Sialia Mexicana Research Articles

The ecological and physiological costs of lead shot and immunological challenge to developing western bluebirds.

We investigated the interacting effects of a nonpathogenic immunological challenge and exposure to lead shot early in the development of nestling western bluebirds (Sialia mexicana). Nestlings were randomly assigned to each of six treatments of an incomplete block design with two antigen treatments, Newcastle disease virus (NDV) or sheep red blood cells (SRBC), and four lead shot treatments (no shot, one shot--0.05 g, 2 shots--0.1 g, or three shots--0.15 g). Survival functions did not differ between the lead treatment groups, and there were no effects of lead treatments on weight, growth rates, fluctuating asymmetry (FA), or antibody response. NDV- and SRBC-treated birds survived better than control birds which may be due to an adjuvant-activation of the entire immune system. However, FA was greater in individuals in the NDV and SRBC treatment groups, suggesting a tradeoff between growth and immunocompetence. Cell-mediated response to phytohemagglutinin of the high-lead treatment groups was significantly less than other groups. Hematocrit increased with age and weight, and was not affected by lead or antigen. While in this study the shape of the growth curve, FA, cell-mediated immunity, and behavior were affected by the higher dose of lead shot, actual lead concentrations in blood are needed to verify this process. Antigenic stress was documented in nestling developmental stability and there were no measured effect of both the lead shot and immunological challenge in combination.

Early reproductive success of western bluebirds and ash-throated flycatchers: a landscape-contaminant perspective

Eggshell quality, clutch size, sex ratio, and hatching success of western bluebirds ( Sialia mexicana) and ash-throated flycatchers ( Myiarchus cinerascens) were studied on a landscape-soil contaminant gradient at Los Alamos National Laboratory (LANL) in New Mexico from 1997 to 1999. A variety of contaminants (heavy metals, chemicals, insecticides, polychlorinated biphenyls (PCBs), organochlorines, and radioactive isotopes) range across different spatial scales and concentrations on LANL land. This study is an example of a monitoring program over a large area with varying degree of contamination that is used to highlight locations of concern for future research. There were two locations where the flycatcher had a lower hatching success. The bluebirds at Sandia wetland, a location of concern for PCBs, had a thinner eggshell thickness index (RATCLIFFE) and the eggs were smaller than at other locations. The flycatcher had thinner eggshells than bluebirds, which could add to sensitivity to exposure to contaminants. There was no variation in clutch size or sex ratio between locations or areas closer to contaminant release sites for both species. Percent females in the clutch ranged from 0 to 100% in the WEBL and from 33 to 67% for ATFL.

Extrapair copulations in western bluebirds ( Sialia mexicana ): female receptivity favors older males

Extrapair paternity has been the focus of numerous studies of socially monogamous birds, but the dynamics of extrapair male-female interactions have been explored in relatively few. In western bluebirds (Sialia mexicana), a species with 20% extrapair paternity, two to four observers monitored extrapair copulations (EPCs) occurring on the female's home territory while her social mate was detained in a cage near the nest. Detention elevated the frequency and visibility of EPCs, so we could score receptivity of females to identifiable extrapair males without the interference of intermale aggression. Most intruding extrapair males were neighboring breeders visiting outside their own mate's fertile period. Local male relatives of the detained male attempted nonincestuous EPCs, but neighboring sons did not attempt to copulate with their mothers and, in some cases, chased away intruders. Female receptivity was not influenced by whether the detained mate was visually present or hidden behind cloth and did not increase with the persistence or relative size of the extrapair male. Female receptivity increased with the age difference between the extrapair male and the female's social mate, suggesting that females prefer older males as extrapair partners. This study suggests that costs of resistance are relatively unimportant in determining female receptivity in western bluebirds, but identifies preferential receptivity to older males as a potential factor in the outcome of inter- and intrasexual conflict over paternity.

