Dendroica Caerulescens Research Articles

Warm Springs, Early Lay Dates, and Double Brooding in a North American Migratory Songbird, the Black-Throated Blue Warbler

Numerous studies have correlated the advancement of lay date in birds with warming climate trends, yet the fitness effects associated with this phenological response have been examined in only a small number of species. Most of these species–primarily insectivorous cavity nesters in Europe–exhibit fitness declines associated with increasing asynchrony with prey. Here, we use 25 years of demographic data, collected from 1986 to 2010, to examine the effects of spring temperature on breeding initiation date, double brooding, and annual fecundity in a Nearctic - Neotropical migratory songbird, the black-throated blue warbler (Setophaga caerulescens). Data were collected from birds breeding at the Hubbard Brook Experimental Forest, New Hampshire, USA, where long-term trends toward warmer springs have been recorded. We found that black-throated blue warblers initiated breeding earlier in warmer springs, that early breeders were more likely to attempt a second brood than those starting later in the season, and that double brooding and lay date were linked to higher annual fecundity. Accordingly, we found selection favored earlier breeding in most years. However, in contrast to studies of several other long-distance migratory species in Europe, this selection pressure was not stronger in warmer springs, indicating that these warblers were able to adjust mean lay date appropriately to substantial inter-annual variation in spring temperature. Our results suggest that this North American migratory songbird might not experience the same fecundity declines as songbirds that are unable to adjust their timing of breeding in pace with spring temperatures.

Indirect effects of an invasive exotic species on a long-distance migratory songbird

The loss of foundational tree species to non-native pests can have far reaching consequences for forest composition and function, yet little is known about the impacts on other ecosystem components such as wildlife. We had the opportunity to observe how the loss of eastern hemlock (Tsuga canadensis), due to the invasive hemlock woolly adelgid (Adelges tsugae), influenced the population ecology of the Black-throated Blue Warbler (Setophaga caerulescens) over a 7 year period. We followed the process of adelgid infestation and subsequent hemlock loss, which allowed us to investigate the patterns and mechanisms of population change. We document a precipitous decline in breeding pairs at one site where hemlock was most abundant in the understory, but not at our other two sites. We observed no changes in reproductive output or apparent survival, yet territory size increased dramatically at the most affected site, suggesting that the decline was due to a lack of colonization by new breeders. Our results demonstrate how an invasive insect pest can indirectly influence wildlife species not believed to be vulnerable and in ways not typically investigated.

Variation in Aggression of Black-throated Blue Warblers Wintering in Jamaica

Abstract. Variation in aggression among birds on different territories has been hypothesized to be influenced by intrinsic factors such as a bird's age, sex, or size and extrinsic factors such as density of neighbors, abundance of food, or territory size. However, little work has examined how and why aggression may vary within a bird's home range. We examined variation in aggressive behaviors of nonbreeding Black-throated Blue Warblers (Setophaga caerulescens) in Jamaican coffee farms by using experimentally simulated intrusions into home ranges. Our multivariate models confirmed that multiple variables associated with aggression included sex, home range size, proximity of intrusion to home range center, and the presence of another aggressive bird in the area. In addition, we found that birds responded more aggressively to intrusions in shade trees, where insects were more abundant, than in the understory of coffee or of small patches of remnant forest. These results suggest the birds responded to variati...

Taxonomic recommendations for British birds: seventh report

Taxonomic recommendations for British birds: seventh report

Demography of Songbird Populations in a Rapidly Changing World: The Importance of Long-term Studies

A major goal of ecological research is to understand how ecological factors, such as food and predator abundance, interact to shape birth and death rates. Case studies describing this research can provide students with an understanding of how ecological conditions affect demographic rates, as well as an opportunity to explore and interpret real data. We have developed a Web-based teaching module based on a long-term study of a migratory songbird, the black-throated blue warbler (Dendroica caerulescens). The website describes this species and the ecological factors that affect its population growth and provides three exercises developed to span a range of student levels.

