebird population in central Kentucky, where state and local Christmas Bird Count and Breeding Bird Survey data indicate bluebirds are declining throughout the Ohio River valley region. We examined several possible hypotheses for this decline, including habitat loss, nest cavity loss, increased competition from other species, and variation in winter temperature using long term nesting data gathered at one study site in central Kentucky. We found that bluebird abundance was positively correlated with mean temperature during the previous winter. Our data suggest bluebirds in central Kentucky are declining because they are susceptible to cold winters and are experiencing increased nest box competition.

Neonatal handling alters the development of the adrenocortical response to stress in a wild songbird (eastern bluebird, Sialia sialis)

Neonatal handling of captive vertebrates can shape the development of their hypothalamo–pituitary–adrenal (HPA) axis and alter their ability to respond to stressful stimuli later in life. However, the long-term effects of such handling on this endocrine axis in free-living species are not well understood. We investigated the effects of age and neonatal handling on corticosterone secretion in response to restraint in eastern bluebird (Sialia sialis) chicks. We found that unhandled (“naïve”) and handled (“experienced”) chicks exhibited no corticosterone response to handling early in development. Thereafter, naïve individuals exhibited the progressive development of a corticosterone response with age, and by day 12 post-hatch, the response resembled that of adult bluebirds. Experienced nestlings, which were handled every other day from the day of hatch, showed a similar pattern of HPA development until day 12 post-hatch, when their corticosterone response was significantly reduced compared to that of naïve nestlings. In contrast, chicks that were handled only once, when 10days old, did not show a reduced corticosterone response at 12days old. Taken together, our data suggest that a certain threshold of accumulated neonatal handling episodes is necessary to depress corticosterone secretion, and/or that the cumulative effects of several handling episodes only manifest themselves once the HPA axis is fully developed. Our findings, in concert with studies on two other wild species, indicate that routine handling of nestlings in the field can alter their responses to stress in a species-specific manner, potentially leading to important fitness consequences.

Cryptic introductions and the interpretation of island biodiversity

Species with cryptic origins (i.e. those that cannot be reliably classed as native or non-native) present a particular challenge to our understanding of the generation and maintenance of biodiversity. Such species may be especially common on islands given that some islands have had a relatively recent history of human settlement. It is likely that select island species considered native might have achieved their current distributions via direct or indirect human actions. As an example, we explore the origins of eastern bluebirds (Sialia sialis bermudensis) on the island of Bermuda. Considered native to the island and a distinct subspecies, this population has diverged in morphology relative to mainland North America. Using microsatellite markers and simulation of island colonization, we show that the Bermuda population of bluebirds is the likely result of a single colonization event that occurred during the 1600s, making this a cryptic invader. To our knowledge, this is one of the youngest examples of a terrestrial vertebrate cryptic invader. We suggest that the eastern bluebird is not an isolated case of cryptic invader on either Bermuda or elsewhere and that caution be exercised when studying present-day distributions of organisms.

Repeatable nest defense behavior in a wild population of Eastern bluebirds (Sialia sialis) as evidence of personality

Consistent individual differences in behavior, or personality, have been demonstrated in a variety of species other than humans, including mammals, birds, and invertebrates. Behavioral consistency has been shown to affect dispersal, foraging, exploration, and antipredator responses, which may have an impact on parental and offspring survival. Despite increasing research in behavioral consistency, the repeatability of nest defense behavior has rarely been assessed in wild bird populations. Furthermore, previous studies investigating nest defense behavior have utilized laboratory studies or mounted predators to elicit defensive behavior. It is important to assess personality in wild populations to fully understand the fitness consequences of behavioral consistency across natural contexts and to utilize live predators or competitors for accurate assessment of defensive behavior. We used an ecologically relevant, live, invasive, nest site competitor, the house sparrow (Passer domesticus), to elicit nest defense behavior in a wild population of Eastern bluebirds (Sialia sialis) to determine if nest defense behavior is repeatable within and between years for males and females (males: 2009, N = 17; 2010, N = 18; both years, N = 9. Females: 2009, N = 22; 2010, N = 15; both years, N = 11). We also determined if individual behavior changes as a function of season, parental age, brood size, or the number of house sparrows around the nest site. We found that females demonstrated repeatable behavior both within and between years. Male nest defense behavior was only repeatable in 1 year and was influenced by season in the other year. Parental age, brood size, and the number of house sparrows around the nest site did not affect nest defense behavior. We conclude that Eastern bluebirds demonstrate consistent nest defense behavior, or personality, although males are more plastic than females.