Enteritis as a Cause of Mortality in the Western Bluebird (Sialia mexicana)

Increased mortalities in adult western bluebirds utilizing nestboxes were noted in western Oregon during 1998 and 1999. A necrohemorrhagic enteritis was found in 8 of 10 birds submitted for necropsy. Acanthocephalan parasites were present in four of eight birds with enteritis. Microscopic changes consistent with necrotic or ulcerative enteritis were commonly present. Anaerobic culture of the intestine yielded Clostridium perfringens in three of three birds. Genotype analysis of two of these isolates revealed them to be C. perfringens type A. Bacterial enteritis is believed to be the cause of the increased mortality rate, but further investigation is required to prove a definitive link to a clostridial agent.

Family-based winter territoriality in western bluebirds, Sialia mexicana: the structure and dynamics of winter groups

Winter residency is characteristic of the majority of cooperatively breeding birds, but the composition and dynamics of winter groups have been examined in relatively few. In 1996–1998, we examined winter territoriality in the western bluebird, a year-round resident that shows a limited degree of helping behaviour in central coastal California, U.S.A. In spring, most western bluebirds breed as socially monogamous pairs, but a small proportion of pairs (3–16%) have additional breeding-age males helping at the nest, usually assisting parents or brothers. We found that year-round residents commonly wintered in family groups that defended territories similar to those used in spring. Winter groups had an even sex ratio and formed early in the autumn, when hatch-year birds dispersed. More females than males left their natal groups to be replaced by an influx of immigrant hatch-year birds. Winter groups typically consisted of breeders and one or two sons from the prior breeding season along with one or more immigrant females. A second period of dispersal occurred in spring when winter groups broke up and most birds other than the breeding pair left the winter territory. When they bred, yearling males and females often bred with unrelated individuals from their winter groups. Sons were more likely to remain on the study area as yearlings when they wintered with both parents than when they wintered with just one parent. We suggest that young males stay the winter due to benefits of remaining in family groups on mistletoe-based winter territories. Subsequent localized dispersal of sons then leads to opportunistic kin-based interactions later in life.

Replacement males in the western bluebird: opportunity for paternity, chick-feeding rules, and fitness consequences of male parental care

In western bluebirds (Sialia mexicana), most pairs remain together for life and share equally in post-hatching parental care. We removed resident males of socially monogamous pairs during laying and after clutch completion to examine chick-feeding rules used by replacement males and current and future fitness consequences of paternal care. Replacement males were not infanticidal and 7 (47%) fed nestlings. Feeding replacement males and the females they joined fed at rates similar to controls. Females without a feeding replacement male compensated by feeding more themselves so that overall feeding rates were not compromised, but they reduced their brooding time. Unlike assisted females, unassisted females exhibited reduced nesting success and their 14-day-old chicks weighed less than controls. Field metabolic rates of unassisted females were 17% higher than those of control females, but the difference was not statistically significant. Older females were better able to raise young without the male's help than were yearling females. Female condition was not affected by male provisioning and unassisted females were as likely to survive to the next breeding season as assisted females. We found no future benefits of provisioning by replacement males; those that fed were no more likely to breed with the female on her subsequent attempt than were males that did not feed, and subsequent clutch sizes were not reduced for females rearing young without the male's help. These experiments suggest that male parental care increases nesting success in western bluebirds and that replacement males use an all-or-none rule to determine whether or not to feed chicks: if they are present during the fertile period they feed at typical rates; if they are not, they usually do not feed at all. Because chick-feeding by males is tied to opportunity for paternity, influences success in the current nest, and does not affect the male's future breeding success, it appears to be parental rather than mating effort.

