Sialia Mexicana Research Articles

Western Bluebird Captures a Western Fence Lizard

I observed a male Western Bluebird (Sialia mexicana) capture a western fence lizard (Sceloporus occidentalis), the first reported incidence of vertebrate-directed raptorial behavior in the Western Bluebird. There are no previously published reports of Western Bluebirds capturing vertebrate prey, although there is one previous report of a Western Bluebird carrying an unidentified lizard in the manner of a prey item, and a few reports of predation on vertebrates by the congeneric Eastern Bluebird (Sialia sialis).

Interspecific Egg-dumping by a Violet-green Swallow in an Active Western Bluebird Nest

Abstract We observed a Violet-green Swallow (Tachycineta thalassina) laying an egg in an active Western Bluebird (Sialia mexicana) nest. The Western Bluebird male and not the female, was aggressive to the Violet-green Swallow but the swallow remained to lay the egg. This is the first documented incidence of which we are aware involving altricial interspecific egg-laying during the nestling phase. We suggest the timing of this event was more consistent with incidental egg deposition, or egg-dumping, than brood parasitism or nest usurpation.

An economical wireless cavity-nest viewer

Inspection of cavity nests and nest boxes is often required during studies of cavity-nesting birds, and fiberscopes and pole-mounted video cameras are sometimes used for such inspection. However, the cost of these systems may be prohibitive for some potential users. We describe a user-built, wireless cavity viewer that can be used to access cavities as high as 15 m and with entrance diameters as small as 2.5 cm. System components include a wireless camera, boom with near-infrared lighting system, hand-held television, and telescoping pole. The unit can be assembled in about one day with commonly available tools, and the total cost was about $850 (US). Our cavity-viewing system was used to monitor the nests of Western Bluebirds (Sialia mexicana) and proved to be reliable and effective. During two field seasons, we attempted to check the status of nests 928 times and were able to determine the nesting stage 901 times (97%). For those willing to assemble their own units, our cavity viewer permits safe, direct examination of cavity nests or open nests for investigators with limited budgets. SINOPSIS. Aparato inhalambrico de bajo costo para observar nidos en cavidades Para examinar nidos en cavidades y cajas de anidamiento, muchas veces es necesario usar instrumentos de fibra ´ optica y cde videos montadas en postes. Sin embargo, el costo de estos sistemas pudiera ser muy elevado para algunos investigadores potenciales. Describimos un aparato inhalque puede ser utilizado para examinar cavidades de hasta 15 m de altura y de 2.5 cm de dien su entrada. Entre los componentes del sistema encontramos una cinhalun sistema de iluminaci´ on infrarojo, un monitor de televisi´ on manual y un poste telescEl sistema puede ser montado en un dia (si hay las herramientas necesarias) y su costo es de $850 dolares. Nuestro sistema fue utilizado para monitorear nidos de Sialia mexicana yp rov´ o ser confiable y efectivo. Durante dos temporadas de campo intentamos determinar el estatus de nidos en 928 ocasiones y pudimos determiner la etapa en que se encontraban en 901 (97%) de estas. Para aquellos que est´ en en la disposicide ensamblar sus propias unidades de monitoreo, nuestro equipo es adecuado y se presta para estudios con presupuesto limitado y le permite al investigador hacer observaciones directas con seguridad.

Nest Densities of Cavity-Nesting Birds in Relation to Postfire Salvage Logging and Time Since Wildfire