Eastern Bluebirds at Prince Albert National Park in 2011

Eastern Bluebirds at Prince Albert National Park in 2011

Effect of Nestling Sex Ratio on the Provisioning Behavior ofSialia sialis(Eastern Bluebird)

To maximize reproductive success, parents may differentially invest in sons and daughters, i.e., sex-biased parental investment. Preferential provisioning behavior has been reported in one population of Sialia sialis (Eastern Bluebird) and attributed to local resource competition. We studied the provisioning behavior of Eastern Bluebirds in Madison County, KY in 2004. We experimentally manipulated brood sex ratios in 24 bluebird nests, creating female-biased (n = 8), male-biased (n = 5), and control (n = 11) nests. Following manipulation, nests were videotaped to record adult provisioning behavior. Brood sex ratio had no effect on the provisioning behavior of either male or female Eastern Bluebirds. Similarly, we found no effect of brood sex ratio on the provisioning rates of either male or female bluebirds for control broods that naturally differed in number of male and female nestlings (n = 9; 5 male-biased and 4 female-biased). Our results contrast with those of a previous study that found that male Eastern Bluebirds fed nestling males less than nestling females, possibly because sons were more likely to compete with them for resources in the future. However, local resource competition seems unlikely because Eastern Bluebirds exhibit low rates of philopatry. Given these conflicting results, additional study is needed to determine if factors such as latitude, food availability, or availability of other resources might influence the provisioning behavior of male Eastern Bluebirds.

WOODEN VERSUS INSULATED METAL NESTBOXES: A COMPARISON OF REPRODUCTIVE SUCCESS AND USE BY SONGBIRDS

ABSTRACTWe compared reproductive success of Tree Swallow (Tachycineta bicolor) and House Wren (Troglodytes aedon) nests in wooden nestboxes to insulated metal nestboxes to determine if a new insulated metal nestbox design is a safe alternative to wooden nestboxes. We also examined the preference of four species of cavity-nesting birds to nest in wooden versus metal nestboxes. No significant differences were found among clutch size, hatching success, and the number of fledglings produced in Tree Swallow and House Wren nests between the two types of nestboxes. Therefore, insulated metal nestboxes appear to be a safe alternative to wooden nestboxes. Metal nestboxes were preferred by Tree Swallows to wooden nestboxes (χ2 = 6.5, n = 25, p = 0.01). However, wooden nestboxes were preferred to metal nestboxes as a first choice for nesting 77.1% of the time for all four species. House Wrens, Eastern Bluebirds (Sialia sialis), and House Sparrows (Passer domesticus) all preferred wooden to metal nestboxes (Fisher's Exact Tests, all p < 0.001). Thus, insulated metal boxes are safe, but may not attract desirable species, such as Eastern Bluebirds, when compared to traditional wooden Peterson-style boxes.

Problems with Using Ptilochronology to Measure the Growth and Nutritional Status of Nestling Bluebirds

Variation in habitat quality can influence birds' survival and reproduction profoundly, but many methods for assessing it are logistically difficult. Ptilochronology is a method of determining the nutritional state of a bird by measuring the width of the growth bars on its feathers and may be a useful proxy for assessing habitat quality. Since each growth bar is thought to represent 24 hr of feather growth, the average size of growth bars provides a record of the bird's nutritional state during feather growth. Ptilochronology has been validated in adult birds but may not be reliable when applied to nestlings. We compared the growth rate of remiges and the average width of the growth bars on them in Eastern Bluebird (Sialia sialis) chicks. On average, the width of growth bars was slightly less than 50% of that expected if each bar corresponded to the daily growth of the feather. The average width of the growth bars also failed to correlate with the concurrent growth rates of the wing and the remiges themselves, although growth rates of the latter were significantly related. Growth bars on nestlings' feathers were also narrower than those of adult bluebirds and their size varied considerably. These data suggest that nestlings' feathers undergo daily pulses of growth that are variable in duration and number, and consequently ptilochronology may not be a reliable method for determining the growth rate of the feathers and hence determining the nutritional status of nestling birds.