Extrapair paternity, inclusive fitness, and within‐group benefits of helping in western bluebirds

In central coastal California, USA, 3–16% of western bluebird (Sialia mexicana) pairs have adult male helpers at the nest. Demographic data on a colour‐ringed population over a 13‐year period indicate that helpers gain a small indirect fitness benefit through increases in the number of young fledged from nests of close kin. A small proportion of adult helpers (16%) that were able to breed and help simultaneously had higher annual inclusive fitness than males that only bred. These males comprised such a minor proportion of helpers that the mean fitness of helpers was still lower than the mean fitness of independent breeders. We used DNA fingerprinting to determine whether extrapair fertilizations alter within‐group benefits enough to tip the balance in favour of helping behaviour. Overall, 19% of 207 offspring were sired by males other than their social father and extrapair fertilizations occurred in 45% of 51 nests. Intraspecific brood parasitism was rare so that mean mother‐nestling relatedness approximated the expected value of 0.5. Extrapair paternity reduced putative father‐offspring relatedness to 0.38. Mean helper‐nestling relatedness was 0.41 for helpers assisting one or both parents and 0.28 for helpers aiding their brothers. Helpers rarely sired offspring in the nests at which they helped. Helping was not conditional on paternity and helpers were not significantly more closely related to offspring in their parents’ nests than to offspring in their own nests. Although helpers may derive extracurricular benefits if helping increases their own or their father's opportunities for extrapair fertilizations, within‐nest inclusive fitness benefits of helping do not compensate males for failing to breed. Breeding failure and constraints on breeding are the most likely explanations for why most helpers help.

A Comparison of the Breeding Ecology of Birds Nesting in Boxes and Tree Cavities

ABSTI~ICT.-We compared laying date, nesting success, clutch size, and productivity of four bird species that nest in boxes and tree cavities to examine whether data from nest boxes are comparable with data from tree cavities. Western Bluebirds (Sinlin nrexicnnn) gained the mast advantage from nesting in boxes. They initiated egg laying earlier, had higher nesting success, lower predation rates, and fledged marginally more young in boxes than in cavities but did not have laracr clutches or hatch more eggs. Plain Titmice (Pnnis iriorrmtrrs) nesting earlier in boxes. ~oise Wrens (~roglo&tes ~eiion) nesting in boxes laid larger clutches, hatched more eggs, and fledged mare young and had marginally higher nesting success and lower predation rates. Ash-throated Flycatchers (Myinrclirrs cincmscens) experienced no apparent benefits from ncsting in bows versus cavities. No significant relationships were found between clutch size and bottom area or volume of cavities for any of these species. These results suggest that researchers should use caution when extrapolating results from nestbox studies of reproductive success, predation rates, and productivity of cavity-nesting birds. Given the different responses of these four species to nesting in boxes, the effects of the addition of nest boxes on community structure also should be considered. Rrcrived 19 September 1996, nccepted 21 April 1997. NEST BOXES MAY BE USED AS A CONSERVATION

Male detention affects extra-pair copulation frequency and pair behaviour in western bluebirds

Male western bluebirds,Sialia mexicanashow patterns of mate following consistent with mate guarding. The hypothesis that males prevent extra-pair copulations by guarding their mates was tested using 1-h male detention experiments. Males were caged on territory in view of the female, visually occluded on territory or removed from the territory altogether (visually+vocally occluded). Extra-pair males were more likely to join females within 15m and attempt to copulate with them in the three detention treatments than in treatments where males were captured and released or not manipulated at all. Detention also increased the number of females accepting one or more extra-pair copulations. The increased frequency of extra-pair copulations was not caused by movement of females away from their nests, nor by increased rates of vocalization by resident males or females. Extra-pair events occurred shortly after the resident male was caught, suggesting that extra-pair copulations can occur during brief lapses in mate following. Intruding males were usually neighbours with mates and nests of their own. Half intruded when their own mates were at peak receptivity, suggesting that males leave their own fertile mates to seek extra-pair copulations. Detained males did not compensate for detention by increasing the frequency with which they copulated with their mates after they were returned to the territory. Acceptance of extra-pair copulations by females did not increase with the number of extra-pair copulation attempts and females were no more likely to participate in extra-pair copulations when resident males were visually occluded than when they were visually present. Only half of the females that experienced extra-pair copulation attempts accepted one or more extra-pair copulations. Although male detention resulted in an increase in the frequency of extra-pair copulations in western bluebirds, females appeared to be selective about participating in extra-pair matings.