Abstract Abstract We monitored the nest densities and nest survival of seven cavity-nesting bird species, including four open-space foragers (American Kestrel [Falco sparverius], Lewis's Woodpecker [Melanerpes lewis], Western Bluebird [Sialia mexicana], and Mountain Bluebird [S. currucoides]) and three wood-foragers (Hairy Woodpecker [Picoides villosus], Black-backed Woodpecker [P. arcticus], and Northern Flicker [Colaptes auratus]), after two wildfires (one partially salvage-logged and one unlogged) in western Idaho from 1994–2004. We estimated the relationship between nest density and time since fire, tested for statistical differences in nest densities and nest survival in the partially salvage-logged vs. unlogged wildfires, and tested for differences in nest survival between early (1–4 years after fire) and late (5–12 years after fire) postfire periods. Nest densities of open-space foragers and Northern Flickers generally increased with time since fire, whereas nest densities of Black-backed and Hairy Woodpeckers peaked 4–5 years postfire. Nest densities of wood-foraging species and Mountain Bluebirds were significantly higher in the unlogged burn, whereas Lewis's Woodpeckers had significantly higher nest densities in the partially logged burn. Kestrels tended to favor the partially logged burn, while Western Bluebird nest densities were nearly equal in both burned areas. For most species, postfire period and treatment (partially logged vs. unlogged) had little influence on nest survival. However, Hairy Woodpecker nest survival was significantly lower in the partially logged burn versus the unlogged wildfire in the early postfire period, and Lewis's Woodpecker nest survival was significantly reduced in the later postfire period versus the early postfire period in the partially logged burn. Importantly, the salvage logging was designed to retain more than half of the snags over 23 cm in diameter, which provided suitable nesting habitat for open-space foragers during the decade following fire.

NEST DENSITIES OF CAVITY-NESTING BIRDS IN RELATION TO POSTFIRE SALVAGE LOGGING AND TIME SINCE WILDFIRE

Abstract ABSTRACT We monitored the nest densities and nest survival of seven cavity-nesting bird species, including four open-space foragers (American Kestrel [Falco sparverius], Lewis's Woodpecker [Melanerpes lewis], Western Bluebird [Sialia mexicana], and Mountain Bluebird [S. currucoides]) and three wood-foragers (Hairy Woodpecker [Picoides villosus], Black-backed Woodpecker [P. arcticus], and Northern Flicker [Colaptes auratus]), after two wildfires (one partially salvage-logged and one unlogged) in western Idaho from 1994–2004. We estimated the relationship between nest density and time since fire, tested for statistical differences in nest densities and nest survival in the partially salvage-logged vs. unlogged wildfires, and tested for differences in nest survival between early (1–4 years after fire) and late (5–12 years after fire) postfire periods. Nest densities of open-space foragers and Northern Flickers generally increased with time since fire, whereas nest densities of Black-backed and Hairy...

PARTITIONING RISK AMONG DIFFERENT CAUSES OF NEST FAILURE

Abstract Nest predation and nest parasitism receive the most attention as causes of nest failure for North American songbirds. Yet for many populations, interspecific competition, adverse weather, abandonment, nestling starvation, and egg failure may also be significant causes of nest failure. Despite the long interest in differential failure, serious challenges remain in the estimation of separate probabilities of nest failure from different causes. Apparent rates of failure suffer from at least two sources of bias: heterogeneous ages at discovery and classification error. We developed maximum-likelihood estimators for cause-specific daily probabilities of nest failure. We further show how the estimators can be extended to include classification error, if known. Finally, we demonstrate a simple application to Loggerhead Shrikes (Lanius ludovicianus), Tree Swallows (Tachycineta bicolor), Violet-green Swallows (T. thalassina), and Western Bluebirds (Sialia mexicana). Daily probabilities of survival were lo...

Forest Stand Characteristics Altered by Restoration Affect Western Bluebird Habitat Quality

Abstract Forest managers are setting Ponderosa pine (Pinus ponderosa) forests in the southwestern United States on a trajectory toward a restored ecosystem by reducing tree densities and managing with prescribed fire. The process of restoration dramatically alters forest stands, and the effects of these changes on wildlife remain unclear. Our research evaluated which aspects of habitat alteration from restoration treatments may be affecting the habitat quality of Western Bluebird (Sialia mexicana), an insectivorous songbird whose populations are declining. Habitat loss resulting from fire‐suppression activities may be partially responsible for their population declines; thus, the bluebird is a good representative species for assessing how the reconstruction of presuppression forest conditions can affect wildlife. We measured habitat variables at 63 successful and 19 unsuccessful Western Bluebird nests in 1999–2001 and 2003. We compared habitat models that represented bluebird biology and habitat changes from restoration. Two models of nest success that included (1) an increased herbaceous and bare ground cover and (2) increased Gambel oak (Quercus gambelii) densities and reduced Ponderosa pine densities were most supported by the data. Increased herbaceous ground cover and Gambel oak density likely represent improved invertebrate assemblages and thus improved forage abundance for nesting bluebirds. Lower Ponderosa pine densities may provide bluebirds with open perches from which to hunt and thereby improve the availability of invertebrates as a food source. We also provide a landscape‐scale example of changes to bluebird habitat quality from treatments, which we recommend as a useful tool in restoration planning.

