Russell D. Dawson

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.

Are avian hematocrits indicative of condition ? American kestrels as a model

Diseased animals or those in poor condition are known to have reduced hematocrits. Many investigators have assumed that hematocrit levels thus reflect condition and disease status of an animal. This study tested these assumptions by examining the relation between hematocrits of American kestrels (Falco sparverius) during several stages of the breeding season, and condition, prey abundance, and blood parasite load. We also examined the potential effects of a number of intrinsic and extrinsic influences on hematocrit. Hematocrits did not differ between the sexes, or between the pre-laying and incubation periods. Among females, hematocrit did not vary with the date of sampling, breeding chronology, prey abundance, condition, age, or molt, although hematocrit increased with ambient temperature during incubation. Hematocrit of males was not related to breeding chronology, prey abundance, condition, age, or molt. During incubation, male hematocrit increased with the date of sampling and ambient temperature. Hematocrits of both sexes declined with the time of day that the sample was taken, and increased with the level of infection of the blood parasite Haemoproteus. The use of hematocrits to assess the health and condition of clinically normal kestrels is therefore questionable, and given the positive association with parasite loads, may even lead to erroneous conclusions.

EFFECTS OF HEMATOZOAN PARASITES ON CONDITION AND RETURN RATES OF AMERICAN KESTRELS

Abstract We evaluated the relationship between blood parasites and body condition of American Kestrels (Falco sparverius) during the breeding season. Females that were infected with at least one species of parasite were in poorer condition than those without parasites during incubation but not prior to egg laying. We suggest that the relationship between parasitism and condition was masked before laying because of large increases in body mass of females during egg formation. Reduced condition of males during incubation also was associated with higher intensity of infections by Haemoproteus in one of two years. The negative association between condition and intensity of infection suggests that blood parasites impose costs on kestrels owing to competition for nutrients or allocation of energy by hosts to immune function or tissue repair. Alternatively, kestrels in poor condition may be more likely to have relapses of chronic infections, or they may be less able to control new infections because of resource limitations. In contrast to results during incubation, during the prelaying period the prevalence of parasites tended to be higher, and in one year infections were more intense, among males in good condition. One possible explanation for these results is that body condition of males during courtship is an important determinant of the quality of mate they are able to obtain, and males may be accumulating body reserves at the expense of decreased immune function. Return rates of female kestrels to the study area declined as the intensity of their Haemoproteus infections increased, suggesting that blood parasitism is associated with reduced survival or increased dispersal probability.

The T-cell-mediated immune response and return rate of fledgling American kestrels are positively correlated with parental clutch size.

Life–history theory predicts that parents face a trade–off between the number and viability of the progeny they produce. We found evidence for an apparent trade–off in a free–living population of American kestrels (Falco sparverius), as larger clutches produced more but lighter fledglings. However, while the body mass of fledglings has traditionally been used as a measure of survival prospect, offspring immuno–competence should also play an important role. We thus measured the T–cell–mediated immune response of fledgling kestrels in relation to brood traits and nest–rearing conditions through a cross–fostering experiment. The immune response was positively correlated with the body condition of fledglings, but was also higher in those hatched from five–egg than four–egg clutches. These results were not influenced by other brood traits, nor by current exposure to stressors and infectious agents, as measured by sero–logical variables. Such ability to resist pathogens may account for why the probability of offspring returning to the study area in subsequent years, when controlling for brood size, was higher for five–egg than four–egg clutches. These results suggest an optimal clutch size through maternal effects on offspring immunocompetence rather than a trade–off between the number and quality of the offspring.

Reproductive success of American Kestrels : The role of prey abundance and weather

Abstract We investigated how natural variation in abundance and availability of the main prey of American Kestrels (Falco sparverius), small mammals, influenced provisioning rates by parents, and offspring size and survival. Provisioning behavior of kestrels was not constrained by the abundance of food in the environment; however, the availability of food, mediated through variation in weather, appeared to significantly influence parental provisioning behavior. Moreover, variation in weather had clear effects on reproductive success because nestlings exposed to inclement weather were smaller and lighter at fledging, and less likely to survive to fledging, compared to nestlings raised during good weather conditions. Prey abundance was not related to offspring size or survival. Our results suggest American Kestrels are limited by the availability, as opposed to abundance, of food on territories. It is likely that during our study, prey abundance was above some minimum threshold necessary to support successful reproduction, and so variation in weather affected reproduction more than variation in prey abundance.

Environmental and genetic variation in T-cell-mediated immune response of fledgling American kestrels

Abstract We investigated genetic and environmental components of variance in avian T-cell-mediated immune response (CMI) through a cross-fostering experiment conducted on wild American kestrels (Falco sparverius). CMI was evaluated in vivo by an experimental challenge with phytohaemagglutinin, a T-cell mitogen, injected intradermally in fledglings. Additionally, we assessed two measures of nutritional condition (body mass and circulating plasma proteins) which could influence the variance components of CMI. A two-way nested ANOVA indicated that CMI of fledgling kestrels was explained more by the nest where the bird was reared (33% of the explained variance) than by the nest of origin (12%). Body mass was explained equally by familial and environmental components, while plasma proteins were only related to the rearing environment. CMI of fledglings was not related to their circulating plasma proteins, but was positively correlated with their body mass. Fledgling body mass seemed to be influenced by pre-hatching or post-hatching maternal effects prior to manipulation since resemblance in body mass of sibships at the age of manipulation was high (h2≤0.58), and body mass at this age predicted body mass at fledging. Therefore, pre-manipulation parental effects on body mass, such as investment in egg size, could have inflated the familial effects on body mass of fledglings and then on its correlated CMI. When controlling for body mass, most of the variation in CMI of fledglings was explained by the nest where the bird was reared (36.6%), while the variance explained by the nest of origin (4%) was not significant. This means that environmental influences are major determinants of offspring CMI. The low proportion of variance explained by the familial component may have been due to the high correlation of CMI to fitness.

