Sialia Mexicana Research Articles

Long-term monitoring reveals the impact of changing climate and habitat on the fitness of cavity-nesting songbirds

Long-term studies on the impacts of global change on reproduction are rare, even though reproductive output can respond more quickly than species' abundances or distributions. We explore the influences of habitat use and weather on fitness, by using an 11-year dataset of 2305 nesting attempts (7174 nestlings) across four species of cavity-nesting songbirds within California's Central Valley. Specifically, we modeled relationships between habitat type, maximum nesting season temperature, and winter/nesting season precipitation and nest site selection, reproductive success, and nestling weight for each species. We found that species selected nest sites based on species-specific habitat variables and that reproductive success and nestling weight peaked in selected habitats, suggesting an absence of ecological traps. Higher maximum nesting season temperatures were negatively associated with clutch size, reproductive success, and nestling weight for all species. For example, models predicted that the probability of successfully fledging tree swallows declined by 39 % and tree swallow nestling weight declined by 19 % when nests experienced the hottest versus the coolest maximum temperatures. In contrast, the effects of precipitation on reproductive outcomes varied by species and timing. We observed strong negative associations between nesting season precipitation and reproductive success/nestling weight in tree swallows and western bluebirds. Our findings suggest that, while habitat conversion to orchards has not resulted in ecological traps, predicted increases in temperature and spring precipitation may reduce the fitness of cavity-nesting songbirds in California. More broadly, our results highlight the importance of long-term monitoring when unravelling impacts of global change on fitness.

Rapid shifts in migration routes and breeding latitude in North American bluebirds

AbstractSpatial and temporal shifts in the migratory patterns of birds have become more frequent as climate change and habitat alteration continue to impact ecosystems and the species dependent on them. In this study, we used eBird community science data collected over ten years to examine potential changes in the migratory patterns of three North American bluebird species: eastern (Sialia sialis), western (Sialia mexicana), and mountain (Sialia currucoides) bluebirds. Community science datasets such as those provided through eBird are a valuable tool for examining population‐level processes, as such data are often costly and time‐consuming to collect through other approaches (e.g., directly tracking individuals). Using generalized additive models, we produced smoothed migration paths for all three species over each season from 2009 to 2018. We asked whether there were changes over this 10‐year period in the timing of spring and fall migration and migration speed, and the population centroids during breeding and migration. In contrast to many species that are experiencing poleward shifts in their distributions, the population centroids during the breeding period of all three bluebird species appear to have shifted southward over the past decade. Perhaps most surprisingly, we also detected strong longitudinal shifts in the population centroids during migration in eastern and western bluebirds, with both species shifting toward the center of the continent. Despite these changes in migratory routes and breeding distributions, we detected no change in the migratory timing or speed of any of the species. Our analysis indicates that bluebirds are rapidly altering the pattern of their migration, likely in response to changing environmental conditions, but not always in the direction predicted.

Competitive events during nest‐building influence offspring growth and behaviour, but not testosterone deposition in the egg yolk in Western Bluebirds (Sialia mexicana)

The environmental stressors that females face before egg‐laying, such as competition for resources, can impact the fitness prospects of the female as well as her offspring through hormone‐mediated maternal effects. In obligate secondary cavity‐nesting species, suitable nest‐holes are a limited resource, so gaining access to nest‐sites may require competition with other individuals. In the Western BluebirdSialia mexicana, we experimentally evaluated whether female exposure to increased competition for nest cavities (reduced available nest‐holes plus exposure to a competitor's decoy and vocalizations) during the period of nest construction affects her offspring's developmental trajectory, survival probability and behaviour through an increase in the concentration of testosterone in the egg yolk. Chicks from experimental females had a lower body mass at age 3 days, followed by a faster growth rate during the rapid growth phase, a slower growth rate during the slow growth phase and larger tarsus at age 15 days (only male offspring). Additionally, experimental chicks exhibited a lower breathing rate. However, female exposure to stronger competition for nest cavities during nest construction did not affect testosterone concentration in the egg yolk. Differential testosterone deposition in yolk is therefore apparently not the underlying mechanism of the effects observed, which may be attributed to changes in females' parental behaviour triggered by competition for breeding sites.