Habitat near nest boxes correlated with fate of eastern bluebird fledglings in an urban landscape

Nest boxes are commonly erected in managed landscapes such as backyards, golf courses and parks, attracting cavity-nesting birds to habitats they would not otherwise occupy. Although studies have shown that habitat around natural nests influences postfledging survival, this relationship has not been examined for artificial nest cavities, which humans may site at inappropriate locations. In Williamsburg, Virginia, we compared vegetation surrounding nest boxes which produced eastern bluebird fledglings that were predated by hawks within 10 days of leaving the nest to vegetation surrounding nest boxes producing surviving fledglings. We found a significant difference between PC scores for forest denseness around nest boxes that produced surviving versus predated fledglings; fledglings that survived tended to come from nest boxes with high forest and canopy cover, close trees and little grass.

Anthropogenic noise is associated with reductions in the productivity of breeding Eastern Bluebirds (Sialia sialis)

Although previous studies have related variations in environmental noise levels with alterations in communication behaviors of birds, little work has investigated the potential long-term implications of living or breeding in noisy habitats. However, noise has the potential to reduce fitness, both directly (because it is a physiological stressor) and indirectly (by masking important vocalizations and/or leading to behavioral changes). Here, we quantified acoustic conditions in active breeding territories of male Eastern Bluebirds (Sialia sialis). Simultaneously, we measured four fitness indicators: cuckoldry rates, brood growth rate and condition, and number of fledglings produced (i.e., productivity). Increases in environmental noise tended to be associated with smaller brood sizes and were more strongly related to reductions in productivity. Although the mechanism responsible for these patterns is not yet clear, the breeding depression experienced by this otherwise disturbance-tolerant species indicates that anthropogenic noise may have damaging effects on individual fitness and, by extraction, the persistence of populations in noisy habitats. We suggest that managers might protect avian residents from potentially harmful noise by keeping acoustically dominant anthropogenic habitat features as far as possible from favored songbird breeding habitats, limiting noisy human activities, and/or altering habitat structure in order to minimize the propagation of noise pollution.

Chapter 6: Constraints on acoustic signaling among birds breeding in secondary cavities: The effects of weather, cavity material, and noise on sound propagation

Increasing evidence suggests that anthropogenic noise from urbanization affects animal acoustic communication. We investigated whether the begging calls of nestling Eastern Bluebirds (Sialia sialis) varied along a disturbance gradient of ambient noise. Contrary to our prediction and the results of a previous study of nestling Tree Swallows (Tachycineta bicolor), we found that nestling Eastern Bluebirds did not increase the amplitude or structural characteris- tics—including frequency, rate, and duration—of their vocalizations in response to ambient noise. However, we found that prevalent temperature and humidity conditions attenuated begging calls. Specifically, in warmer, more humid weather, vocalizations of nestling Eastern Bluebirds attenuated outside the nest box; this is consistent with research conducted on the propagation of sound in various mediums and temperatures. Finally, our results indicate that increased ambient noise is associated with a decrease in the signal-to-noise ratio of nestling vocalizations. In other words, loud ambient noise likely masks chick begging calls, which suggests that chicks and par- ents may experience communication difficulties in noisy environments. We suggest that future studies explore the effects of ambient noise on parental behavior and aspects of parent-offspring communication and conflict that are related to raising a brood of nestlings.

