Seasonal changes in male and female bill knob size in the mute swan Cygnus olor

Sexual selection theory predicts that sexually selected ornaments are costly to maintain and, as condition-dependent signals, are likely to vary in attractiveness with season and age. Mute swans Cygnus olor possess a black, fleshy knob at the base of the bill, which is present in both sexes. Using measures calculated from digital photographs taken over two years we monitored changes in the size of the bill knob in individual swans throughout the breeding season. Our longitudinal data show that bill knob size is highly dynamic. Relative bill knob size was larger in males than females and was consistently greater for breeding males than for non-breeders. For males, relative bill knob size peaked during cygnet hatching, when male protection of the brood is most important, and was smallest during moult. In females, breeders had larger bill knobs than non-breeders at all times apart from immediately after egg-laying and incubation, when the reverse was true, presumably reflecting the costs of reproduction. Body mass was a highly significant predictor of relative bill knob size in both sexes, as was age, with an initial increase and then later a decline in relative ornament size across the lifetime of male birds. The bill knob ornament in mute swans thus appears to be a condition-dependent, highly malleable trait. It accurately reflects the differing pressures experienced by individual birds as they progress through the breeding season, suggesting selection by both intra- and inter-sexual forces.     

Assessment of body condition in live birds; measurements of protein and fat reserves in the mute swan, Cygnus olor

Condition in animals is often measured in relation to various demands, such as reproduction, migration and cold weather. Usually, indices of protein and fat reserves have to be measured on dead animals. Measurements on live animals enable variations in the condition of individuals to be monitored over time, and can be related to their life histories.
In birds, the size of the breast muscle is the most commonly used measure of protein reserves. A technique for measuring breast muscle thickness was developed, using ultrasound, and was tested on mute swans. The measurements were accurate and highly repeatable. Breast muscle thickness was positively related to the lean dry weight of the breast muscle, and is likely to be a good indicator of total protein reserves.
A system of fat scoring by colour was developed to assess the size of the subcutaneous fat layer. Although the method relies on subjective judgements, the results were highly correlated with the percentage of fat in the subcutaneous layer, and with other measures of fat reserves.
Measurements were made on live, non-breeding swans to monitor seasonal variations in reserves. The seasonal pattern of fat reserves was similar to the normal cycle of weight changes; high in winter and low in summer. There was no change in relation to the moult. No distinct seasonal variation in muscle thickness was found.
There is an unusual pattern of change in the relative size of reserves during emaciation in swans. Muscle reserves appear to be depleted to a greater extent than fat reserves. This is possibly due to the effects of lead poisoning, causing protein necrosis and preventing muscle regeneration.     

Macrophages,a normal component in seasonally involuting testes of the swan,Cygnus olor

Summary In the swan, Cygnus olor, spermatogenesis shows a pronounced seasonal cycle. During the long involution phase a transient invasion of macrophages into the seminiferous tubules regularly takes place at the height of regression prior to total rest. These macrophages are involved in the additional disposal of supernumerary and degenerating premature germ cells, at a time when the Sertoli cells are already filled with phagolysosomes.     