Roosting behaviour of a migratory songbird on Jamaican coffee farms: landscape composition may affect delivery of an ecosystem service

SummaryRecent evidence indicates that insectivorous birds in shade coffee farms provide economically significant ecosystem services by reducing insect pests, which should encourage cultivation practices favouring birds and other wildlife. However, the provisioning of this service may be dependent on landscape composition and movement patterns of these mobile consumers. Very little information is currently available on bird movements in coffee-dominated landscapes. We examined roosting behaviour in Black-throated Blue Warblers Dendroica caerulescens on two Jamaican coffee farms using radio telemetry in order to determine whether birds commute between nocturnal roosts and diurnal foraging ranges in coffee habitat. Nocturnal tracking revealed most birds moved outside diurnal foraging ranges on the farms to roost in forested habitat patches, sometimes up to 1 km away. Of 42 roost locations, only eight roosts were within the coffee farm perimeter: one individual in a coffee bush, and seven others in shade trees or vegetated riparian strips. Logistic regression confirmed that birds strongly selected continuous forest and individual trees over coffee for roosting. These findings reflect the ecological connectivity between coffee habitats and the adjacent landscape and raise the possibility that the provisioning of pest reduction services could be at least partly dependent on a farm’s proximity to forest patches.

Low-amplitude song predicts attack in a North American wood warbler

Many animal signals given during agonistic interactions are thought to convey levels of aggressive motivation of signallers, but few studies have experimentally tested whether such signals actually predict subsequent aggressive behaviour. In songbirds, the predictive content of most putatively aggressive vocal signals remains largely unresolved. Prior studies of agonistic vocal signalling in birds have generally assessed the reliability of vocal displays using indirect response measures such as approach to a playback speaker. However, this method does not allow aggression to be assessed directly, because subjects have no opportunity to engage in actual attacks. In this study, we assessed the aggressive content of vocal signals in black-throated blue warblers, Dendroica caerulescens , using a direct measure of aggression. We presented territorial males with a brief period of conspecific song playback, recorded their vocal responses, and then revealed a taxidermic mount, giving subjects an opportunity to attack. We found that one song feature, low-amplitude ‘soft’ song, was an extremely reliable predictor of attack; a logistic regression model in which the occurrence of soft song was the sole predictor correctly classified 93% of males as attackers or nonattackers. By contrast, four other vocal behaviours (total number of songs, rate of song type switching and numbers of two nonsong vocalizations) failed to predict whether birds would attack the mount. These data provide evidence that soft song in black-throated blue warblers conveys extremely reliable information about impending aggression. We suggest that the reliability of soft song observed in this study and recent studies in emberizid sparrows may be stabilized by vulnerability costs because the limited distance with which it can be transmitted forces signallers to place themselves at risk of retaliatory attack.

Relative influence of male and female care in determining nestling mass in a migratory songbird

Biparental care is common in birds, with the allocation of effort being highly variable between the sexes. In most songbird species, the female typically provides the most care early in the breeding cycle with both parents providing care when provisioning young. Food provisioning should be directly related to offspring quality; however, the relative influence each parent has on offspring quality has rarely been assessed at the nest level. Consequently, we were interested in assessing the relative influence male and female provisioning has on one measurement of offspring quality, nestling mass, in the black‐throated blue warbler Dendroica caerulescens. Over a six year period, 2003–2008, we collected information on average nestling mass per brood on day 6 of the nestling cycle and parental provisioning rates on day 7 of the nestling cycle from 182 first brood nests on three different study plots. We found that average nestling mass was directly related to male provisioning rate, while it was not related to female provisioning rate. On the other hand, estimated biomass provisioned had little influence on average nestling mass, calling into question its utility in assessing parental quality. Finally, there was some indication that parental influence on average nestling mass was dependent on the other parent's provisioning rate, suggesting that parents work in concert to influence nestling quality.