NESTING SUCCESS OF WESTERN BLUEBIRDS (SIALIA MEXICANA) USING NEST BOXES IN VINEYARD AND OAK-SAVANNAH HABITATS OF CALIFORNIA

Abstract Loss of oak woodlands to vineyard development in California is a growing concern to conservationists. Analyzing breeding performance of birds that nest in and around vineyards versus those that nest in nearby native habitat can provide information on the suitability of vineyard environments to birds. We placed predator-protected nest boxes in vineyard and oak-savannah habitats and monitored nest-box occupancy, nesting success, and life history characteristics of Western Bluebirds (Sialia mexicana) that used the boxes. Western Bluebirds were common occupants in both habitats, occupying >50% of available nest boxes. Analysis using program MARK revealed that nest survival was not associated with habitat type; however, clutch size was greater and nests were initiated earlier in vineyard than in oak-savannah habitat. Our results suggest that when naturally occurring nest sites are limiting, vineyards could be converted to good breeding habitat for Western Bluebirds with the addition of nest boxes. Nes...

Behavioral correlations across breeding contexts provide a mechanism for a cost of aggression

Identifying correlations among behaviors is important for understanding how selection shapes the phenotype. Correlated behaviors can indicate constraints on the evolution of behavioral plasticity or may reflect selection for functional integration among behaviors. Obligate cavity-nesting birds provide an opportunity to examine these correlations because males must defend limited nest cavities while also competing for mating opportunities and providing parental care. Here, I investigated the role of behavioral correlations in producing a counterintuitive relationship between nest defense and reproductive success in western bluebirds (Sialia mexicana) such that males that defended their nests most intensely had the lowest reproductive success, measured as the number of within and extrapair offspring that fledged. By experimentally measuring aggression across contexts, I show that this cost of nest defense was due to the correlated expression of aggression across the contexts of nest defense and male‐male competition coupled with a trade-off between male‐male aggression and parental care. In particular, more aggressive males provisioned their females less during incubation and this led to disrupted incubation patterns and fewer fledged offspring. However, aggressive males did not benefit from avoiding parental investment by gaining extrapair fertilizations, and thus, it is unclear how high levels of aggression are maintained in this population despite apparent costs. These results suggest that there are constraints to the evolution of plasticity in aggression and emphasize the importance of considering the integrated behavioral phenotype to understand how variation in behavior is linked to fitness. Key words: aggression, behavioral syndrome, fitness cost, parental care, Sialia mexicana. [Behav Ecol]

Aggressive behaviour affects selection on morphology by influencing settlement patterns in a passerine bird

The importance of behaviours as instigators or inhibitors of evolutionary change remains largely unresolved and this is in part because there are very few empirical examples of how behaviours affect evolutionary processes. By determining the environment of breeding, aggressive interactions over territories have the potential to strongly impact selection pressures experienced by individuals. Western bluebirds (Sialia mexicana) provide a unique opportunity to investigate the evolutionary importance of aggression, since their highly variable breeding habitat favours distinct foraging techniques and they also compete aggressively for nest boxes, a resource that is easy to manipulate. Here, I show experimentally that more aggressive males compete more effectively for territories with a high density of nest boxes and, as a consequence, aggressive and non-aggressive males are sorted into distinct breeding habitats that differ in the strength of selection on morphological traits. Specifically, males with longer tails and tarsi were favoured in open habitats where high agility is required to forage efficiently, whereas in forested habitats, where agility is less important, selection was weak. These results show that aggression can affect selection on a local scale by determining individual settlement patterns. More generally, because territorial interactions are important across a wide variety of taxa, these results suggest that aggressive behaviour has the potential to impact the evolutionary trajectory of many animal populations.