Population‐level influence of a recurring disease on a long‐lived wildlife host

Despite a heightened interest regarding the role of infectious diseases in wildlife conservation, few studies have explicitly addressed the impacts of chronic, persistent diseases on long‐term host population dynamics. Using mycoplasmal upper respiratory tract disease (URTD) within natural gopher tortoise Gopherus polyphemus populations as a model system, we investigated the influence of chronic recurring disease epizootics on host population dynamics and persistence using matrix population models and Markov chain models for temporally autocorrelated environments. By treating epizootics as a form of environmental stochasticity, we evaluated host population dynamics across varying levels of outbreak duration (ρ), outbreak recurrence (f), and disease‐induced mortality (μ). Baseline results indicated a declining growth rate (λ) for populations under unexposed or enzootic conditions (λ_Enzootic= 0.903, 95% CI: 0.765–1.04), and a median time to quasi‐extinction of 29 years (range: 28–30 years). Under recurring epizootics, stochastic growth rates overlapped with baseline growth rates, and ranged between 0.838–0.902. Median quasi‐extinction times under recurring epizootics also overlapped for most scenarios with those of baseline conditions, and ranged between 18–29 years, with both metrics decreasing as a function of f and μ. Overall, baseline (enzootic) conditions had a greater impact on λ than epizootic conditions, and demographic vital rates were proportionately more influential on λ than disease‐ or outbreak‐associated parameters. Lower‐level elasticities revealed that, among disease‐ and outbreak‐associated parameters, increases in μ, force of infection (φ), and f negatively influenced λ. The impact of disease on host population dynamics depended primarily on how often a population underwent an epizootic state, rather than how long the epizootic persisted within the exposed population. The modeling framework presented in this paper could be widely applied to a range of wildlife disease systems in which hosts suffer from persistent recurring diseases.     

Age‐related variation and temporal patterns in the survival of a long‐lived scavenger

Although senescence has been described for various fitness components in a wide range of animal species, few studies have studied senescence in long‐lived species, and little is known about its interactions with varying environmental conditions. Using a 32 year capture–mark–recapture dataset on the griffon vulture Gyps fulvus, we examined the demographic patterns of actuarial senescence and the patterns of year‐to‐year variation in survival rates. We found a significant, surprisingly late, decrease of annual survival probabilities from the age of 28 years onward and divided individual lifetimes into to three categories (juvenile, mid‐age and senescent birds). In agreement with the environmental canalization hypothesis, our analyses uncovered 1) higher temporal variation of annual survival probabilities in both the juvenile and senescent age classes compared to the mid‐age class and 2) low sensitivity of the population growth rate to the survival of both the juvenile and senescent age classes. Our results further suggested that the temporal variation in the survival of senescent birds might be related to intra‐annual changes in air temperature amplitudes. Finally, using population dynamics modeling, we revealed contrasting effects of the inclusion of the senescent age class on predicted population growth, depending on how survival rates were modeled. Altogether, our results demonstrate the existence of a class of senescent birds that exhibit distinct demographic properties compared to juvenile and mid‐age classes.     

Parasite‐associated mortality in a long‐lived mammal: Variation with host age,sex, and reproduction

Parasites can cause severe host morbidity and threaten survival. As parasites are generally aggregated within certain host demographics, they are likely to affect a small proportion of the entire population, with specific hosts being at particular risk. However, little is known as to whether increased host mortality from parasitic causes is experienced by specific host demographics. Outside of theoretical studies, there is a paucity of literature concerning dynamics of parasite‐associated host mortality. Empirical evidence mainly focuses on short‐lived hosts or model systems, with data lacking from long‐lived wild or semi‐wild vertebrate populations. We investigated parasite‐associated mortality utilizing a multigenerational database of mortality, health, and reproductive data for over 4,000 semi‐captive timber elephants (Elephas maximus), with known causes of death for mortality events. We determined variation in mortality according to a number of host traits that are commonly associated with variation in parasitism within mammals: age, sex, and reproductive investment in females. We found that potentially parasite‐associated mortality varied significantly across elephant ages, with individuals at extremes of lifespan (young and old) at highest risk. Mortality probability was significantly higher for males across all ages. Female reproducers experienced a lower probability of potentially parasite‐associated mortality than females who did not reproduce at any investigated time frame. Our results demonstrate increased potentially parasite‐associated mortality within particular demographic groups. These groups (males, juveniles, elderly adults) have been identified in other studies as susceptible to parasitism, stressing the need for further work investigating links between infection and mortality. Furthermore, we show variation between reproductive and non‐reproductive females, with mothers being less at risk of potentially parasite mortality than nonreproducers.     