Short-term benefits of prescribed fire to bird communities of dry forests

BackgroundLow-severity prescribed fire is an important tool to manage fire-maintained forests across North America. In dry conifer forests of the western USA, prescribed fire is often used to reduce fuel loads in forests characterized historically by mixed- and low-severity fire regimes. Understanding the ecological effects of prescribed fire treatments is important for predicting the impacts of these management actions on wildlife communities. Few studies, however, have estimated small landbird responses to forest treatments at spatial scales relevant to their ecology or have examined potential differences in treatment effects applied within historically mixed- vs. low-severity fire regimes. Therefore, we evaluated prescribed fire treatment effects and relationships with burn severity for avian communities in dry conifer forests dominated by ponderosa pine (Pinus ponderosa) located on seven national forests in the interior western USA. We surveyed birds for 1–4 years and 1–3 years before and after prescribed fire treatments at mixed- and low-severity fire regime locations, respectively, following a before-after, control-impact study design — 8 paired control-treatment units in mixed-severity locations (16 total study units with 320 survey points) and 4 paired control-treatment units in low-severity locations (10 total study units with 278 survey points). Using a Bayesian hierarchical multi-species occupancy model, we investigated responses to prescribed fire treatments by a community of 95 bird species.ResultsWe found statistically supported treatment effects and/or burn severity relationships for 33 species primarily in mixed-severity locations. The data supported positive treatment effects at mixed-severity locations for 9 species (American robin [Turdus migratorius], western bluebird [Sialia mexicana], hairy woodpecker [Dryobates villosus], black-backed woodpecker [Picoides arcticus], American three-toed woodpecker [Picoides dorsalis], house wren [Troglodytes aedon], dusky flycatcher [Empidonax oberholseri], western wood-pewee [Contopus sordidulus], gray flycatcher [Empidonax wrightii]), whose occupancy was more likely after treatment at the most severely burned units, and a negative effect for one species (ruby-crowned kinglet [Corthylio calendula]), whose occupancy was less likely after treatment at the most severely burned units. At low-severity locations, only two species exhibited treatment effects, both negative (red-faced warbler [Cardellina rubrifrons] and lark sparrow [Chondestes grammacus]). We also found supported occupancy relationships with burn severity post-treatment (i.e., regardless of species distribution before treatment) for 29 species, most of which were consistent with their life histories (e.g., patterns of positive relationships for cavity-nesting, bark insectivores and negative relationships for open-nesting, foliage insectivores). Stronger responses to prescribed fire treatments at mixed-severity locations were unexpected because prescribed fire applications were more similar to historical wildfires characteristic of low-severity fire regimes.ConclusionsBird populations in historically low-severity locations may be relatively unresponsive to prescribed fire because fire there is typically more frequent and regular. By comparison, fire events in forests characterized by a mixed-severity regime are less common, potentially eliciting more responses to an infrequent opportunity, even by species that are strongly associated with recently burned forests by wildfire. Our results suggest that fire management activities intended to reduce fuels and lower the risk of high-severity wildfire can also be effective in creating habitat for some fire specialists at least in the short term.

Dynamic Changes in Begging Signal Short-Term Information on Hunger and Need.

AbstractDynamic signals can convey distinct information to a receiver on different timescales, making assessment of how quickly signal strength changes important for understanding signal function. Here, we combine repeated measures of offspring begging behavior of western bluebirds with assessments of fitness as well as quantitative genetic analyses of cross-fostered offspring to investigate whether variation in begging behavior conveys information about hunger, need, or quality or has no signaling function. Begging intensity increased with food deprivation, supporting the signal-of-hunger hypothesis. However, after controlling for this variation, multiple lines of evidence showed that begging also signaled need but not quality. Specifically, begging intensity was repeatable only on short timescales, and nestlings that begged more intensely were in poorer condition. Moreover, variation in mean begging intensity was not strongly related to measures of fitness. In general, we found that begging behavior is a highly flexible trait that appears to be unconstrained by both genetic and early developmental influences, as indicated by the cross-fostering experiment that confirmed that the nest environment, not genetic relatedness, explained variation in begging behavior. Together, these results support the idea that begging dynamically signals shorter-term information: hunger and need. More generally, they show the importance of assessing the timescale of signal change to understand its function.