Winter resource wealth drives delayed dispersal and family-group living in western bluebirds

Delayed dispersal, where offspring remain with parents beyond the usual period of dependence, is the typical route leading to formation of kin-based cooperative societies. The prevailing explanations for why offspring stay home are variation in resource wealth, in which offspring of wealthy parents benefit disproportionately by staying home, and nepotism, where the tendency for parents to be less aggressive and share food with offspring makes home a superior place to wait to breed. These hypotheses are not strict alternatives, as only wealthy parents have sufficient resources to share. In western bluebirds, Sialia mexicana, sons usually delay dispersal until after winter, gaining feeding advantages through maternal nepotism in a familial winter group. Experimentally reducing resource wealth (mistletoe) by half on winter territories caused sons to disperse in summer, even though their parents remained on the territory during the winter. Only 8% of sons remained with their parents on mistletoe-removal territories compared to 50% of sons on control territories (t(9,10)=3.33, p<0.005). This study is the first to demonstrate that experimentally reducing wealth of a natural food resource reduces delayed dispersal, facilitating nepotism and family-group living. The results clarify the roles of year-round residency, resource limitation and relative wealth outside the breeding season in facilitating the formation of kin-based cooperative societies.

Facultative sex ratio adjustment by western bluebird mothers with stay-at-home helpers-at-the-nest

Although population sex ratios rarely deviate from unity, the sex ratio of individual mothers should be labile, allowing them to bias their sex ratios in favour of the more successful sex when they can expect a difference in the mean reproductive value of their daughters and sons. Just how mothers should bias sex allocation is particularly complicated in cooperative breeders, because the adaptive sex ratio may be influenced by conflicting impacts of helpers on their parents' fitness via ‘local resource enhancement’ and ‘local resource competition’. In western bluebirds, Sialia mexicana, breeding-age helpers-at-the-nest are exclusively male and increase their parents' nesting success in the current year. As such, they are viewed as helpers, not hinderers, even though by failing to gain a breeding position they reduce their parents' annual inclusive fitness below that of parents with a breeding son. In this study, I tested the hypothesis that nonbreeding, stay-at-home helpers indicate poor breeding prospects for locally competing sons, and investigated one prediction of this hypothesis that such helpers also indicate a tendency for parents to produce substandard sons. A female removal experiment showed that female mates were in short supply. Pairs had daughter-biased broods when they had a stay-at-home helper son that failed to get a mate in the first place, but not when they had a helper that had been breeding on his own, but returned home to help sometime after his mother finished laying her eggs. Stay-at-home sons were not developmentally delayed relative to breeders of the same cohort, suggesting that helpers may not be inferior to breeders. Although the population sex ratio fluctuates about unity, a multiyear data set showed a negative relationship between the frequency of helping and the annual brood sex ratio for the population. These results suggest that local resource competition outweighs local resource enhancement in driving individual and population sex ratio variation in western bluebirds, a pattern that would not be predicted based on the simple question of whether helpers help or hinder at the nest.

A test of the importance of direct and indirect fitness benefits for helping decisions in western bluebirds

In cooperative breeders, the extent to which helpers at the nest adjust their contributions in accordance with direct and indirect (kin-selected) fitness payoffs remains an open question. In a long-term study of the western bluebird, Sialia mexicana, helpers were exclusively male and helped at nests of both parents, a parent and stepparent, or a brother and unrelated female. This natural variation in the context of helping facilitated comparison of observational data on groups in which one type of fitness benefit (current direct, future direct, or indirect) varied, whereas the other two were constant. Helpers reduced their share of provisioning as they got older, so comparisons were restricted to groups with yearling helpers. When potential direct fitness benefits were identical, but relatedness was reduced by half owing to the presence of a stepparent, yearling helpers failed to reduce their share of feeding trips to the nest. The potential for future direct fitness benefits via possible mate and territory inheritance was low, and did not influence the helper’s share of provisioning in a comparison of groups with similar relatedness and opportunities for current direct fitness benefits. Even though cobreeding to gain current direct fitness benefits was infrequent (17% of nests with brother-helpers), it was associated with an increase in the helper’s share of provisioning, suggesting that a helper’s feeding allocation responds positively to increased opportunity for parentage in the nest. The current study demonstrates a useful framework for separating direct and indirect benefits with respect to helping decisions, and indicates that western bluebird helpers adjust their feeding rates in response to the potential for direct fitness benefits in the current nest, not indirect benefits or future direct fitness payoffs. Although past studies of this population showed that indirect benefits play a role in whether or not helpers help, the current study indicates that they do not play a role in how frequently helpers feed at the nest.