Observations on a hybrid between the whooper swan (<Emphasis Type="Italic">Cygnus cygnus</Emphasis>) and the mute swan (<Emphasis Type="Italic">Cygnus olor</Emphasis>)

The swan obtained by hybridization between Cygnus cygnus and C.olor was delivered to the Moscow Zoo from the Leningrad Zoo in 1989. Between 2002 and 2004, it permanently remained with a female whooper swan in a large pond. The two swans behaved as a breeding pair, although the sex of the hybrid bird remained unknown. We observed no courtship displays addressed by the hybrid swan either to the female whooper swan or to other swans living in the pond. The hybrid swan was exteriorly closer to C. cygnus rather than to C. olor and showed increased aggressiveness. Its threatening behavior was intermediate between parental species in terms of threat displays and motor activity, although behavioral patterns characteristic of C.olor prevailed. The most frequent sounds in the vocal repertoire of the hybrid swan were close to the vibrant throbbing call of C.olor in terms of duration and frequency range. In contrast to these sounds, the call of the hybrid swan was noiselike, with distinct harmonic structure (as in the whooper swan except for extremely narrow interharmonic intervals). These vocal signals of the hybrid swan were shown to be intermediate between those of the parental species. In addition, the acoustic repertoire of the hybrid swan included rarely emitted sounds closely resembling elements of vocalizations of both C. cygnus and C. olor.     

Spatiotemporal variation in local adaptation of a specialist insect herbivore to its long‐lived host plant

Local adaptation of interacting species to one another indicates geographically variable reciprocal selection. This process of adaptation is central in the organization and maintenance of genetic variation across populations. Given that the strength of selection and responses to it often vary in time and space, the strength of local adaptation should in theory vary between generations and among populations. However, such spatiotemporal variation has rarely been explicitly demonstrated in nature and local adaptation is commonly considered to be relatively static. We report persistent local adaptation of the short‐lived herbivore Abrostola asclepiadis to its long‐lived host plant Vincetoxicum hirundinaria over three successive generations in two studied populations and considerable temporal variation in local adaptation in six populations supporting the geographic mosaic theory. The observed variation in local adaptation among populations was best explained by geographic distance and population isolation, suggesting that gene flow reduces local adaptation. Changes in herbivore population size did not conclusively explain temporal variation in local adaptation. Our results also imply that short‐term studies are likely to capture only a part of the existing variation in local adaptation.     

Low hydrogen peroxide production in mitochondria of the long‐lived Arctica islandica: underlying mechanisms for slow aging

The observation of an inverse relationship between lifespan and mitochondrial H₂O₂ production rate would represent strong evidence for the disputed oxidative stress theory of aging. Studies on this subject using invertebrates are surprisingly lacking, despite their significance in both taxonomic richness and biomass. Bivalve mollusks represent an interesting taxonomic group to challenge this relationship. They are exposed to environmental constraints such as microbial H₂S, anoxia/reoxygenation, and temperature variations known to elicit oxidative stress. Their mitochondrial electron transport system is also connected to an alternative oxidase that might improve their ability to modulate reactive oxygen species (ROS) yield. Here, we compared H₂O₂ production rates in isolated mantle mitochondria between the longest‐living metazoan—the bivalve Arctica islandica—and two taxonomically related species of comparable size. In an attempt to test mechanisms previously proposed to account for a reduction of ROS production in long‐lived species, we compared oxygen consumption of isolated mitochondria and enzymatic activity of different complexes of the electron transport system in the two species with the greatest difference in longevity. We found that A. islandica mitochondria produced significantly less H₂O₂ than those of the two short‐lived species in nearly all conditions of mitochondrial respiration tested, including forward, reverse, and convergent electron flow. Alternative oxidase activity does not seem to explain these differences. However, our data suggest that reduced complex I and III activity can contribute to the lower ROS production of A. islandica mitochondria, in accordance with previous studies. We further propose that a lower complex II activity could also be involved.     