Asymmetric discrimination of geographical variation in song in a migratory passerine

Variation in sexual signals across populations is a common phenomenon, and most research to date has found that individuals discriminate against nonlocal signals. However, the strength of nonlocal signal discrimination can vary asymmetrically across populations, a possibility that has received less attention. Such asymmetries can be due to recognition errors in some populations, variation across populations in the ‘quality’ of local individuals, such that populations with high-quality individuals respond more strongly, and/or variation in the perception of signals, with signals from some populations being perceived as more intimidating or aggressive. Here, we examine song differences and male responsiveness to local and nonlocal songs in two populations of the black-throated blue warbler, Dendroica caerulescens, and also explore possible reasons for geographical variation in responsiveness. In the northern population, male responses were stronger to local songs than to nonlocal songs, whereas in the southern population, male responses to local and nonlocal songs did not differ. Overall responsiveness did not differ between the populations, and songs from one population were not responded to more weakly (or strongly) across populations. Overall, our results fit best with a model of asymmetric recognition error. Asymmetries in nonlocal responsiveness across populations may be a common phenomenon, and can have profound effects on patterns of dispersal, mate choice and gene flow.

Lidar remote sensing variables predict breeding habitat of a Neotropical migrant bird

A topic of recurring interest in ecological research is the degree to which vegetation structure influences the distribution and abundance of species. Here we test the applicability of remote sensing, particularly novel use of waveform lidar measurements, for quantifying the habitat heterogeneity of a contiguous northern hardwoods forest in the northeastern United States. We apply these results to predict the breeding habitat quality, an indicator of reproductive output of a well-studied Neotropical migrant songbird, the Black-throated Blue Warbler (Dendroica caerulescens). We found that using canopy vertical structure metrics provided unique information for models of habitat quality and spatial patterns of prevalence. An ensemble decision tree modeling approach (random forests) consistently identified lidar metrics describing the vertical distribution and complexity of canopy elements as important predictors of habitat use over multiple years. Although other aspects of habitat were important, including the seasonality of vegetation cover, the canopy structure variables provided unique and complementary information that systematically improved model predictions. We conclude that canopy structure metrics derived from waveform lidar, which will be available on future satellite missions, can advance multiple aspects of biodiversity research, and additional studies should be extended to other organisms and regions.

Settlement in Novel Habitats Induced by Social Information May Disrupt Community Structure

Abstract. Birds may colonize new habitats because of introduction, changing environmental conditions, and/or altered social or environmental cues. However, aside from introduced (often invasive) species, little is known about the consequences of such colonizations for members of existing communities. If the realized niche is influenced by the presence or absence of heterospecific competitors, then addition of a species to a novel habitat or location could result in extirpation or avoidance if members of the existing community are subdominant. Alternatively, if for some species heterospecific cues are the primary means for collecting information about a site's quality, heterospecific attraction could occur. To test these predictions, we experimentally induced free-living Black-throated Blue Warblers (Dendroica caerulescens) to colonize a novel environment within their existing range. We used dynamic occupancy modeling to test for the dynamics of colonization and local extinction as a function of our experi...

Scale-Dependent Mechanisms of Habitat Selection for a Migratory Passerine: An Experimental Approach

Habitat selection theory predicts that individuals choose breeding habitats that maximize fitness returns on the basis of indirect environmental cues at multiple spatial scales. We performed a 3-year field experiment to evaluate five alternative hypotheses regarding whether individuals choose breeding territories in heterogeneous landscapes on the basis of (1) shrub cover within a site, (2) forest land-cover pattern surrounding a site, (3) conspecific song cues during prebreeding settlement periods, (4) a combination of these factors, and (5) interactions among these factors. We tested hypotheses with playbacks of conspecific song across a gradient of landscape pattern and shrub density and evaluated changes in territory occupancy patterns in a forest-nesting passerine, the Black-throated Blue Warbler (Dendroica caerulescens). Our results support the hypothesis that vegetation structure plays a primary role during presettlement periods in determining occupancy patterns in this species. Further, both occupancy rates and territory turnover were affected by an interaction between local shrub density and amount of forest in the surrounding landscape, but not by interactions between habitat cues and social cues. Although previous studies of this species in unfragmented landscapes found that social postbreeding song cues played a key role in determining territory settlement, our prebreeding playbacks were not associated with territory occupancy or turnover. Our results suggest that in heterogeneous landscapes during spring settlement, vegetation structure may be a more reliable signal of reproductive performance than the physical location of other individuals.

