INTRASPECIFIC NEST PARASITISM IN BIRDS

(1) The incidence of intraspecific nest parasitism in birds is reviewed. This phenomenon can be identified by various methods, ranging from biochemical examination of protein and enzyme polymorphisms to observations on the sequence of appearance of eggs in the nest as well as on the shape and colour of the eggs. The phenomenon has been reported for at least 53 species (Table I), mostly among precocial birds.
(2) The evidence suggests that the parasites are ( a ) young, unmated birds, ( b ) females which have lost their nests and (c) mated females which also lay in the nests of other females. High proportions of such females, and scarcity of suitable nest sites increase the rate of parasitism.
(3) Breeding success in parasitized nests is reduced by factors such as egg loss, desertion, inefficient incubation, reduced clutch size and late layings. The upper limit of the rate of intraspecific parasitism is discussed. It is argued that its incidence will be higher in the tropics than in extreme zones, for example, high latitudes and deserts, because there is higher synchronization of breeding in the latter, and increased predation in the former zones. Also, it will be more common in species which lay large clutches, such as precocial species.     

FREQUENCY-DEPENDENT FITNESS CONSEQUENCES OF INTRASPECIFIC NEST PARASITISM IN SNOW GEESE

The reproductive efficiency, defined as the number of breeding recruits produced per egg laid; of intraspecific nest parasites; of hosts in parasitized nests; and of unparasitized nesting females, was measured for 14 years for lesser snow geese Anser caerulescens caerulescens nesting near Churchill, Manitoba, Canada. Relative efficiencies were 0.71–0.88, 0.91, and 1.0 for eggs of parasites, hosts, and unparasitized birds, respectively. Differences in the hatching probabilities of the three classes of eggs produced the efficiency differences. Parasitic success was limited by the parasites' failure to place more eggs than expected by chance into nests at the appropriate time relative to host incubation. Host nesting success was lower when more than one parasitic egg was added to the clutch. No differences in gosling survival and breeding recruitment probabilities were detected among any categories of goslings. Thus, hatching parasitic young are at no disadvantage relative to parental young, and there is no support for the hypothesis that increased success of host young at later stages of reproduction might offset negative effects at the egg stage. The hatching efficiency of parasitic eggs declined more rapidly than that of parental eggs as the parasitism rate increased. Efficiencies were similar when 3–4% of the eggs laid per year were parasitic, but relative parasitic efficiency was significantly lower when the parasitism rate was 8–9% or more. Using ancillary information and assumptions about the fecundity, viability, and behavioral flexibility of parasitic and parental females, we conclude that intraspecific nest parasitism could compete with nesting as a reproductive strategy in this population. The conditional use of parasitism by a large component of the population in certain years, however, combined with negative-frequency dependent success, limits the potential spread of a purely parasitic strategy in this population.     

A REANALYSIS OF NEST STRUCTURE IN THE WEAVERS (PLOCEINAE) USING NUMERICAL TAXONOMIC TECHNIQUES

Variables of nest architecture and construction behaviour of 62 species of ploceine finches studied by Crook (1963) were reanalysed using multivariate statistical techniques. Phenetic assessments were based on 38 two-state characters for 48 OTUs. Product moment correlation coefficients and average distance coefficients were used as measures of similarity, and species were clustered using the unweighted pair-group method with arithmetic averages. A three-dimensional model was constructed of species plotted with respect to the first three principal components extracted from a matrix of correlations among characters. Principal component and factor analytic techniques were employed to determine trends in character variation. The phenetic affinities expressed in the 3-D model and two phenograms show many similarities to the phenetic groupings deduced by Crook. However, there were also some differences in interpretation. Crook's Modes C and D were shown as distinct clusters in my analyses, but Modes A and B did not separate completely. In particular, the globular nest of P. baglafecht of Mode A, Type IV, grouped with kidney-shaped nests of species of Mode B, Type Va. The treatment of P. pelzelni, P. madagascariensis, and P. philippinus in my analysis differed somewhat from Crook's. The resulting cluster of F. sechellarum and M. malimbicus brought out some effects of standardising data on resulting interpretations of similarity. Species within the subtypes of Crook usually remained close together, but the affinities between subtypes (and types) often differed between our studies. The results of factor and principal component analyses indicate that the main trends in character variation concern those traits found together in (a) M. rubriceps; (b) A. unicolor and Pa. superciliosus; (c) forest birds; and (d) non-forest birds. These groups also are the main clusters. Differences in Crook's and my results are discussed and some of the advantages of utilising numerical techniques to describe similarity are mentioned.     