Do candidate genes for migration and behavior explain migratory variation in bluebirds (Sialia spp.)?

Avian migration is a widespread, complex behavior that has long fascinated both scientists and the general public. Nonetheless, we have not identified the genetic and environmental controls of migration. Comparisons of migratory strategies within and among species suggest that these differences are associated with genetic and personality variation. Recently, studies have identified possible candidate genes for personality and migration in several avian taxa. Partially migratory species—those in which only some individuals migrate, while others remain resident year round—provide good study systems to test whether variation at candidate loci covaries with variation in migratory behavior. Using both a population genetic and a linear model approach, we tested whether genetic variation at 2 candidate genes (adenylate cyclase activating polypeptide 1, ADCYAP1, and dopamine receptor, DRD4) underlies variation in migratory behavior in the Western Bluebird (Sialia mexicana), a partial migrant, and the closely related Mountain Bluebird (S. currucoides), an obligate migrant, breeding in the Okanagan Valley, British Columbia. Our analyses suggested that DRD4, a gene usually associated with personality, might explain some of the propensity to migrate, but provided no evidence that ADCYAP1, a gene usually associated with the propensity to migrate, explained variation in migratory behavior in these species. Our results join a growing body of literature suggesting the effects of candidate genes are not generalizable across species or populations.

Do candidate genes for migration and behavior explain migratory variation in bluebirds (Sialia spp.)?

Avian migration is a widespread, complex behavior that has long fascinated both scientists and the general public. Nonetheless, we have not identified the genetic and environmental controls of migration. Comparisons of migratory strategies within and among species suggest that these differences are associated with genetic and personality variation. Recently, studies have identified possible candidate genes for personality and migration in several avian taxa. Partially migratory species—those in which only some individuals migrate, while others remain resident year round—provide good study systems to test whether variation at candidate loci covaries with variation in migratory behavior. Using both a population genetic and a linear model approach, we tested whether genetic variation at 2 candidate genes (adenylate cyclase activating polypeptide 1, ADCYAP1, and dopamine receptor, DRD4) underlies variation in migratory behavior in the Western Bluebird (Sialia mexicana), a partial migrant, and the closely related Mountain Bluebird (S. currucoides), an obligate migrant, breeding in the Okanagan Valley, British Columbia. Our analyses suggested that DRD4, a gene usually associated with personality, might explain some of the propensity to migrate, but provided no evidence that ADCYAP1, a gene usually associated with the propensity to migrate, explained variation in migratory behavior in these species. Our results join a growing body of literature suggesting the effects of candidate genes are not generalizable across species or populations.

Individual Nest Site Preferences Do Not Explain Upslope Population Shifts of a Secondary Cavity-Nesting Species.

Simple SummaryEnvironmental changes such as climate change have affected wildlife species behavior and geographic ranges globally. We analyzed nesting data of western bluebirds to determine whether the link between geographic range shifts of a western bluebird population in New Mexico, USA is due to individual adaptations or changes occurring at a larger scale. We looked at location data of marked and recaptured nestlings and adults that nested within our study area. We found that individual choices have no impact on the geographic range shift being observed in this population, suggesting that population-level processes, such as emigration and immigration, may be the main cause of these shifts.Geographic ranges of plants and animals are shifting due to environmental change. While some species are shifting towards the poles and upslope in elevation, the processes leading to these patterns are not well known. We analyzed 22 years of western bluebird (Sialia mexicana) data from a large nest box network in northern New Mexico at elevations between 1860 m and 2750 m. This population has shifted to higher elevations over time, but whether this is due to changes in nesting behavior and preference for higher elevation within the population or driven by immigration is unclear. We banded adults and nestlings from nest boxes and examined nesting location and elevation for individual birds captured two or more times. Most recaptured birds nested at the same nest boxes in subsequent years, and the number of birds that moved upslope did not significantly differ from the number that moved downslope. Fledglings moved greater distances and elevations than adults, but these movements were not upslope specific. Female fledglings showed greater changes in elevation and distance compared to male fledglings, but again, movements were not consistently upslope. The upslope shift in this population may be due to birds immigrating into the population and not from changes in individual nesting behavior.

