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.     

Causes of mortality in the Mute swan Cygnus olor on the River Thames

Over the last 20 years there has been an unprecedented decline of the Mute swan on the River Thames. Between August 1979 and October 1981 a systematic attempt was made to collect as many swan corpses as possible. Of 94 swan corpses examined at post-mortem 57% died from lead poisoning due to ingestion of anglers' weights. Birds dying from lead poisoning had significantly higher liver and kidney lead levels. Kidney proved to be the most useful indicator tissue, the median lead level in birds with lead weights in their gizzard was 908 μg/g DM compared to 8 μg/g DM in birds without weights. More immature birds die from lead poisoning than other factors and significantly more corpses were found between the months of July-October.
Ultra-structure examinations revealed electron dense intra-nuclear inclusions in the cells of proximal convulated tubules of the kidney and X-ray analysis confirmed that these structures consisted of lead.     

The epidemiology underlying age‐related avian malaria infection in a long‐lived host: the mute swan Cygnus olor

Quantifying the factors that predict parasite outbreak and persistence is a major challenge for both applied and fundamental biology. Key to understanding parasite prevalence and disease outbreaks is determining at what age individuals show signs of infection, and whether or not they recover. Age‐dependent patterns of the infection of a host population by parasites can indicate among‐individual heterogeneities in their susceptibility to, or rate of recovery from, parasite infections. Here, we present a cross‐sectional study of avian malaria in a long‐lived bird species, the mute swan Cygnus olor, examining age‐related patterns of parasite prevalence and modelling patterns of infection and recovery. One‐hundred and fifteen swans, ranging from one to nineteen years old, were screened for infection with Plasmodium, Haemoproteus and Leucocytozoon parasites. Infections with three cytochrome‐b lineages of Haemoproteus were found (pooled prevalence 67%), namely WW1 (26%), which is common in passerine birds, and two new lineages closely related to WW1: MUTSW1 (25%) and MUTSW2 (16%). We found evidence for age‐related infection in one lineage, MUTSW1. Catalytic models examining patterns of infection and recovery in the population suggested that infections in this population were not life‐long – recovery of individuals was included in the best fitting models. These findings support the results of recent studies that suggest hosts can clear infections, although patterns of infection‐related mortality in older birds remain to be studied in more detail.     

Density-dependent decline of breeding success in an introduced, increasing mute swan Cygus olor population

An increasing population of the mute swan Cygnus olor , was studied from its very establishment in 1976 until 1998. As the number of pairs increased, there was a decline in all production parameters, including the average number of clutches per pair, the average number of broods per clutch, and the average number of fledged young per brood. In a multiple regression analysis covering the whole breeding season, the number of pairs and the average number of clutches per pair explained 71% of the variation in the average number of fledglings per pair. During the brood season, the average number of fledglings per brood was an additionally important parameter in explaining fledgling production. Habitat quality seemed to affect breeding: in places occupied earlier more cygnets fledged from a clutch than in habitats inhabited later. The decreased production of young very likely reflects density-dependent effects on reproduction. This density dependence seems to operate on the breeding grounds, since winter harshness did not affect breeding success. Density-dependent processes started acting already when pairs were beginning to nest, and continued during the brood period. Density dependence has apparently not been detected at the pair stage in earlier studies of bird populations.     

Intracellular localization of lead in tissues of the mute swan

The aim of this study was to examine the ultrastructure of kidney, liver, blood and intestine of a lead-poisoned bird and compare these with tissues of a healthy bird. From these results it was hoped to reveal that any abnormalities observed were due to lead poisoning and consequently suitable sections were also selected for X-ray analysis. Renal intranuclear inclusions were present in the lead-poisoned swan. X-ray analysis clearly demonstrated that these granules consist of an amorphous mass of lead probably as lead phosphate. Large numbers of electron dense granules were observed in the liver of the lead-poisoned bird and occasionally in the healthy swans liver. X-ray analysis demonstrated that these granules contain iron. Some of the red blood cells of the lead-poisoned swan have opaque granules associated with the plasma membrane, these were not observed in the healthy swan.     

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.     

Winter territoriality of Mute Swans Cygnus olor

There was considerable variation in winter territoriality among Mute Swan pairs around Oxford, though most pairs maintained their territories for at least half the winter. Variation was associated with temperature and with territory quality pairs left their territories most commonly during the coldest months of the winter, and pairs on good territories, with abundant vegetation and accessible pasture. stayed longer on territory than those on poor territories with sparse vegetation and little pasture. As in other species, exclusive territorial defence was maintained at intermediate levels of resource availability, and flocks were able to settle on a few particularly good territories. Circumstantial evidence suggested that winter territories may be maintained not only to provide an adequate food supply for the owners but also to ensure access to a brerding site by preventing other individuals from taking over the area.     