Coexistence and population genetic structure of the whooper swan <Emphasis Type="Italic">Cygnus cygnus</Emphasis> and mute swan <Emphasis Type="Italic">Cygnus olor</Emphasis> in Lithuania and Latvia

Two closely related swan species, the mute swan Cygnus olor and the whooper swan Cygnus cygnus, were formerly allopatric throughout their breeding ranges, but during the last decades a sympatric distribution has become characteristic of these species in the Baltic Sea region. The whooper swan has gradually replaced the mute swan in many suitable habitats in Lithuania and Latvia. Marked differences in the genetic population structure of both species may partially explain the dominance of the whooper swan, as genetic population divergence can be a major factor affecting inter-specific competition. A homogenous genetic population structure was defined for mute swans breeding in Lithuania, Latvia, Poland and Belarus. Breeding mute swans in this region are mostly of naturalised origin. A diverse population genetic structure characterizes whooper swans breeding in Lithuania and Latvia.     

Small‐scale resource tracking in a population of a long‐lived insect

How plant‐feeding insects distribute themselves and utilize their host plant resources is still poorly understood. Several processes may be involved, and their relative roles may vary with the spatial scale considered. Herein, we investigate small‐scale patterns, namely how population density of a gall midge is affected by individual growth form, phenology, and microsite characteristics of its herb host. The long‐lived plant individuals vary much with regard to number of shoots, flower abundance, and flowering phenology. This variation is connected to site characteristics, primarily the degree of sun exposure. The monophagous insect galls the flowers of the host plant – an easily defined food resource. It is a poor disperser, but very long‐lived; diapausing larvae can stay in the soil for many years. Galls were censused on individual plants during 5 years; from a peak to a low in gall population density. Only a very small fraction of the flowers produced (<0.5%) were galled even in the peak year. Nevertheless, most plant individuals had galls at least 1 year. In a stepwise multiple regression, plant size (number of shoots) was found to be the most important predictor of gall density (galls/flower). However, gall density decreased more than one order of magnitude over the plant size range observed. There was also a weak effect of plant phenology. Early flowering plants had lower gall densities than those starting later. Sun exposure had no direct effect on gall density, but a path analysis revealed indirect effects via the timing of flowering. Gall population change was highly synchronous in different parts of the study area with no significant decrease in synchrony with distance.     

Contrasting heterozygosity‐fitness correlations across life in a long‐lived seabird

Selection is a central force underlying evolutionary change and can vary in strength and direction, for example across time and space. The fitness consequences of individual genetic diversity have often been investigated by testing for multilocus heterozygosity‐fitness correlations (HFCs), but few studies have been able to assess HFCs across life stages and in both sexes. Here, we test for HFCs using a 26‐year longitudinal individual‐based data set from a large population of a long‐lived seabird (the common tern, Sterna hirundo), where 7,974 chicks and breeders of known age were genotyped at 15 microsatellite loci and sampled for life‐history traits over the complete life cycle. Heterozygosity was not correlated with fledging or post‐fledging prospecting probabilities, but was positively correlated with recruitment probability. For breeders, annual survival was not correlated with heterozygosity, but annual fledgling production was negatively correlated with heterozygosity in males and highest in intermediately heterozygous females. The contrasting HFCs among life stages and sexes indicate differential selective processes and emphasize the importance of assessing fitness consequences of traits over complete life histories.     

Mating dynamics and multiple paternity in a long‐lived vertebrate

Multiple paternity is relatively common across diverse taxa; however, the drivers and implications related to paternal and maternal fitness are not well understood. Several hypotheses have been offered to explain the occurrence and frequency of multiple paternity. One set of hypotheses seeks to explain multiple paternity through direct and indirect benefits including increased genetic diversity or enhanced offspring fitness, whereas another set of hypotheses explains multiple paternity as a by‐product of sexual conflict and population‐specific parameters such as density. Here, we investigate mating system dynamics in a historically studied population of the American alligator (Alligator mississippiensis) in coastal South Carolina. We examine parentage in 151 nests across 6 years and find that 43% of nests were sired by multiple males and that male reproductive success is strongly influenced by male size. Whereas clutch size and hatchling size did not differ between singly sired and multiply sired nests, fertility rates were observed to be lower in multiply sired clutches. Our findings suggest that multiple paternity may exert cost in regard to female fitness, and raise the possibility that sexual conflict might influence the frequency of multiple paternity in wild alligator populations.     