Genetics, local environment and health as factors influencing plasma carotenoids in wild American kestrels ( Falco sparverius )

Carotenoids are important as pigments for bright coloration of animals, and as physiologically active compounds with a wide array of health–related functions. Carotenoid–dependent coloration may have evolved as a signal to conspecifics; however, factors that may limit availability of carotenoids are poorly known. We investigated how the acquisition of carotenoids may be constrained by availability in the environment, diet, genetic make–up and health status of wild American kestrels (Falco sparverius). Plasma concentrations of siblings at the time of fledging showed a high degree of resemblance; however, a cross–fostering experiment revealed that variance was largely explained by nest of rearing, rather than nest of origin, thus indicating a low genetic component. A multivariate analysis of attributes of nestlings (sex, size, plasma proteins, immune function), parental reproduction (laying date, clutch size) and rearing conditions (brood size, size hierarchy, nestling mortality) showed only a small significant effect of leucocyte differentials on carotenoid concentrations of nestlings. A strong environmental effect on plasma carotenoids was demonstrated by levels of adult kestrels being correlated within mated pairs, and having a significant association with the abundance of voles, the primary prey species, per territory.

EFFECTS OF HATCHING DATE AND EGG SIZE ON GROWTH, RECRUITMENT, AND ADULT SIZE OF LESSER SCAUP

Abstract Evidence suggests that birds breeding early in the season or laying larger eggs are at a selective advantage because quality and survival of their offspring are higher. We tested whether wild Lesser Scaup (Aythya affinis) hatching early in the season or from larger eggs had enhanced growth as ducklings and larger size or higher probability of recruiting to the local breeding population in years after hatching. After correcting for age at capture, body mass, head length, and culmen of ducklings were inversely related to hatching date, but were unrelated to egg volume. As adults, late-hatched birds had shorter wing lengths (second-year birds only) and tended to have smaller head lengths (all after-hatch year birds) than early hatching birds. We suggest that later-hatching birds are smaller due to increased competition for food during brood rearing. Recruitment probability increased as natal egg volume increased and decreased among birds with later natal hatching dates. We speculate these results are due to higher mortality of ducklings hatched from small eggs, and because early-hatched birds have more time to acquire nutrient reserves that would reduce vulnerability to costs of migration.

Variation in Hematocrit and Total Plasma Proteins of Nestling American Kestrels (Falco sparverius) in the Wild

Abstract Hematocrits and total plasma proteins were determined at 24 days old for 86 female and 85 male nestling American kestrels ( Falco sparverius ) from the wild in northern Saskatchewan. No sex differences were detected in either hematocrit or plasma protein. For females, hematocrit and plasma protein were not related to time of sampling, temperature at sampling, mass of nestlings or length of the tenth primary flight feather. In males, hematocrit correlated only with the length of tenth primary. For both sexes, concentrations of plasma protein declined as the season progressed. Differences in brood size, natural and experimental, did not affect hematocrit or plasma protein levels. Of six nestlings that died before leaving the nest, three showed depressed hematocrit values compared with those that successfully left the nest, whereas none showed significant deviations in plasma protein levels. For 42 birds, we took a second blood sample 2–5 days after the first. Changes in blood parameters of males between these sampling periods were not related to changes in weight, growth of tenth primary or changes in temperature. Among females, there were trends for changes in weight to be correlated with changes in both hematocrit and plasma protein. Our results suggest that hematocrits and plasma proteins of nestling kestrels are not robust indicators of nutritional status and condition.

Plumage brightness and age predict extrapair fertilization success of male tree swallows, Tachycineta bicolor

In socially monogamous passerines, extrapair paternity can increase the variance in male reproductive success. If gaining extrapair fertilizations is linked to specific secondary sexual ornaments, the opportunity for sexual selection is enhanced. Therefore, to understand the evolution of male phenotypic characteristics, it is important to identify traits that predict male extrapair mating success. Tree swallows show among the highest rates of extrapair paternity known to occur in birds, yet it is unclear whether male extrapair mating success is associated with phenotypic traits that honestly advertise individual quality. We compared morphological characteristics and plumage coloration of male tree swallows that sired extrapair offspring with those that sired only within-pair offspring in the same breeding population to identify the characteristics that predict extrapair fertilization success. Males who produced extrapair offspring had brighter plumage, and were more likely to be returning breeders, than males that did not have extrapair young. In paired comparisons, however, there was no difference between extrapair males and the male they cuckolded. These results suggest that female tree swallows may prefer brighter or older individuals as extrapair mates, but also, that older males may invest more energy in pursuing extrapair copulations. Furthermore, since females had extrapair offspring in their nest regardless of their social mates morphometric or plumage attributes, we suggest that extrapair mating may be a reproductive strategy allowing females to increase the genetic diversity, while maintaining genetic quality, of their offspring.