CONTROL OF COLONY AND UNICELL FORMATION IN A SYNCHRONIZED SCENEDESMUS12

Colony formation in a synchronized Scenedesmus could be controlled by the addition of 0.05% Na₃ citrate or 85 μM EDTA to modified Bristol's medium. No unicells were formed; only S. quadricauda-like or S. longispina-type colonies were observed in young cultures grown in that medium. A colony population could be made completely unicellular in 2 days if grown in soil-Bristol's medium and transferred daily. The pleomorphic Scenedesmus was examined in synchronized culture. When the organism was grown in a defined medium in a 15-hr light /9-hr dark cycle on a roller tube rotator at 2-6 rpm and transferred daily, synchrony of cell division and release of the products were achieved. In a synchronized culture 2 doublings/day were recorded, with most cytoplasmic cleavages and all releasing of daughter cells taking place in the dark period. In many observations with several synchronized strains of Scenedesmus, no fixed pattern of release of daughter products from mother cells or colonies was detected. Colonies or unicells had their full spine complement at the time of release. As a cell or colony aged the spines sometimes increased in thickness. Other Scenedesmus strains were examined to provide supporting data.     

COOPERATIVE BREEDING IN A POPULATION OF WHITEBROWED SPARROW WEAVERS PLOCEPASSER MAHALI

This paper reports on a three year study describing cooperative breeding and demography of a sampled population of approximately 108 colour-marked White-browed Sparrow Weavers in Zambia. Twenty-seven communal groups with two to 11 individuals in a group made up this population and occupied a l km² area of mopane woodland. Each group had only a single reproductive pair and the pair-bond appeared strong. Non-reproductives were generally siblings or half-siblings and they contributed a major share of nestling feeding, nest building, intruder chasing and guarding. Groups cooperatively built nests in certain preferred trees and they maintained their nests year round. Feeding range encircled the nesting site and was used almost exclusively by the communal group. Overall population density was relatively stable, and between-year variation in group size was significantly greater than within-year variation. Variation in annual reproductive success among groups was high and correlated positively with size. Mortality rates for nestlings, six-month-olds and juveniles were not correlated significantly with group size. Frequency of disappearance for reproductives of both sexes was correlated negatively with group size.     

EPIDEMIOLOGY AND SOCIAL JUSTICE IN LIGHT OF SOCIAL DETERMINANTS OF HEALTH RESEARCH

The present article identifies how social determinants of health raise two categories of philosophical problems that also fall within the smaller domain of ethics; one set pertains to the philosophy of epidemiology, and the second set pertains to the philosophy of health and social justice. After reviewing these two categories of ethical concerns, the limited conclusion made is that identifying and responding to social determinants of health requires inter-disciplinary reasoning across epidemiology and philosophy. For the reasoning used in epidemiology to be sound, for its scope and (moral) purpose as a science to be clarified as well as for social justice theory to be relevant and coherent, epidemiology and philosophy need to forge a meaningful exchange of ideas that happens in both directions.     

BIOCHEMICAL HETEROZYGOSITY AND MORPHOLOGIC VARIATION IN A COLONY OF PAPIO HAMADRYAS HAMADRYAS BABOONS

This analysis examines the association between genetic heterozygosity and individual morphologic variation in a captive population of Papio hamadryas hamadryas consisting of 403 juveniles and adults. The population structure of the colony was artificially generated and maintained and is thus rigorously defined. Subpopulations delimited by age, sex, and degree of inbreeding are also explored. Heterozygosity, as enumerated from six simple Mendelian biochemical loci, is compared with the residual morphologic variation of each individual for each of 20 quantitative traits. Use of a sequential Bonferroni technique nullifies all significant correlations. Principal-components analysis reduces the morphometrics to a single or few significant axes in each population. The first axis of the total population contains 86.07% of the variation in the sample and the absolute values of the factor scores exhibit a significant positive correlation with heterozygosity at P < 0.05. Correcting for age- and sex-related variation in the total population with a linear model subsequently demonstrates that no significant correlation between heterozygosity and morphologic variation exists. No significant relationship is found in the inbred animals or subpopulations when age and sex are controlled. Previous studies have indicated that individuals proximal to the population mean for a specific polygenic trait exhibit a higher biochemical heterozygosity than individuals distant from the mean. The results presented here, which are based on more loci than many studies and a well-defined population, do not support this relationship. Substructuring of a population by age and sex can lead to spurious correlations with univariate or multivariate techniques. Comprehensive indices of genetic variation and rigorous statistical techniques should be used in future analyses. Studies that fail to recognize these design elements should be interpreted with caution.     