A field guide for aging passerine nestlings using growth data and predictive modeling

BackgroundAccurate nestling age is valuable for studies on nesting strategies, productivity, and impacts on reproductive success. Most aging guides consist of descriptions and photographs that are time consuming to read and subjective to interpret. The Western Bluebird (Sialia mexicana) is a secondary cavity-nesting passerine that nests in coniferous and open deciduous forests. Nest box programs for cavity-nesting species have provided suitable nesting locations and opportunities for data collection on nestling growth and development.MethodsWe developed models for predicting the age of Western Bluebird nestlings from morphometric measurements using model training and validation. These were developed for mass, tarsus, and two different culmen measurements.ResultsOur models were accurate to within less than a day, and each model worked best for a specific age range. The mass and tarsus models can be used to estimate the ages of Western Bluebird nestlings 0–10 days old and were accurate to within 0.5 days for mass and 0.7 days for tarsus. The culmen models can be used to estimate ages of nestlings 0–15 days old and were also accurate to within less than a day. The daily mean, minimum, and maximum values of each morphometric measurement are provided and can be used in the field for accurate nestling age estimations in real time.ConclusionsThe model training and validation procedures used here demonstrate that this method can create aging models that are highly accurate. The methods can be applied to any passerine species provided sufficient nestling morphometric data are available.

Sex ratio of Western Bluebirds Sialia mexicana is mediated by phenology and clutch size.

Mothers may produce more of one sex to maximize their fitness if there are differences in the cost of producing each sex or there are differences in their relative reproductive value. Breeding date and clutch size are known to influence offspring sex ratios in birds through sex differences in dispersal, social behaviours, differential mortality, and available food resources. We tested if breeding date, clutch size and drought conditions influenced offspring sex ratios in a sexually size-monomorphic species, the Western Bluebird, by interrogating a 21-year dataset. After controlling for differential mortality, we found that hatch dates late in the breeding season were associated with the production of more females, suggesting that the value of producing males declines as the breeding season progresses. When clutch size was taken into account, small clutches yielded significantly more females late in the breeding season compared to the early and middle parts of the breeding season that produced significantly more males. Large clutches early in the season tended to produce more females, although this was not significant. Drought severity was not correlated with sex ratio adjustment. We propose and discuss several explanations for these patterns, including male offspring, but not female offspring, acting as helpers, increased female nestling provisioning late in the breeding season, differences in food abundance, and egg-laying order. Future work will help to uncover the mechanisms leading to these patterns. Identifying patterns and mechanisms of sex ratio skew from long-term datasets is important for informing predictions regarding life-history trade-offs in wildlife populations.

Eggshell consumption in different reproductive stages and broods of the Western Bluebird, Sialia mexicana

Calcium is an essential micronutrient to egg production and reproductive success in birds. Its availability at the start of the reproductive cycle is especially important to passerines (perching birds, namely songbirds) since they do not store calcium when not reproducing. To date, no data has been published to illustrate whether Western Bluebirds would consume provisioned eggshells, during which phases and to what degree in the reproductive cycle consumption might occur, and whether the amount consumed would vary between broods in a single season. We hypothesized that Western Bluebirds would consume the highest amount of eggshells during nest-building and egg-laying and that consumption would be greater during the first brood. Western Bluebirds breeding in nest boxes hanging in trees were provided eggshells in small containers attached to the roof of the box. Differences in average consumption between reproductive stages were significant. The greatest consumption was seen during the Pre-hatch phase (nest-building, egg-laying, and incubation), and less during the Post-hatch phase (nestling and fledgling). This was likely in preparation for and to replenish calcium needed during egg-laying in both broods. Average consumption between two broods produced by a single breeding pair was not significantly different. However, there was significant variation in the amount of consumption between breeding pairs. This high variation suggests that birds may have different strategies for obtaining and consuming calcium.