Spatial determinants of infection risk in a multi‐species avian malaria system

Spatially‐variable processes can be an important element of host–parasite interactions, but their longer term demographic and evolutionary effects depend on the magnitude of variation in space, the scale at which variation occurs and the degree to which such processes are temporally stable. Here, we use multiple years of data from a study of two closely related tit species (Paridae), infected with two congeneric species of avian malaria (Plasmodium), to evaluate the roles of extrinsic and intrinsic factors in driving spatial heterogeneity in infection risk, and to address questions of scale and temporal stability in these vector‐driven host–parasite interactions. We show that the two malaria parasite species exhibit markedly different spatial epidemiology: P. relictum infections are effectively randomly distributed in space, with no temporal consistency, whereas P. circumflexum infections exhibit pronounced spatial structuring that is stable over the six years of this study and similar in both host species. We show that both conspecific and heterospecific host density contribute to elevated infection risk, but that the main determinants of elevated risk of P. circumflexum infection risk are habitat features probably associated with vector distribution and abundance. We discuss the implications of these findings, both for our understanding of the epidemiology of malaria in the wild, but also in terms of the longer‐term evolutionary and demographic consequences that spatially variable parasite‐mediated selection may have on host populations.     

A longitudinal study of age‐related changes in Haemoproteus infection in a passerine bird

Blood parasites such as malaria and related haemosporidians commonly infect vertebrate species including birds. Understanding age‐specific patterns of parasite infections is crucial for quantifying the fitness consequences of parasitism for hosts and for understanding parasite transmission dynamics. We analyzed longitudinal and cross‐sectional infection data in house martins Delichon urbica, a migratory bird suffering from intense haemosporidian infection. We separated within‐ from among‐individual effects of age on prevalence. Our results showed that the probability of blood parasite infection increased as individual house martins aged. We also showed that the prevalence of infection decreased with age at last reproduction when controlling for age, showing a selective disappearance of infected birds from the population (i.e. selection). The estimated effect of age on prevalence was underestimated two‐ to three‐fold if not accounting for such selection. This study highlights the importance of taking among‐individual heterogeneity in the capacity to fight a disease into account because such heterogeneity can mask age‐related patterns of infection. These findings emphasize the relevance of considering within‐ and among‐individual patterns of infection in order to understand parasite‐induced mortality and the potential for parasite transmission.     

The epidemiology of the highly pathogenic H5N1 avian influenza in Mute Swan (Cygnus olor) and other Anatidae in the Dombes region (France), 2006

In February 2006, a highly pathogenic avian influenza (HPAI) H5N1 virus was isolated from Common Pochards (Aythia ferina) in the Dombes region of France, an important migrating and wintering waterfowl area. Thereafter, HPAI H5N1 virus was isolated from 39 swab pools collected from dead waterfowl found in the Dombes, but only from three pooled samples collected outside of this area but located on the same migration flyway. A single turkey farm was infected in the Dombes. The epizootic lasted 2 mo and was restricted to the Dombes area. Virus-positive pools were detected in 20 of 1,200 ponds and infected Mute Swans (Cygnus olor) represented 82% of the virus-positive pools. Other infected species included Common Pochard (n=4), Grey Heron (Ardea cinerea, n=1), Eurasian Buzzard (Buteo buteo, n=1), and Greylag Goose (Anser anser, n=1). Despite intensive monitoring during and after the outbreak, HPAI H5N1 virus was not isolated from healthy wild birds. Our results are consistent with an HPAI H5N1-virus introduction into the Dombes via migrating ducks. These birds could have been pushed west by a severe cold spell in central Europe where the virus had already been detected. The Mute Swan served as an excellent epidemiologic sentinel during this outbreak; swans appear to be highly sensitive to infection with these viruses and swan mortality was easy to detect. During the outbreak, the mortality rates for wild birds remained moderate and the virus affected a limited number of species.     