THE EFFECTS OF HOST GENOTYPE AND SPATIAL DISTRIBUTION ON TREMATODE PARASITISM IN A BIVALVE POPULATION

A basic assumption underlying models of host-parasite coevolution is the existence of additive genetic variation among hosts for resistance to parasites. However, estimates of additive genetic variation are lacking for natural populations of invertebrates. Testing this assumption is especially important in view of current models that suggest parasites may be responsible for the evolution of sex, such as the Red Queen hypothesis. This hypothesis suggests that the twofold reproductive disadvantage of sex relative to parthenogenesis can be overcome by the more rapid production of rare genotypes resistant to parasites. Here I present evidence of significant levels of additive genetic variance in parasite resistance for an invertebrate host-parasite system in nature. Using families of the bivalve mollusc, Transennella tantilla, cultured in the laboratory, then exposed to parasites in the field, I quantified heritable variation in parasite resistance under natural conditions. The spatial distribution of outplanted hosts was also varied to determine environmental contributions to levels of parasite infection and to estimate potential interactions of host genotype with environment. The results show moderate but significant levels of heritability for resistance to parasites (h² = 0.36). The spatial distribution of hosts also significantly influenced parasite prevalence such that increased host aggregation resulted in decreased levels of parasite infection. Family mean correlations across environments were positive, indicating no genotype-environment interaction. Therefore, these results provide support for important assumptions underlying coevolutionary models of host-parasite systems.     

THE SOCIABLE WEAVER,PART 2: NEST ARCHITECTURE AND SOCIAL ORGANIZATION

Maclean, G. L. 1973. The Sociable Weaver, Part 2: Nest architecture and social organization. Ostrich 44:191-218.

Sociable Weavers build nest masses in a number of indigenous tree species (especially on Acacia giraffae branches) and on artificial nest sites like telephone poles. They never build in exotic trees. Nest masses are built of grass straws and roofed over with a superstructure of coarser material such as thorn twigs. The grass substructure contains the nest chambers which do not interconnect. The substructure may be divisible into two or more levels, each forming a social unit comprising the birds inhabiting it.

Each social level of birds is confined to its own structural level at all times, but a bird may roost in any chamber within its own level. The superstructure is not divisible into social units and any bird may build or perch on any part of the superstructure. Movements of birds from one colony to another are rare. The colony at one nest mass leaves the nest at about sunrise in summer, a little later in winter, and flies to the feeding grounds; the birds return to the nest mass for a siesta lasting from about 10:00 hours to 14:00 hours in hot weather, less than this in cool weather. They depart again for their feeding grounds until about sunset.

The internal temperature and RH of the nest chambers were not found to be significantly different from ambient temperature and RH when ambient temperatures were > 21,7°C. At ambient temperatures < 26,7°C the RH of the nest chambers was significantly lower than ambient Rh, but temperatures were not significantly different during the day.     

THE EFFECTS OF DIURNAL AND TIDAL PERIODICITIES IN THE NUMBERS AND ACTIVITIES OF HERRING GULLS LARUS ARGENTATUS IN A COLONY

The average numbers of Herring Gulls Larus argentatus present in a breeding colony on Walney Island, Cumbria, were found to vary with the tidal cycle but to remain effectively constant with time of day through the breeding season. An activity survey, based on 50 Herring Gulls observed at half-hourly intervals during March and April 1973, showed that sleep and rest varied inversely with each other with sleep increasing to 50 per cent at midday. After a peak in the proportion of gulls asleep four hours before low tide, sleeping progressively decreased until low tide; seemingly a result of resident gulls waking and remaining more alert as others left the colony in search of food. Preening was constant throughout the day and tide cycle. Other behaviours (mostly courtship and agonistic behaviour associated with territory defence) increased slightly during low tide and were more common early and late in the day. Night observations of the gulls' activities showed that there was a peak of sleeping between midnight and 02.00 hours. It is suggested that Herring Gulls have a bimodal diel sleep pattern.     

BERGMANN'S RULE,SEASONALITY, AND GEOGRAPHIC VARIATION IN BODY SIZE OF HOUSE SPARROWS

Body size of House Sparrows is positively correlated with seasonality (annual temperature range) in both North America and Europe. Previously used climatic variables gave inconsistent or contrasting results for the two continental samples. The physiological advantage of large size in seasonal environments appears to be increased fasting ability, but this may be offset somewhat, particularly for females, by other factors, e.g., competitive interactions that influence rates of energy intake or ability to initiate breeding in the spring. It is proposed that minimization of maintenance energy costs on a per individual basis, and thus small size, is advantageous in equable (and predictable) environments.