Effects of urbanization and nest-box design on reproduction vary by species in three cavity-nesting passerines in the Okanagan Valley, British Columbia, Canada

Artificial nest boxes provide an important resource for secondary cavity-nesting passerines, whose populations may be limited by the availability of nesting sites. However, previous studies have demonstrated that the design and placement of boxes may affect the reproductive success of the birds that use them. In this study, we asked whether the habitat surrounding a nest box or the type of box influenced reproduction in three cavity-nesting passerines. We studied Western Bluebirds (Sialia mexicana Swainson, 1832), Mountain Bluebirds (Sialia currucoides (Bechstein, 1798)), and Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) breeding in artificial nest boxes at sites across 70 km of the Okanagan Valley of British Columbia, Canada. Sites varied in their degree of urbanization, from relatively undisturbed ranchland, to cultivated vineyards, to frequently disturbed “suburban” habitat, and boxes varied in type of entrance (slot or hole). Western Bluebirds nested earlier in vineyards, and Tree Swallows produced significantly fewer fledglings in suburban habitat. In addition, Tree Swallows nested earlier and produced more fledglings in slot boxes. Our results suggest that conservation actions for cavity-nesting passerines may depend on the target species, which in turn should dictate the appropriate box type and habitat when erecting or replacing nest boxes.

Extraterritorial visits in a cooperatively breeding songbird are consistent with multiple functions

Cryptic interactions among individuals in animal societies can be challenging to detect due to their rarity or their covert nature, but they can have important fitness consequences for the individuals involved. Using radiofrequency identification (RFID) technology, we examined the pattern and function of visits to the nests of conspecifics in western bluebirds, Sialia mexicana. Western bluebirds live in kin neighbourhoods year-round and exhibit a low frequency of cooperative breeding. Various functions have been proposed for extraterritorial visits, but few studies explore multiple potential functions. We asked whether extraterritorial visits in western bluebirds are consistent with (1) seeking extrapair copulations (EPCs), (2) prospecting for future nest sites or mates and/or (3) maintaining relationships with kin. We found some evidence supporting all three functions of visits. Postdawn visits made by males to unrelated females were significantly more likely to occur during the period in which males seek within-pair matings than outside this period. We found that visiting a given nestbox or a given potential mate was associated with an increased probability of nesting in that box or with the potential mate the subsequent year, suggesting that some visits are linked to prospecting. Finally, males preferentially visited kin: the proportion of visits to relatives was greater than the expected mean proportion of visits if males visited boxes randomly while travelling the same distance. Our work supports multiple functions of extraterritorial visits and highlights the importance of rare and cryptic behaviours to understanding social behaviour.

Sex bias in parental care is associated with brood age and fledglings’ growth rate in Western Bluebirds Sialia mexicana

Parental care is defined as any behavioral trait of the parents that improves the fitness of the offspring, and that probably evolved or is currently maintained by this function. There is variation in the type and amount of care allocated by parents depending on fluctuation in brood demands that respond to growth and developmental environment. Each sex, within the breeding pair, may respond differently to changing brood demands, and such variation might impact offspring development and fitness. Even though there is a plethora of studies evaluating sex differences in parental care, proximate factors influencing distribution of parental duties within the breeding pairs have received far less attention. In a wild population of Western Bluebirds, we assessed sex differences in parental care (i.e. brood attendance, food deliveries, nest cleaning) through nestling development, and evaluated whether distribution of parental care between sexes is associated with offspring growth. We found that females adjusted their parental behavior according to changes in brood demands more often than males, and fed, attended, and cleaned more than males. Moreover, a more female-biased distribution of care through nestling development was positively associated with the growth of fledglings. We conclude that sex differences in parental care may be a measure of specialization in parental duties within breeding pairs favored by natural selection. These findings are in marked contrast to earlier studies in Northern populations of Western Bluebirds that have failed to detect differences in parental care between sexes, and suggest that differences in the social environment or ecological conditions between populations of the same species might lead to differences in parental cooperation between males and females.