Life-History Variation and Demography in Western Bluebirds (Sialia Mexicana) in Oregon

Abstract During the seven-year period from 1995 to 2001, we conducted a detailed, demographic study of a population of Western Bluebirds (Sialia mexicana) in Oregon. Using a data set consisting of individual capture histories for ≈8,000 birds, we estimated age-specific and study-year specific survival estimates of adult males and females. High juvenile recruitment enabled us to generate accurate estimates of juvenile survival from fledging to first breeding attempt. We analyzed fecundity patterns, including nesting phenology, clutch success, and overall reproductive success for nearly 1,200 nesting attempts. Temporal variation in survival and fecundity had strong demographic consequences. Asymptotic population growth analysis suggested a slowly growing population. However, variation in vital rates led to population declines even over the relative short timescale of our study, making the conservation status of this species uncertain. In addition to those results, we discuss the costs and benefits of using data collected by volunteers to study demographic processes.

Life-History Variation and Demography in Western Bluebirds (Sialia mexicana) in Oregon

Life-History Variation and Demography in Western Bluebirds (Sialia mexicana) in Oregon

LIFE-HISTORY VARIATION AND DEMOGRAPHY IN WESTERN BLUEBIRDS (SIALIA MEXICANA) IN OREGON

Abstract During the seven-year period from 1995 to 2001, we conducted a detailed, demographic study of a population of Western Bluebirds (Sialia mexicana) in Oregon. Using a data set consisting of individual capture histories for ≈8,000 birds, we estimated age-specific and study-year specific survival estimates of adult males and females. High juvenile recruitment enabled us to generate accurate estimates of juvenile survival from fledging to first breeding attempt. We analyzed fecundity patterns, including nesting phenology, clutch success, and overall reproductive success for nearly 1,200 nesting attempts. Temporal variation in survival and fecundity had strong demographic consequences. Asymptotic population growth analysis suggested a slowly growing population. However, variation in vital rates led to population declines even over the relative short timescale of our study, making the conservation status of this species uncertain. In addition to those results, we discuss the costs and benefits of using ...

IMMUNE AND GROWTH RESPONSE OF WESTERN BLUEBIRDS AND ASH‐THROATED FLYCATCHERS TO SOIL CONTAMINANTS

During the past two decades, a critical need has developed to determine how exposure to contaminants in the environment affects individual and population processes. In this study, the immunocompetence of Western Bluebirds (Sialia mexicana) and Ash‐throated Flycatchers (Myiarchus cinerascens) was studied on a landscape–soil contaminant gradient at Los Alamos National Laboratory (LANL) in New Mexico during 1997–1999. A variety of contaminants (heavy metals, chemicals, insecticides, polychlorinated biphenyls, and radioactive isotopes) range across different spatial scales and concentrations on LANL land. The two species have similar life‐history traits, except that the Ash‐throated Flycatcher has a faster rate of development and fledges 4–5 days earlier than the bluebird. The number of active nest boxes increased from 1997 to 1998 and 1999 for the Western Bluebird, but not for the Ash‐throated Flycatcher. Survival of nestling flycatchers was lower in areas within 60 m of a potential contaminant release site, with a higher survivorship function associated with boxes farther away. The two species did not differ in their response to antigens, and there was no difference between locations for immunocompetence for either species. Flycatcher nestlings had a higher average cell‐mediated response than bluebird nestlings, as predicted by the faster rate of development of the flycatchers. Phytohemagglutinin response varied between locations for both species. The cell‐mediated effects were dynamic in that, in general, the same locations showed similar patterns for each year. Hematocrits steadily increased with age for both species and varied between locations for the bluebird, but not the flycatcher.