Temporal Changes in Migratory Bird Use of Edges During Spring and Fall Seasons in Pennsylvania

Edge created through forest fragmentation can have significant impacts on the avian community, increasing predation and nest-parasitism rates and changing species richness and abundance patterns near edges. Although considerable research has demonstrated edge effects during the breeding season, few studies have considered how proximity to an edge affects migrant communities in stopover habitat. We studied bird communities in southern Pennsylvania (Adams, Chester, and Montgomery counties) to determine if richness and abundance of migrants were impacted by edges during spring and fall 1999–2001. The three categories of study sites were north-facing forest-herbaceous edges, south-facing forest-herbaceous edges, and interior forest (>300 m from an edge); sites were located in Gettysburg National Military Park-Eisenhower National Historic Site (GETT-EISE) and Valley Forge National Historical Park. During fall migration, Nearctic-Neotropic migrant species richness was significantly (P = 0.03) higher in interior forest compared to edges, whereas species richness of other guilds was not significantly different among edge types. During spring migration, richness of both permanent residents (P < 0.001) and temperate migrants (P < 0.001) were higher at both edges compared to interior sites. Only two species, Dendroica caerulescens (Black-throated Blue Warbler) (P = 0.03 during fall, P = 0.002 during spring) and Vireo olivaceus (Red-eyed Vireo) (P = 0.03 during fall, P = 0.05 during spring), showed differences among edge types during both spring and fall migration. Although the parks differed in amount of forest and in landscape composition, differences in richness and abundance patterns of birds between the parks also may be based on active management of Odocoileus virginianus Zimmerman (White-tailed Deer) at GETT-EISE that has resulted in vegetation differences. Overall, we found differential impacts of fragmentation (edge) on guilds and during different migratory periods, illustrating the importance of considering habitat use during each season individually for conservation of migratory songbirds.

Effects of spatial habitat heterogeneity on habitat selection and annual fecundity for a migratory forest songbird

Understanding how spatial habitat patterns influence abundance and dynamics of animal populations is a primary goal in landscape ecology. We used an information-theoretic approach to investigate the association between habitat patterns at multiple spatial scales and demographic patterns for black-throated blue warblers (Dendroica caerulescens) at 20 study sites in west-central Vermont, USA from 2002 to 2005. Sites were characterized by: (1) territory-scale shrub density, (2) patch-scale shrub density occurring within 25 ha of territories, and (3) landscape-scale habitat patterns occurring within 5 km radius extents of territories. We considered multiple population parameters including abundance, age ratios, and annual fecundity. Territory-scale shrub density was most important for determining abundance and age ratios, but landscape-scale habitat structure strongly influenced reproductive output. Sites with higher territory-scale shrub density had higher abundance, and were more likely to be occupied by older, more experienced individuals compared to sites with lower shrub density. However, annual fecundity was higher on sites located in contiguously forested landscapes where shrub density was lower than the fragmented sites. Further, effects of habitat pattern at one spatial scale depended on habitat conditions at different scales. For example, abundance increased with increasing territory-scale shrub density, but this effect was much stronger in fragmented landscapes than in contiguously forested landscapes. These results suggest that habitat pattern at different spatial scales affect demographic parameters in different ways, and that effects of habitat patterns at one spatial scale depends on habitat conditions at other scales.