Effects of mute swan grazing on a keystone macrophyte

1. This study describes the early summer foraging behaviour of mute swans (Cygnus olor) on the River Frome, a highly productive chalk stream in southern England in which Ranunculus penicillatus pseudofluitans is the dominant macrophyte. 2. A daily maximum of 41 ± 2.5 swans were present along the 1.1 km study reach during the study period (late May to the end of June). The river was the primary feeding habitat. Feeding activity on the river at dawn and dusk was much lower than during daylight, but we cannot rule out the possibility that swans fed during the hours of darkness. 3. The effects of herbivory on R. pseudofluitans biomass and morphology were quantified. Biomass was lower in grazed areas and swans grazed selectively on leaves in preference to stems. A lower proportion of stems from grazed areas possessed intact stem apices and flowering of the plant was reduced in grazed areas. 4. A model, based on the swans’ daily consumption, was used to predict the grazing pressure of swans on R. pseudofluitans. The model accurately predicted the number of bird days supported by the study site, only if grazing was assumed to severely reduce R. pseudofluitans growth. The proportion of the initial R. pseudofluitans biomass consumed by a fixed number of swans was predicted to be greater when the habitat area was smaller, initial R. pseudofluitans biomass was lower and R. pseudofluitans was of lower food value. 5. We concluded that the flux of N and P through the study reach was largely unaffected by swan activity. The quality of R. pseudofluitans mesohabitat (the plant as habitat for invertebrates and fish) was significantly reduced by grazing which also indirectly contributed to reduced roughness (Manning's n) and by inference water depth. Wetted habitat area for fish and invertebrates would also be lowered over the summer period as a consequence of the reduction in water depth. It was estimated that, while grazing, an individual swan may eat the same mass of invertebrates per day as a 300‐g trout. 6. There is a need to manage the conflict between mute swans and the keystone macrophyte, R. pseudofluitans, in chalk streams, and the modelling approach used here offers a potentially useful tool for this purpose.     

Effects of seasonality,isolation and patch quality for habitat selection processes by mute swans Cygnus olor in a fishpond landscape

Foragers in patchy environments do not only select sites for single patch characteristics, but also have to consider the local environment of such patches. We studied habitat selection by mute swans Cygnus olor in a wide and heterogeneous fishpond region (the Dombes, eastern France). In this study, we considered fishpond isolation, resource quality within fishponds and breeding status of mute swans during both summer and winter. Mute swans did not select aquatic habitat randomly within the landscape. During summer, the population spread preferentially on medium to large fishponds, in subregions with numerous or closely related waterbodies, without generating a clumped distribution of birds. In addition to a positive effect of local fishpond number (2 km radius), breeding birds also responded positively to fishpond size. Non‐breeders selected fishponds mainly according to their size. Intraspecific territoriality did not appear to limit the presence of non‐breeders (i.e. moulting flocks), since both breeders and non‐breeders could coexist on the larger fishponds. During winter, mute swans used medium to large reflooded fishponds after summer drainage. The surrounding aquatic environment of fishponds played a minor role in determining flocking, compared to actual patch quality. Flocking occurred on large fishponds that had reflooded after having dried the summer before, whatever the agricultural cultivation practiced in the summer following drainage. The results suggest that geographical aspects should be taken into account when considering the potential impact of this expanding species within such ecosystems, and also in more general management policies dealing with aquatic habitats for waterbird populations.     

Lead levels in the blood of Mute swans Cygnus olor on the River thames

In this paper blood lead levels in three categories of Mute swan are examined, (i) flock birds (ii) breeding birds and (iii) cygnets. From these regional, seasonal and sex variation for both 1980 and 1981 was examined. In addition a portable haematofluorometer was assessed to determine its possible use as an alternative to atomic absorption and spectrophotometry for determining a measure of lead exposure.
Very few swans on the River Thames had blood lead levels below the maximum acceptable level of 40 mgg/100 ml. In general lead levels increased with proximity to London and the swans on the tributaries consistently had the lowest levels which were always below the maximum acceptable level. Blood lead levels in flock birds were shown to be highest during the coarse fishing season and it was only during the close season that levels dropped to around 40 g/1OO ml. Breeding females had significantly higher lead levels than males and females with lead levels in excess of 200 g/1OO ml seemed to have a poor chance of producing cygnets or surviving to the next breeding season. Cygnet mortality was significantly higher on the lower Thames where blood lead levels were also known to be at their highest.     

An analysis of weight changes in the Mute Swan Cygnus olor

The weights of Mute Swans contain useful information on individuals' prospects for survival and breeding but to interpret them one needs to know what 'typical' weights would be. This paper uses multiple linear regression to predict such 'typical' weights: 1968 zveighings of 957 Mute Swans, from south Staffordshire, England, are analysed with respect to sex, age, moultstage, month of capture and breeding status. Because of marked sexual dimorphism, knowing a swan's sex is fundamental to interpreting its weight; age from hatching and stage of primary feather growth affect cygnets' weights near fledging; weights of young swans change with age during their first years of life; primary feather moult significantly depresses the weights of first year males but not, in this study area, of older males or females; monthly, seasonal changes are generally small; breeding adults are significantly heavier. The results and their biological implications agree well with other, less comprehensive, analyses. The method described could facilitate both the identification of other factors that should be investigated and comparisons between different study areas.     

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.     

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.     

The effect of breeding status on the timing of moult in Mute Swans Cygnus olor

It is well known that in breeding Mute Swans Cygnus olor males initiate moult 4–6 weeks after their mate, and that in non-breeding flocks males and females moult at about the same time. Here we show that mated pairs that have lost their young moult at the same time, implying that the male is able to avoid the delay in moult at very short notice.