Invasive Ants Alter Foraging and Parental Behaviors of a Native Bird

Introduced species can exert outsized impacts on native biota through both direct (predation) and indirect (competition) effects. Ants frequently become established in new areas after being transported by humans across traditional biological or geographical barriers, and a prime example of such establishment is the red imported fire ant (Solenopsis invicta). Introduced to North America in the 1930's, red imported fire ants are now firmly established throughout the southeastern United States. Although these invasive predators can dramatically impact native arthropods, their effect on vertebrates through resource competition is essentially unknown. Using a paired experimental design, we compared patterns of foraging and rates of provisioning for breeding eastern bluebirds (Sialia sialis) in unmanipulated (control) territories to those in adjacent (treated) territories where fire ants were experimentally reduced. Bluebirds inhabiting treated territories foraged nearer their nests and provisioned offspring more frequently than bluebirds inhabiting control territories with unmanipulated fire ant levels. Additionally, nestlings from treated territories were in better condition than those from control territories, though these differences were largely confined to early development. The elimination of significant differences in body condition towards the end of the nestling period suggests that bluebird parents in control territories were able to make up the food deficit caused by fire ants, potentially by working harder to adequately provision their offspring. The relationship between fire ant abundance and bluebird behavior hints at the complexity of ecological communities and suggests negative effects of invasive species are not limited to taxa with which they have direct contact.

Structural coloration signals condition, parental investment, and circulating hormone levels in Eastern bluebirds (Sialia sialis)

Many of the brilliant plumage coloration displays of birds function as signals to conspecifics. One species in which the function of plumage ornaments has been assessed is the Eastern bluebird (Sialia sialis). Studies of a population breeding in Alabama (USA) have established that plumage ornaments signal quality, parental investment, and competitive ability in both sexes. Here we tested the additional hypotheses that (1) Eastern bluebird plumage ornamentation signals nest defense behavior in heterospecific competitive interactions and (2) individual variation in plumage ornamentation reflects underlying differences in circulating hormone levels. We also tested the potential for plumage ornaments to signal individual quality and parental investment in a population breeding in Oklahoma (USA). We found that Eastern bluebirds with more ornamented plumage are in better condition, initiate breeding earlier in the season, produce larger clutches, have higher circulating levels of the stress hormone corticosterone, and more ornamented males have lower circulating androgen levels. Plumage coloration was not related to nest defense behavior. Thus, plumage ornamentation may be used by both sexes to assess the physiological condition and parental investment of prospective mates. Experimental manipulations of circulating hormone levels during molt are needed to define the role of hormones in plumage ornamentation.

Female‐biased offspring sex ratios in birds at a mercury‐contaminated river

Mercury is a ubiquitous environmental pollutant that can negatively impact physiology and behavior of vertebrates, causing sub‐lethal changes in condition and reducing fitness. Here we examine its effect on offspring sex ratio. Previous studies demonstrate the ability of environmental contaminants to skew sex ratios in wild populations toward the production of females, and research in humans has demonstrated a decrease in male births following mercury exposure. We therefore hypothesized that female birds inhabiting the floodplain of a mercury‐contaminated river would produce broods more biased towards the production of females relative to birds from uncontaminated areas. We examined complete broods of three species: the aquatic‐feeding belted kingfisher Megaceryle alcyon, the terrestrial‐feeding eastern bluebird Sialia sialis, and the tree swallow Tachycineta bicolor, which feeds from both aquatic and terrestrial sources. Nestling sex ratios were shifted toward the production of females in all three species on mercury‐contaminated sites when compared to uncontaminated reference sites. These results may be explained by endocrine disruption or the Trivers–Willard theory of sex allocation. Our study is the first to examine the impact of mercury on offspring sex ratios in birds, and therefore contributes to our understanding of the potential for this persistent biomagnifying contaminant to alter fitness and effective population size in wildlife.