Mate attendance and copulatory behaviour in western bluebirds: evidence of mate guarding

The western bluebird, Sialia mexicana, is a socially monogamous passerine with extra-pair offspring in over one-third of nests. Copulatory behaviour and mate attendance were observed for 5 years to determine whether males protect their paternity by mate guarding. Western bluebirds copulated frequently but the frequency of copulation did not increase in the hour after laying, when fertilization is thought to occur. Males attempted to copulate with their mates at a steady rate from 60 days before laying until the onset of incubation. Female acceptance of within-pair copulations increased to 80% 10 days before laying and remained high throughout the laying period. Proximity of paired males and females corresponded with changes in the frequency of within-pair copulation attempts, but following of females by their mates corresponded with peak receptivity of females to copulation. Neither proximity nor following corresponded with temporal changes in the rate ofcourtship feeding. Pairs were closer and following of the female more frequent when nearest neighbour distances were reduced, as expected with mate guarding if the threat of extra-pair copulation increases with local density. Pairs were also closer when vegetative cover around the nest increased, suggesting that one function of mate attendance may be to keep the female in view. Male western bluebirds may ensure their paternity by thwarting extra-pair copulations and by increasing copulatory access; however, paternity assurance provides a more comprehensive explanation than copulatory access for spatial and temporal variation in associative behaviours of pairs.

Nestling Sex-Ratio Variation in Western Bluebirds

ABsTRAcr.-We tested five hypotheses for facultative manipulation of sex ratios in a population of Western Bluebirds (Sialia mexicana) studied for 13 years in California. Based on 2,187 nestlings from 549 broods, the overall mean (? SE) proportion of males was 51.9 + 1.1%, not significantly different from 50%. Sex ratios of nestlings were significantly biased in only 1 of 13 years, and we failed to detect any significant bias related to brood reduction, breeder female age, presence of helper males, first-egg date, condition of the breeder female, or annual differences in environmental conditions. These results allow us to reject all five hypotheses, including: (1) nestling size dimorphism and local resource competition, both of which predict a female-biased sex ratio; (2) local resource enhancement via the repayment model of Emlen et al. (1986), which predicts a more male-biased sex ratio than we observed; and (3) seasonal changes in either maternal condition or nestling condition, both of which predict shifts in sex ratios not observed in our data. These results prompt us to urge caution when reporting sex-ratio biases in natural populations and when interpreting published studies, many of which suffer from small sample sizes, post hoc analyses, and insufficiently conservative statistical tests. Sex-ratio biases in birds are uncommon, and considerable data will be necessary to determine which hypotheses (if any) are sufficiently robust to regularly select for avian sex ratios that deviate significantly from 50:50. Received 22 January 1996, accepted 20 June 1996.

Fitness consequences of helping behavior in the western bluebird

We examined the fitness consequences of helping behavior in the western bluebird (Sialia mexicand) at Hastings Reservation in Carmel Valley, California, USA, and tested hypodieses for how helpers benefit from engaging in alloparental behavior. Both juvenile and adult western bluebirds occasionally help at the nest. During a 12 year period, all adult helpers and most juvenile helpers were male. Helpers usually fed at nests of both their parents and rarely helped when only one parent was present. The frequency of pairs with adult helpers was only 7%, but nearly one-third of adult males helped among those with both parents on the study area. At least 28% were breeders whose nests failed. The propensity to help appears to depend upon parental survival, male philopatry, and die breeding success of potential helpers. Feeding rates were not increased at nests with juvenile helpers, apparendy because breeding males reduced their feeding rates. In contrast, adult helpers increased die overall rates of food delivery to die nest in spite of a reduction in die number of feeding trips made by both male and female parents. Helpers did not derive any obvious direct fitness benefits from helping, but diey had greater indirect fitness than nonhelpers due to increases in nesding growdi rates and fledging success at dieir parents' nests. Helpers fledged fewer offspring in their first nests dian did nonhelpers, suggesting diat they were birds widi reduced reproductive potential. Aldiough we have not yet measured the effect of extrapair fertilizations on die fitness benefits of helping, we calculated die difference in fitness between helpers and nonhelpers as a function of die potential helper's paternity when breeding independendy and his fadier's paternity in die nest at which he might help. In conjunction with constraints on breeding and indirect fitness benefits, we predict that relatedness of males to die young in dieir own as well as dieir parents' nests will influence helping behavior in western bluebirds.