Parental Care in the Multi-Brooded Black-Throated Blue Warbler

Maximizing reproductive output often entails a trade-off between energy spent on current breed- ing attempts and that saved for future reproductive opportunities. For species with biparental care, energy spent on the current breeding attempt represents not only a trade-off with future breeding opportunities but also an interac- tion with the energetic effort of one's mate. In most songbird species, the female typically invests the most in the early stages of breeding. Consequently, the male's contribution to provisioning young may free the female from this energetically costly activity and aid her ability to attempt a second brood. We investigated parental provision- ing in the Black-throated Blue Warbler (Dendroica caerulescens) to see if males and females altered their provi- sioning rates with respect to first and second broods. Using parental provisioning rates from 239 nests from three study sites over 6 years, we show that females provisioned young of first broods at a rate lower than that for second broods, while males' provisioning rate did not differ. Males' provisioning rate was inversely associated with that of females, with males increasing their provisioning when the number of young in a nest increased while females' provisioning decreased. Consequently, we believe our results highlight both the trade-off in energy females spend on current and future reproduction and the role of males' care in helping to maintain reproductive output through increased effort when conditions for feeding are difficult.

Bet‐hedging and the orientation of juvenile passerines in fall migration

1. Bet-hedging of innate migratory orientation of juvenile passerines may be a fitness-enhancing strategy for fall migration. Experimental studies support the view that juvenile passerines on their first migration to unknown winter grounds orient on a predetermined vector programme and make little or no adjustment for wind displacement. This trait, coupled with the unpredictable profile of wind speed and direction that the juvenile will encounter during migration, suggests that the fitness of a parent's juvenile offspring will be highly variable from year to year. Under these circumstances, within-clutch phenotypic variation in migratory orientation may be evolutionarily favoured. 2. To explore this hypothesis, a migration model is developed for a small passerine with breeding grounds in New England and winter grounds in the Caribbean. Parameterization is based on life history data of the neotropical migrant Dendroica caerulescens, the black-throated blue warbler. The model is simulated for the offspring of 20 000 adult females under each of a wide range of potential orientation programmes, incorporating stochastic wind profiles along potential migratory routes, based on 7 years of wind data for eastern North America. 3. Under these simulations, bet-hedging in the form of within-clutch variation of migratory orientation strongly dominates within-clutch homogeneity, yielding higher geometric mean fitness in all vector programmes considered. 4. The simulation results provide a potential explanation for the variation observed in the tracks of juvenile passerines. Bet-hedging also explains the extensively-documented 'coastal effect' in which fall banding stations along the Atlantic coast of the United States consistently capture a much higher percentage of juvenile birds than do more inland stations. 5. Bet-hedging is consistent with the published finding that slower flying birds exhibit greater variation in their migratory orientation than faster flying birds. 6. The bet-hedging model of migratory orientation presented in this paper provides a theoretical structure capable of organizing a diverse collection of field and laboratory observations as predictable consequences of an evolutionarily favoured strategy. This theory may constitute a major advance in our understanding of bird migration and thus justifies the design and execution of new laboratory and field experiments to assess its power and predictive reach.

Mesoscale patterns of altitudinal tenancy in migratory wood warblers inferred from stable carbon isotopes