Multiple Environmental Stressors Induce an Adaptive Maternal Effect.

AbstractEvolution of adaptation requires predictability and recurrence of functional contexts. Yet organisms live in multifaceted environments that are dynamic and ever changing, making it difficult to understand how complex adaptations evolve. This problem is particularly apparent in the evolution of adaptive maternal effects, which are often assumed to require reliable and discrete cues that predict conditions in the offspring environment. One resolution to this problem is if adaptive maternal effects evolve through preexisting, generalized maternal pathways that respond to many cues and also influence offspring development. Here, we assess whether an adaptive maternal effect in western bluebirds is influenced by maternal stress pathways across multiple challenging environments. Combining 18 years of hormone sampling across diverse environmental contexts with an experimental manipulation of the competitive environment, we show that multiple environmental factors influenced maternal corticosterone levels, which, in turn, influenced a maternal effect on aggression of sons in adulthood. Together, these results support the idea that multiple stressors can induce a known maternal effect in this system. More generally, they suggest that activation of general pathways, such as the hypothalamic-pituitary-adrenal axis, may simplify and facilitate the evolution of adaptive maternal effects by integrating variable environmental conditions into preexisting maternal physiological systems.

Combined effect of anthropogenic noise and artificial night lighting negatively affect Western Bluebird chick development

Abstract Sensory pollutants such as anthropogenic noise and night lighting now expose much of the world to evolutionarily novel sound and night lighting conditions. An emerging body of literature has reported a variety of deleterious effects caused by these stimuli, spanning behavioral, physiological, population, and community-level responses. However, the combined influence of noise and light has received almost no attention despite the co-occurrence of these stimuli in many landscapes. Here we evaluated the singular and combined effects of these stimuli on Western Bluebird (Sialia mexicana) reproductive success using a field-based manipulation. Nests exposed to noise and light together experienced less predation than control and light-exposed nests, and noise-exposed nests experienced less predation than control nests, yet overall nest success was only higher in noise-exposed nests compared to light-exposed nests. Although exposure to light decreased nestling body condition and evidence was mixed for the singular effects of noise or light on nestling size, those nestlings exposed to noise and light together were smaller across several metrics than nestlings in control nests. Our results support previous research on the singular effects of either stimuli, including potential benefits, such as reduced nest predation with noise exposure. However, our results also suggest that noise and light together can negatively affect some aspects of reproduction more strongly than either sensory pollutant alone. This finding is especially important given that these stimuli tend to covary and are projected to increase dramatically in the next several decades.

Does Vitamin E Supplementation Enhance Growth Benefits of Breeding Helpers at No Oxidative Costs?

During early postnatal development, biomolecules are particularly exposed to the detrimental actions of unneutralized reactive oxygen species. These prooxidant molecules have been claimed to mediate the trade-off between growth and somatic maintenance. Vitamin E is a key exogenous antioxidant that plays an important role in protecting biological membranes against oxidative damage. However, evidence of the effect of vitamin E supplementation during early life on growth and oxidative status in wild populations is equivocal. We tested the effect of supplementing western bluebird nestlings (Sialia mexicana) with vitamin E on growth rate, antioxidant capacity, and oxidative damage to lipids. During the period of accelerated growth (5-8 d), bill growth rate was 21% higher in supplemented nestlings from nests with breeding helpers than in supplemented nestlings from unassisted nests. Vitamin E also boosted tarsus growth rate during the period of slow growth (11-18 d), and this effect was independent of the presence of breeding helpers. Differences in body size and mass, oxidative damage to lipids, and antioxidant capacity were not evident between supplemented and control nestlings at 18 d. Therefore, we conclude that vitamin E promoted faster bill and tarsus growth, but this transient effect disappeared as soon as the supplementation ceased. Our experimental study also supports the idea that tocopherols are rapidly metabolized, since we failed to detect any evident increase of vitamin E in supplemented nestlings at age 18 d. These results provide partial support for the hypothesis that growth rate is constrained by its costs in terms of increased susceptibility to oxidative stress.