Male share of provisioning is not influenced by actual or apparent loss of paternity in western bluebirds

Approximately 45% of western bluebird (Sialia mexicana) females have some chicks in the nest that are not sired by their social mates. Extrapair fertilizations account for 42% of offspring in these nests and 19% of nestlings overall. I tested the hypothesis that males reduce nestling provisioning when their certainty of paternity or share of paternity is reduced. Capture and detention of socially monogamous males for 1 h or 24 h during the laying period reduced males' copulatory access and their ability to mate guard, increasing the frequency with which extrapair males intruded and attempted to copulate with resident females. Males detained during laying did not reduce their share of feeding trips compared to control males detained during incubation, compared to unmanipulated males, or compared to males that were captured but not detained. Males detained on territory for 1 h during the laying period did not reduce their share of feeding trips when they observed male intrusion, nor when they observed their mates accepting extrapair copulations. Males that witnessed their mates accepting extrapair copulations did not reduce their share of risk in provisioning. Genetic fingerprinting at nonexperimental nests indicated that males also failed to reduce their feeding contributions when their estimated share of paternity was reduced, even when a helper male was present to reduce the impact on nestlings. These results suggest that male western bluebirds do not make significant adjustments in their share of provisioning when they have evidence of partial paternity loss. Together with prior results, this study suggests that western bluebird males use an all-or-none rule, contributing approximately half of the parental provisioning at nests, as long they have some copulatory access to the female during egg laying.

Natal dispersal and personalities in great tits (Parus major).

Dispersal is a major determinant of the dynamics and genetic structure of populations, and its consequences depend not only on average dispersal rates and distances, but also on the characteristics of dispersing and philopatric individuals. We investigated whether natal dispersal correlated with a predisposed behavioural trait: exploratory behaviour in novel environments. Wild great tits were caught in their natural habitat, tested the following morning in the laboratory using an open field test and released at the capture site. Natal dispersal correlated positively with parental and individual exploratory behaviour, using three independent datasets. First, fast-exploring parents had offspring that dispersed furthest. Second, immigrants were faster explorers than locally born birds. Third, post-fledging movements, comprising a major proportion of the variation in natal dispersal distances, were greater for fast females than for slow females. These findings suggest that parental behaviour influenced offspring natal dispersal either via parental behaviour per se (e.g. via post-fledging care) or by affecting the phenotype of their offspring (e.g. via their genes). Because this personality trait has a genetic basis, our results imply that genotypes differ in their dispersal distances. Therefore, the described patterns have profound consequences for the genetic composition of populations.

Forest Restoration Treatment Effects on the Nesting Success of Western Bluebirds (Sialia mexicana)

AbstractWe examined the effects of presettlement forest restoration treatments on the nesting success of Western Bluebirds in ponderosa pine forests of northwestern Arizona, U.S.A. From 1998 to 2001 we monitored 97 active Western Bluebird nests, 41 in current‐condition untreated forest and 56 in restoration‐treated forest. We found no effect of restoration treatments on clutch size and little effect on the number of nestlings per nest. However, in treated forest stands number of fledglings per nest averaged 1.6 times greater, and probability of a nest surviving to successfully fledge at least one young was up to 4.2 times greater than in untreated forest. Probability of a nest succeeding averaged 0.39 ± 0.11 (SE) and 0.75 ± 0.06 from 1999 to 2001 in untreated and treated forests, respectively. In addition, in treated forest, average number of nests infested with the blowfly parasite Protocalliphora sialia was up to 4.3 times greater, and number of parasites per fledgling was up to 10.7 times greater than in untreated forest. Overall, the data suggest that in treated forest Western Bluebirds have a higher probability of successfully fledging young, but they are at greater risk of parasitic infestations, of which the ultimate effects on post‐fledging survival are unknown.