We analyzed carbon isotope ratios (delta13C) of liver and pectoral muscle of Black-throated Blue Warblers (Dendroica caerulescens) to provide a mesoscale perspective on altitudinal tenancy in the Appalachian Mountains, North Carolina, U.S.A. Movements of males are poorly understood, particularly the degree to which yearlings (first breeding season) and older males (second or later breeding season) wander altitudinally during the breeding season. Liver and muscle delta13C values of warblers exhibited significant year and altitude effects, but yearling and older males were isotopically indistinguishable. Liver delta13C values increased with altitude at the rate of approximately 0.5% per hundred per 1000 m. The altitudinal lapse rate of muscle delta13C (approximately l1.1% per hundred per 1000 m) was nearly identical to the average rate of increase reported in several groups of C3 plants (approximately 1.1% per hundred per 1000 m). This suggests that the majority of males foraged within relatively narrow altitudinal zones during the breeding season. We caution, however, that the discrimination of altitudinal trends in carbon isotope ratios depends on relatively large multiyear samples. Given the scatter in data, it is unlikely that individuals can be accurately assigned to a particular altitude from carbon isotope values. Rapid adjustment of liver and muscle delta13C values to local altitudinal environments is consistent with the results of experimental dietary studies that show carbon turnover rates are relatively rapid in small migratory passerines. In a broader context, carbon isotope data have been increasingly used as proxies for wintering habitat use of Nearctic-Neotropical migratory passerines. However, tissues with high metabolic rates are unlikely to retain much isotopic signal of wintering habitat use by the time migrants reach their breeding territories.

Depredation of Black-Throated Blue Warbler Nestlings by an Introduced Slug (Arionidae)

Black-throated Blue Warblers (Dendroica caerulescens) are subject to nest predation by a variety of avian and mammalian species. I present evidence that slugs (Gastropoda: Mollusca) can also function as nest predators. On two occasions, slugs were observed feeding on 6–7 day-old nestling Black-throated Blue Warblers at Hubbard Brook Experimental Forest, New Hampshire, USA. This is apparently the first report documenting that slugs can function as avian nest predators.

Geographic Variation in the Mitochondrial Control Region of Black-throated Blue Warblers (Dendroica caerulescens)

ABSTRACT.— We investigated the genetic population structure of the Black-throated Blue Warbler (Dendroica caerulescens), a Nearctic-Neotropic migrant passerine that breeds in cool mixed deciduous-coniferous forests in eastern North America. A cline in plumage color in breeding populations in the central Appalachian Mountains suggests either a contact zone between two formerly allopatric populations or the presence of a strong contemporary selection gradient. Analysis of 333 base pairs of the mitochondrial control region from 287 individuals sampled from 14 populations revealed relatively high haplotype diversity, low nucleotide diversity, and limited but significant phylogeographic structure across the breeding range (analysis of molecular variance [AMOVA], variation among populations = 2.7%; P < 0.01) and between northern and southern population groups (AMOVA, variation among groups = 2.9%; P < 0.01). Genetic differentiation among populations did not conform to an isolation-by-distance model. Nucleotide ...

Dynamic occupancy models reveal within‐breeding season movement up a habitat quality gradient by a migratory songbird

The timing of settlement decisions likely influences the quality of breeding site choices.This is particularly the case in migratory birds, because the conditions that enhance breeding success are often not apparent upon arrival after migration. A strategy that addresses this problem is to adjust settlement decisions when reliable information becomes available.We used a new indirect method – dynamic site occupancy modeling – to estimate apparent movement of black‐throated blue warblersDendroica caerulescensamong sites within a breeding season. Because individuals should disperse to sites that maximize their fitness, we hypothesized that warblers would move up a habitat quality gradient when opportunities arose. For our study species, that would involve moving into sites with greater shrub density and at higher elevation within northern hardwoods forest, as these two features are positively correlated with reproduction and apparent survival in this species.Although the probability of site occupancy in our study landscape remained consistent throughout the breeding season (range: 0.66–0.69), occupancy models revealed substantial support for apparent movement of individuals within the breeding season. The mean probability of emigration from a point count site was 0.21 (±0.03 SE), and the mean probability of immigration to a site not previously occupied was 0.51 (±0.05 SE). The spatial distribution of this movement was a function of habitat quality. A portion of the black‐throated blue warbler population appears to arrive on the breeding grounds and settle initially in sub‐optimal habitat, moving subsequently into high quality densely shrubbed habitat at higher elevations. This modeling approach provides a new means to test hypotheses about habitat selection and movement by using presence–non‐detection data.