An Experimental Test of Offspring Recognition in Western Bluebirds

An Experimental Test of Offspring Recognition in Western Bluebirds

Provisioning in western bluebirds is not related to offspring sex

Provisioning in western bluebirds is not related to offspring sex

Daily Allocation of Time and Energy of Western Bluebirds Feeding Nestlings

Daily Allocation of Time and Energy of Western Bluebirds Feeding Nestlings

Environmental effects on variation and covariation in reproductive traits of Western Bluebirds

Environmental conditions can influence the expression and correlations of phenotypic traits. I studied phenotypic plasticity in reproductive traits of Western Bluebirds breeding in northern Arizona. Data collected over 4 years on two contrasting habitats identified significant spatial and temporal variation in bluebird reproduction. Clutch size was similar over different environmental conditions whereas timing of clutch initiation, percent fledging success, and frequency of second nest attempts were flexible. Correlations between traits varied widely-often changing sign-among samples from different years or habitats. Correlations of traits with reproductive success were also dependent on environmental conditions. Variation in traits reflected behavioral responses by nesting adults to differences in time for breeding and feeding conditions. Density of trees differed between habitats and had opposing effects on these environmental variables; breeding seasons were generally longer, but feeding rates to nestlings were lower on the more open habitat. Late Spring snows delayed reproduction and increased the importance of limited time for breeding; feeding conditions were more influential following a dry Spring. This and other studies illustrate that data on phenotypic plasticity are important when evaluating the ecological and evolutionary forces underlying life histories.

Energetics of Growth and Maturation in Sympatric Passerines That Fledge at Different Ages

We studied the energetics of growth and maturation of sympatric Western Bluebird (Sialia mexicana) and Ash-throated Flycatcher (Myiarchus cinerascens) nestlings in the San Jacinto Mountains, Riverside County, California. Both species fledge at a body mass of 27-28 g, but the flycatcher leaves the nest 4-5 days earlier than the bluebird. The growth curves of these two species become asymptotic by 14 days of age. Energy budgets, derived from estimates of CO2 production and energy accumulation in tissues, demonstrate that the average daily total metabolizable energy requirement of the nestling flycatcher is approxi- mately 22% (range: 16-32%) higher than the nestling bluebird. Most of this difference is due to a significantly higher field metabolic rate throughout the nestling period of the flycatcher. We attribute this higher energy expenditure to a greater daily allocation of energy to the metabolic processes and activity associated with maturation, which allows the flycatcher to fledge at a younger age than the bluebird. Because of the nestling flycatcher's higher daily energy requirements, adult flycatchers must gather up to 20 g more food each day than bluebird parents with the same number of young. Nestling mortality indicates that Ash- throated Flycatchers may be more susceptible to nest failure than Western Bluebirds during periods of poor food availability. Received 7 October 1988, accepted 7 July 1990. THE ENERGY requirements of young in rela- tion to the parents' ability to provide sufficient food to meet those requirements is an intrinsic relationship that affects the reproductive strat- egies of birds. Many factors influence the en- ergy requirements of growing birds. Differ- ences in body size, growth pattern, and the thermal environment of the nest site can affect the peak energy demands of a brood (Fiala and Congdon 1983, Ricklefs and White 1981, Mer- tens 1969, O'Connor 1975, Bryant and Gardiner 1979, Drent and Daan 1980). Our purpose was to compare the energy ex- penditure of sympatric nestling Western Blue- birds (Sialia mexicana) and Ash-throated Fly- catchers (Myiarchus cinerascens). The adult mass of both species is similar (range: 25-30 g), and nestlings fledge at a body mass ca. 27-28 g. The age at fledging differs by 4-5 days (bluebird: 20-21 days; flycatcher: 16-17 days). These two species have very similar breeding ecologies. The species share the same habitat, both are cavity nesters, and both are insectivorous dur- ing the breeding season. Because the breeding

