L. Scott Johnson

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases

Abstract. The widespread use of artificial nestboxes has led to significant advances in our knowledge of the ecology, behaviour and physiology of cavity nesting birds, especially small passerines. Nestboxes have made it easier to perform routine monitoring and experimental manipulation of eggs or nestlings, and also repeatedly to capture, identify and manipulate the parents. However, when comparing results across study sites the use of nestboxes may also introduce a potentially significant confounding variable in the form of differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. However, the use of nestboxes may also introduce an unconsidered and potentially significant confounding variable due to differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. Here we review to what extent the characteristics of artificial nestboxes (e.g. size, shape, construction material, colour) are documented in the ‘methods’ sections of publications involving hole-nesting passerine birds using natural or excavated cavities or artificial nestboxes for reproduction and roosting. Despite explicit previous recommendations that authors describe in detail the characteristics of the nestboxes used, we found that the description of nestbox characteristics in most recent publications remains poor and insufficient. We therefore list the types of descriptive data that should be included in the methods sections of relevant manuscripts and justify this by discussing how variation in nestbox characteristics can affect or confound conclusions from nestbox studies. We also propose several recommendations to improve the reliability and usefulness of research based on long-term studies of any secondary hole-nesting species using artificial nestboxes for breeding or roosting.

Effects of Haematophagous Ectoparasites on Nestling House Wrens, Troglodytes aedon: Who Pays the Cost of Parasitism?

We experimentally investigated the effects of two haematophagous ectoparasites, larvae of the blow fly Protocalliphora parorum (Diptera: Calliphoridae) and the mite Dermanyssus hirundinis (Mesostigmata: Dermanyssidae), on the survival, health and growth of nestling house wrens Troglodytes aedon. Nestlings in broods with relatively heavy infestations (> 10 larvae, and hundreds to thousands of mites, per nestling) did not have higher mortality, reduced growth of tarsi and primary feathers, or lower pre-fledging hematocrits than nestlings occupying nests in which parasites were killed with heat

Genotype and extra–pair paternity in the house wren: a rare–male effect?

Females in socially monogamous species may select extra–pair (EP) mates to increase the heterozygosity, and hence fitness, of their offspring. We tested this hypothesis in the house wren (Troglodytes aedon), a largely monogamous songbird in which EP young are common. We typed paired males and females, nestlings, and males on neighbouring territories, at five to seven microsatellite loci over 2 years in a Wyoming, USA, population. We identified EP sires at 20 nests with EP young. In pairwise comparisons, we found no significant differences between cuckolded within–pair (WP) males and EP sires in three measures of heterozygosity (mean d2, standardized heterozygosity and internal relatedness). However, EP sires had fewer alleles that were common within the population than did the WP males they cuckolded. Nearby males who were EP sires also had fewer common alleles than did nearby males who did not sire EP young. Females in our population may be more prone to accept copulations from males with rare genotypes than from males with common genotypes. Alternatively, selection of rare–male sperm may occur within the female reproductive tract. Because mating with rare males is likely to increase offspring heterozygosity, our data suggest that EP mating may provide genetic benefits to females.

Neonatal body condition, immune responsiveness, and hematocrit predict longevity in a wild bird population

Measures of body condition, immune function, and hematological health are widely used in ecological studies of vertebrate populations, predicated on the assumption that these traits are linked to fitness. However, compelling evidence that these traits actually predict long-term survival and reproductive success among individuals in the wild is lacking. Here, we show that body condition (i.e., size-adjusted body mass) and cutaneous immune responsiveness to phytohaemagglutinin (PHA) injection among neonates positively predict recruitment and subsequent longevity in a wild, migratory population of house wrens (Troglodytes aedon). However, neonates with intermediate hematocrit had the highest recruitment and longevity. Neonates with the highest PHA responsiveness and intermediate hematocrit prior to independence eventually produced the most offspring during their lifetime breeding on the study site. Importantly, the effects of PHA responsiveness and hematocrit were revealed while controlling for variation in body condition, sex, and environmental variation. Thus, our data demonstrate that body condition, cutaneous immune responsiveness, and hematocrit as a neonate are associated with individual fitness. Although hematocrits effect is more complex than traditionally thought, our results suggest a previously underappreciated role for this trait in influencing survival in the wild.