Reproductive success of three passerine species exposed to dioxin-like compounds near Midland, Michigan, USA

Nests of three passerine birds, house wren (HOWR), tree swallow (TRES), and eastern bluebird (EABL) were monitored daily (2005–2007) at study areas (SAs) downstream of Midland, Michigan where soil and sediment concentrations of polychlorinated dibenzofurans (PCDFs) were significantly greater than the regional background concentrations and upstream reference areas (RAs). Similarly, TRES research conducted at sites contaminated with dioxin-like compounds indicated that concentrations of polychlorinated dibenzo-p-dioxins and PCDFs, expressed as ΣPCDD/DFs and 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents observed in the diet and eggs of these three species would be predicted to cause significant effects on reproduction. However, site-specific reproductive parameters including hatching success and fledging success at downstream SAs were similar to or greater than those at upstream RAs. Specifically, hatching success was not significantly different among years, species, locations, or between early and late nesting attempts. Of all initiated clutches, 66% (n = 427), 73% (n = 245), and 64% (n = 122) successfully fledged at least one nestling for HOWR, TRES, and EABL, respectively. Overall reproductive performance was similar between SAs and RAs. The reason for these unexpected results is consistent with the fact that there are species-specific and congener-specific differences in sensitivities to the effects of aryl hydrocarbon receptor agonists.Electronic supplementary materialThe online version of this article (doi:10.1007/s10646-012-0869-4) contains supplementary material, which is available to authorized users.

Parasites and Pathogens of Eastern Bluebirds (Sialia sialis): A Field Survey of a Population Nesting Within a Grass-Dominated Agricultural Habitat in Georgia, U.S.A., with a Review of Previous Records

The parasite community associated with a population of eastern bluebirds (Sialia sialis) nesting in a grass-dominated habitat in northern Georgia, U.S.A., was surveyed by live examination of adults and nestlings; examination of feces, nesting materials, and blood smears; necropsy; and polymerase chain reaction techniques. During the course of 5 breeding seasons, we found 10 macroparasite species, which included 1 tick species (Ixodes brunneus), 1 louse species (Philopterus sialii), 1 species of hematophagous mite (Dermanyssus prognephilus), and 5 other species not previously reported from eastern bluebirds, including an acanthocephalan (Plagiorhynchus cylindraceus), a species of strongyloid nematode, a cestode (Hymenolepis sp.), a nestling bird fly (Carnus floridensis), and a nest mite (Campephilocoptes sp.); a description of feather mites associated with the population was not included in this report. Overall prevalence of macroparasites was 16.1%. We also detected 5 species of protistan parasites, including Haemoproteus fallisi, Plasmodium relictum, Trypanosoma avium, and 2 others, an Atoxoplasma sp. and a Sarcocystis sp., which represent new host–parasite records. One bird tested positive for West Nile virus and for St. Louis encephalitis virus. More than 50% of the birds examined were infected with 1 or more microparasites. Prevalence of microparasites varied by examination method employed. An updated list of known parasites of eastern bluebirds gathered from published literature and database searches is provided.

Artificial Nest Cavity Used Successfully By Native Species and Avoided by European Starlings

We describe a weather-durable cavity design used successfully by cavity-nesting species native to the eastern USA and, although accessible, avoided by European Starlings (Sturnus vulgaris). The artificial nest cavity was constructed using 9.5-cm inside diameter polyvinyl chloride tubes cut to 27.5-cm lengths. The tubes were mounted horizontally with 5.1-cm entry holes drilled through one of the capped ends. Eastern Bluebirds (Sialia sialis), House Wrens (Troglodytes aedon), and Tree Swallows (Tachycineta bicolor) nested in 49 of 100, newly mounted tubes on utility poles in north-central Ohio, USA from April through June 2009. These species nested in 85% of the tubes during the same period in 2010 and fledged young from 94.1% of nests. We added 10 nest tubes (27.5-cm long × 17-cm inside diam) at sites similar to the smaller tubes in 2010. Two of the larger tubes were used by nesting starlings and six by native species. Cavity vertical depth has been shown to be an important feature in starling nest site selection, but our data from the larger tubes indicate that other factors are likely important. The smaller design could offer nesting opportunities for a range of native cavity-nesting species while limiting use by starlings.