Intersexual variation and factors affecting parental care in Western Bluebirds: a comparison of nestling and fledgling periods

We compared factors affecting parental feeding rates in Western Bluebirds (Sialia mexicana) between the nestling and fledgling periods to evaluate intersexual variation in parental care. Parents did not adjust the frequency of feeding visits between modal (five young) and below-modal (four or less young) broods during the nestling period. The frequency of parental feeding visits also was not significantly affected by offspring age during the nestling period. Males and females exhibited no significant differences in provisioning offspring, although males maintained a constant level of care throughout the nestling period, whereas females increased feeding visits following the brooding period. During the fledgling period, offspring from below-modal broods were fed at higher rates with increasing age than offspring from modal broods. Broods remained together (siblings averaged 8 m apart) within 200 m of the nest box for a week after fledging. As fledglings became more mobile, they would pursue parents while parents were foraging; parental feeding rates thus tended to increase with fledgling age and distance moved from the nest. Offspring sex did not influence parental care during the fledgling period. Both adults fed fledglings, with males taking sole care of fledglings if females initiated a second clutch soon (7–10 days) after fledging of the first brood. Parental feeding rates increased by 60% during the fledgling period compared with the nestling period. No evidence for brood division during the fledgling period was found. Although brood division represents one way of reducing energetic costs attributable to feeding fledglings, Western Bluebirds exhibit an alternate behavior in which the energetics of raising a brood are shared equally between the parents throughout both the nestling and fledgling periods. This is further facilitated by a close association of young during the fledgling period that may reduce energetic costs related to locating and feeding young.

Population Biology of Cavity Nesters in Northern Arizona: Do Nest Sites Limit Breeding Densities?

Breeding densities of secondary (i.e., nonexcavating) cavity-nesting birds are often assumed to be limited by availability of nest sites. We investigated this assumption for species breeding in northern Arizona's ponderosa pine forests. In 1979, we installed nest boxes on three treatment plots that differed in habitat structure and monitored breeding densities of six species through the 1983 breeding season. The effect of nest boxes was evaluated by comparing breeding densities on three treatment plots from 1980 to 1983 with: (1) pretreatment densities (1973 to 1975, 1979), and (2) densities on control plots from 1980 to 1983. We observed variation in the importance of nest-site limitation among treatment plots and species. Overall breeding densities (all species combined) increased significantly on only two treatment plots. Individual species' responses were influenced by habitat structure, and breeding densities of only three species were apparently limited by nest sites before boxes were installed; Violet-green Swallows (Tachycineta thalassina), Pygmy Nuthatches (Sitta pygmaea), and Western Bluebirds (Sialia mexicana). A given species' breeding density in northern Arizona is nest-site limited if it is locally common and reliant on dead trees for nest sites. Availability of food or foraging substrate and territoriality may determine an upper limit to breeding densities if nest sites are in ample supply.

Defense of Nest Boxes by Western Bluebirds during the Post-Breeding Period

Observation de Sialia mexicana defendant un nichoir artificiel, comportement qui doit indiquer un manque de cavites, car cet oiseau niche habituellement dans les cavites