Female house wrens (Troglodytes aedon) increase the size, but not immunocompetence, of their offspring through extra-pair mating.

House wrens are typically socially monogamous, but frequently engage in extra‐pair matings leading to multisired broods. Because females do not appear to acquire direct material benefits from their extra‐pair mates, we tested the hypothesis that female house wrens derive indirect genetic benefits, such as enhanced immunocompetence (cutaneous immune activity, humoral immunity, and plasma bactericidal activity) and condition (size and haematoserological traits) for their offspring, by mating polyandrously. We predicted that extra‐pair young (EPY) should show greater immune responsiveness and better body condition than their within‐pair maternal half‐siblings (WPY). Contrary to our prediction, WPY had higher cutaneous immune activity than their EPY brood‐mates in two of three years, and EPY and WPY did not differ in measures of innate and humoral immunity. WPY also had higher albumin to γ‐globulin ratios than EPY; however, they were not in better condition based on other measures. EPY had consistently longer tarsi (a measure of long‐bone size) than their WPY half‐siblings, suggesting that females engage in extra‐pair copulations with larger males. The benefits of large structural size in the study population is unknown, but based on evidence from other passerines, we suggest that structural size may be an important fitness‐related trait in house wrens. We conclude that our results are not consistent with the hypothesis that females gain immune‐related benefits for their offspring by engaging in extra‐pair matings. Further study of the fitness consequences of differences in tarsus length is needed to determine whether females acquire size‐related benefits for their offspring from extra‐pair mates.

Sexual selection in a socially monogamous bird: Male color predicts paternity success in the mountain bluebird, sialia currucoides

Ornamental traits are thought to evolve because they give individuals an advantage in securing multiple mates. Thus, the presence of ornamentation among males in many monogamous bird species presents something of a conundrum. Under certain conditions, extra-pair paternity can increase the variance in reproductive success among males, thus increasing the potential for sexual selection to act. We addressed this possibility in the mountain bluebird (Sialia currucoides), a socially monogamous songbird in which males possess brilliant ultraviolet (UV)-blue plumage. Specifically, we asked whether a male’s success at siring offspring within his own nest and within the nests of other males was related to his coloration. In pairwise comparisons, males that sired extra-pair offspring were not more colorful than the males that they cuckolded. However, males that sired at least one extra-pair offspring were, on average, brighter and more UV-blue than males that did not sire extra-pair offspring. Brighter, more UV-blue males sired more offspring both with their own mate and tended to sire more offspring with extra-pair mates and thus sired more offspring overall. Our results support the hypothesis that the brilliant UV-blue ornamental plumage of male mountain bluebirds evolved at least in part because it provides males with an advantage in fertilizing the eggs of multiple females.

CHANGES IN EGG SIZE AND CLUTCH SIZE WITH ELEVATION IN A WYOMING POPULATION OF MOUNTAIN BLUEBIRDS

Abstract Few studies have examined how avian life-history traits vary within populations as elevation increases and climate becomes more severe. We compared egg and clutch sizes of Mountain Bluebirds (Sialia currucoides) nesting at two elevations (1500 m and 2500 m above sea level) in the Bighorn Mountains of Wyoming over two years. Eggs laid by females at the high-elevation site were, on average, significantly (6%) smaller in volume than eggs laid by their lower-elevation counterparts. Across elevations, egg size showed a significant positive correlation with female body condition (weight relative to size), and high-elevation females had significantly lower indices of condition than low-elevation females. Temperatures during clutch formation were colder at the high-elevation site, and egg size was negatively related to temperature after controlling for the effects of female condition. Clutches of females at high elevations were, on average, marginally smaller (by 5%, 0.3 eggs) than clutches of low-elevation females. Unlike egg size, clutch size was unrelated to either female condition or temperature during clutch formation. This suggests that, when under energetic or nutritional stress at high elevations, females sacrifice egg size before sacrificing clutch size.