Long-term phenology of two North American secondary cavity-nesters in response to changing climate conditions

Wildlife populations can respond to changes in climate conditions by either adapting or moving to areas with preferred climate regimes. We studied nesting responses of two bird species, western bluebird (Sialia mexicana) and ash-throated flycatcher (Myiarchus cinerascens), to changing climate conditions (i.e., rising temperatures and increased drought stress) over 21 years in northern New Mexico. We used data from 1649 nests to assess whether the two species responded to changing climate conditions through phenological shifts in breeding time or shifts in nesting elevation. We also examined changes in reproductive output (i.e., clutch size). Our data show that western bluebirds significantly increased nesting elevation over a 19-year period by approximately 5 m per year. Mean spring temperature was the best predictor of western bluebird nesting elevation. Higher nesting elevations were not correlated with hatch dates or clutch sizes in western bluebirds, suggesting that nesting at higher elevations does not affect breeding time or reproductive output. We did not observe significant changes in nesting elevation or breeding dates in ash-throated flycatchers. Nesting higher in elevation may allow western bluebirds to cope with the increased temperatures and droughts. However, this climate niche conservatism may pose a risk for the conservation of the species if climate change and habitat loss continue to occur. The lack of significant changes detected in nesting elevation, breeding dates, and reproductive output in ash-throated flycatchers suggests a higher tolerance for changing environmental conditions in this species. This is consistent with the population increases reported for flycatchers in areas experiencing dramatic climate changes.

Long-term variation in environmental conditions influences host-parasite fitness.

Long‐term data on host and parasite fitness are important for predicting how host–parasite interactions will be altered in an era of global change. Here, we use data collected from 1997 to 2013 to explore effects of changing environmental conditions on bird–blowfly interactions in northern New Mexico. The objectives of this study were to examine what climate variables influence blowfly prevalence and intensity and to determine whether blowflies and climate variables affect bird fledging success. We examined how temperature, precipitation, and drought affect two parasitic blowflies and their hosts, Western Bluebirds (Sialia mexicana) and Ash‐throated Flycatchers (Myiarchus cinerascens). We found that blowfly prevalence did not change over time. Blowfly intensity increased over time in bluebird nests, but not in flycatcher nests. More blowflies result in slightly higher fledging success in bluebirds, but not flycatchers. There was a significant interaction between blowflies and precipitation on bluebird fledging success. For flycatchers, there was a significant interaction between blowflies and temperature and between blowflies and drought severity on fledging success. Given that the southwest is projected to be hotter and have more frequent and prolonged droughts, we predict that flycatchers may be negatively impacted by blowflies if these trends continue. Future work should focus on investigating the role of both blowflies and climate on fledging success. Climate patterns may negatively impact host fitness through altered parasite pressure.

Correlates of alternative migratory strategies in western bluebirds

Partial migration occurs when only some animals in a population migrate. While evidence suggests that migratory strategies are partially controlled by genes, individual and environmental conditions which alter the cost‐benefit trade‐off of migration among individuals are also likely to play a role. Three hypotheses have been advanced to explain condition‐dependent partial migration: the arrival time, dominance and body size hypotheses. In this study, we asked whether these hypotheses explained differences in migratory strategy among individuals in a partially migratory population of western bluebirds Sialia mexicana breeding in southern British Columbia, Canada. We used stable hydrogen isotope signatures in claw tissue to determine migratory strategy of individual bluebirds, and examined patterns of migration at both individual and population levels. The proportion of resident bluebirds varied significantly over the three years of the study, and across study sites. Several migrants switched to the resident strategy between years; however, we found no evidence of strategy switching in the opposite direction. Young birds were significantly more likely to be resident than older birds, a pattern which could arise if early arrival is particularly important for birds obtaining a territory for the first time. Furthermore, young females were the most likely of all sex–age classes to be resident, which may reflect a survival advantage of residency for young females. Finally, birds mated assortatively by migratory strategy and isotopic evidence suggests that members of a pair often wintered in the same place. Our results provided no support for the dominance or body size hypotheses, and only limited support for the arrival time hypothesis in bluebirds. However, taken together, we suggest that our findings indicate that social factors may influence migratory strategies in this system.