Cryptic inbreeding depression in a growing population of a long‐lived species

Genetic effects are often overlooked in endangered species monitoring, and populations showing positive growth are often assumed to be secure. However, the continued reproductive success of a few individuals may mask issues such as inbreeding depression, especially in long‐lived species. Here, we test for inbreeding depression in little spotted kiwi (Apteryx owenii) by comparing a population founded with two birds to one founded with 40 birds, both from the same source population and both showing positive population growth. We used a combination of microsatellite genotypes, nest observations and modelling to examine the consequences of assessing population viability exclusively via population growth. We demonstrate (i) significantly lower hatching success despite significantly higher reproductive effort in the population with two founders; (ii) positive growth in the population with two founders is mainly driven by ongoing chick production of the founding pair; and (iii) a substantial genetic load in the population founded with two birds (10–15 diploid lethal equivalents). Our results illustrate that substantial, cryptic inbreeding depression may still be present when a population is growing, especially in long‐lived species with overlapping generations.     

Investigating the effects of age‐related spatial structuring on the transmission of a tick‐borne virus in a colonially breeding host

Higher pathogen and parasite transmission is considered a universal cost of colonial breeding due to the physical proximity of colony members. However, this has rarely been tested in natural colonies, which are structured entities, whose members interact with a subset of individuals and differ in their infection histories. We use a population of common guillemots, Uria aalge, infected by a tick‐borne virus, Great Island virus, to explore how age‐related spatial structuring can influence the infection costs borne by different members of a breeding colony. Previous work has shown that the per‐susceptible risk of infection (force of infection) is different for prebreeding (immature) and breeding (adult) guillemots which occupy different areas of the colony. We developed a mathematical model which showed that this difference in infection risk can only be maintained if mixing between these age groups is low. To estimate mixing between age groups, we recorded the movements of 63 individually recognizable, prebreeding guillemots in four different parts of a major colony in the North Sea during the breeding season. Prebreeding guillemots infrequently entered breeding areas (in only 26% of watches), though with marked differences in frequency of entry among individuals and more entries toward the end of the breeding season. Once entered, the proportion of time spent in breeding areas by prebreeding guillemots also varied between different parts of the colony. Our data and model predictions indicate low levels of age‐group mixing, limiting exposure of breeding guillemots to infection. However, they also suggest that prebreeding guillemots have the potential to play an important role in driving infection dynamics. This highlights the sensitivity of breeding colonies to changes in the behavior of their members—a subject of particular importance in the context of global environmental change.     

Diet–demography relationships in a long‐lived predator: from territories to populations

Understanding the mechanisms that shape animal population dynamics is of fundamental interest in ecology, evolution and conservation biology. Food supply is an important limiting factor in most animal populations and may have demographic consequences. Optimal foraging theory predicts greater consumption of preferred prey and less diet diversity when food is abundant, which may benefit key fitness parameters such as productivity and survival. Nevertheless, the correspondence between individual resource use and demographic processes in populations of avian predators inhabiting large geographic areas remains largely unexplored, particularly in complex ecosystems such as those of the Mediterranean basin. Based on a long‐term monitoring program of the diet and demography of Bonelli's eagle Aquila fasciata in western Europe, here we test the hypothesis that a predator's diet is correlated to its breeding productivity and survival at both the territorial and population levels, and ultimately to its population growth rate. At the territorial level, we found that productivity increased with greater consumption of European rabbits Oryctolagus cuniculus, the Bonelli's eagle's preferred prey, and pigeons, an important alternative prey for this predator. The survival of territorial pairs was negatively affected by higher diet diversity, which probably reflected the inability to find sufficient high quality prey. Diet effects at the population level were similar but more noticeable than at the territorial level, i.e. a greater consumption of rabbits, together with lesser consumption of small‐to‐medium avian species (‘other birds’; non‐preferred prey), increased productivity, while greater diet diversity and lower consumption of rabbits was associated with reduced survival and population growth rate. Overall, our study illustrates how the diet of a predator species can be closely related to key individual vital rates, which, in turn, leave a measurable fingerprint on population dynamics within and among populations across large spatial scales.