Extra-pair young in house wren broods are more likely to be male than female.

Sex-allocation theory predicts that females should preferentially produce offspring of the sex with greater fitness potential. In socially monogamous animal species, extra-pair mating often increases the variance in fitness of sons relative to daughters. Thus, in situations where offspring sired by a females extra-pair mate(s) will typically have greater fitness potential than offspring sired by the within-pair mate, sex-allocation theory predicts that females will bias the sex of offspring sired by extra-pair mates towards male. We examined the relationship between offspring sex and paternity over six breeding seasons in an Illinois population of the house wren (Troglodytes aedon), a cavity-nesting songbird. Out of the 2345 nestlings that had both sex and paternity assigned, 350 (15%) were sired by extra-pair males. The sex ratio of extra-pair offspring, 0.534, was significantly greater than the sex ratio of within-pair offspring, 0.492, representing an increase of 8.5 per cent in the proportion of sons produced. To our knowledge, this is the first confirmed report of female birds increasing their production of sons in association with extra-pair fertilization. Our results are consistent with the oft-mentioned hypothesis that females engage in extra-pair mating to increase offspring quality.

Mechanisms of evolutionary change in structural plumage coloration among bluebirds (Sialia spp.)

Combinations of microstructural and pigmentary components of barbs create the colour displays of feathers. It follows that evolutionary changes in colour displays must reflect changes in the underlying production mechanisms, but rarely have the mechanisms of feather colour evolution been studied. Among bluebirds in the genus Sialia, male rump colour varies among species from dark blue to light blue while breast colour varies from blue to rusty. We use spectrometry, transmission electron microscopy and Fourier analysis to identify the morphology responsible for these divergent colour displays. The morphology of blue rump barbs is similar among the three species, with an outer keratin cortex layer surrounding a medullary ‘spongy layer’ and a basal row of melanin granules. A spongy layer is also present in blue breast barbs of mountain bluebirds Sialia currucoides and in rusty breast barbs of western Sialia mexicana and eastern bluebirds Sialia sialis. In blue barbs melanin is basal to the spongy layer, but is not present in the outer cortex or spongy layer, while in rusty barbs, melanin is present only in the cortex. The placement of melanin in the cortex masks expression of structural blue, creating a rusty display. Such shifts in microstructures and pigments may be widespread mechanisms for the evolutionary changes in the colours of feathers and other reflective structures across colourful organisms.

Manipulation of offspring sex ratio by second-mated female house wrens

In 1973, Trivers and Willard proposed that offspring sex ratio should be associated with the quality of parental care likely to be provided to the offspring. We tested this hypothesis by comparing fledgling sex ratios in nests of first–and second-mated female house wrens (Troglodytes aedon). In our Wyoming population, second–mated females typically receive little or no male parental assistance and fledge fewer and lower–quality young compared with first–mated females. Assuming that being of lower quality has stronger negative effects on the future reproductive success of males than that of females in this polygynous population, we predicted that fledgling sex ratios in the nests of second–mated females would be female–biased compared with the fledgling sex ratios of first-mated females. Additionally, we asked whether any sex bias at fledging could have resulted from male–biased nestling mortality caused by sex–biased parental provisioning. As predicted, mean fledgling sex ratios in nests of second–mated females were more female–biased than fledgling sex ratios in nests of first–mated females. However, we found no evidence of either sex-biased nestling mortality or sex–biased parental provisioning. These findings suggest that females are responding to their status as second–mated females and to the associated low–quality parental care that their young are likely to receive by producing female–biased clutches rather than manipulating the offspring sex ratio through sex–biased nestling mortality.