Foraging strategies of Great Cormorants Phalacrocorax carbo carbo wintering north of the Arctic Circle

This study describes how 30 Great Cormorants Phalacrocorax carbo carbo managed to catch sufficient food for their daily energetic needs under conditions of reduced daylight and cold while wintering north of the Arctic Circle. Activity observations showed that the Great Cormorants' daily foraging pattern was generally bimodal, with morning and evening feeding peaks. They compensated for shorter daylengths in midwinter by starting to forage later and ending progressively earlier at lower light intensities. Fishing constituted only a minor part of their time–activity budget, and was one of the most efficient reported in marine birds. The Great Cormorants spent less than 60 minutes a day fishing in midwinter. Although subzero ambient temperatures and blizzards contributed to increased heat loss in midwinter, this potential energy loss did not seem to be compensated for by an increase in fish intake. Instead the Great Cormorants seemed to economize energy expenditure by halving the time spent at sea, and halving the number but doubling the mass of each fish taken.     

Great Cormorants Phalacrocorax carbo feed on larger fish in late winter

Capsule: Great Cormorants Phalacrocorax carbo prey on larger fish in late winter compared to early winter.

Aims: This study aimed to describe Cormorant diet and to discover whether prey size changed during winter.

Methods: The diet of Cormorants was studied using regurgitated pellets collected from roosting places at the upper Elbe River, Czech Republic, during winter (from December to March).

Results: The diet consisted of 24 fish species from 6 fish families. Roach Rutilus rutilus dominated in the diet (over 50% of biomass). Size of fish in the diet increased over time during the whole winter. Except for European Chub Squalius cephalus, the increased size applied for the majority of the most frequently consumed fish species: Roach, European Perch Perca fluviatilis and White Bream Abramis brama. Cormorants consumed mostly fish species of lower commercial and angling value.

Conclusion: Fisheries management should reflect on the fact that competition for larger-sized fish is the highest in late winter and in early spring. For that reason, stocking of potentially vulnerable fish could be delayed to the time when overwintering birds leave the area and recreational fishing could be restricted in late winter.     

Late‐breeding Great Cormorants Phalacrocorax carbo sinensis produce fewer young of the more vulnerable sex

We examined the brood sex ratio and offspring body mass in relation to the timing of breeding and brood size in the Great Cormorant Phalacrocorax carbo sinensis. The brood sex ratio was not related to brood size but it was significantly related to the hatching date, with a decreasing proportion of males in the brood in the course of the season. Male chicks had significantly lower body mass if they hatched later in the season, whereas there was no such relationship for female offspring. Assuming that environmental conditions deteriorate with progress of the breeding season, and male offspring may be more vulnerable to poor environmental conditions, the observed decline in the proportion of male offspring late in the season may be adaptive.     

The relationship between sea surface temperature and population change of Great Cormorants Phalacrocorax carbo breeding near Disko Bay,Greenland

Arctic seas have warmed and sea ice has retreated. This has resulted in range contraction and population declines in some species, but it could potentially be a boon for others. Great Cormorants Phalacrocorax carbo have a partially wettable plumage and seem poorly suited to foraging in Arctic waters. We show that rates of population change of Cormorant colonies around Disko Bay, Greenland, are positively correlated with sea surface temperature, suggesting that they may benefit from a warming Arctic. However, although Cormorant populations may increase in response to Arctic warming, the extent of expansion of their winter range may ultimately be limited by other factors, such as sensory constraints on foraging behaviour during long Arctic nights.     

Stable isotopes indicate the extent of freshwater feeding by cormorants Phalacrocorax carbo shot at inland fisheries in England

1. The numbers of cormorants Phalacrocorax carbo feeding at English freshwater fisheries during winter have increased rapidly over the last 20 years, causing concern among fishery managers and anglers.
2. In order to assess the extent of freshwater feeding, stable isotope ratios of carbon and nitrogen (δ¹³C and δ¹⁵N) in feathers of wild cormorants from inland freshwater fisheries were compared with those in the feathers of piscivorous birds with marine diets (captive 'marine-fed' cormorants, free-ranging shags Phalacrocorax aristotelis ) and freshwater diets (juvenile goosanders Mergus merganser ).
3. Isotope signatures of feathers represent the diet at the time of growth. Feathers grown at different times of the year were taken from wild cormorants; each feather type therefore represented the diet over a different temporal scale.
4. Isotopic analyses of feathers indicated that, when shot, nearly all of the cormorants had been feeding entirely on freshwater prey. The mean δ¹³C value of primary feathers growing when birds were shot was –22·2‰, indicative of an entirely freshwater diet.
5. The move to freshwater habitats from coastal breeding grounds occurred over several months, but once established cormorants appear to have fed at freshwater sites throughout the autumn and winter.
6. The suitability of using a two-source isotopic mixing model in order to quantify the extent of freshwater feeding in piscivorous birds is discussed.
7. Although the results indicate long-term residency and feeding in freshwater systems, they do not indicate whether birds were feeding regularly at the sites at which they were shot, or the composition of the diet. It is recommended that further studies using telemetry and multiple isotope analyses be carried out in order to address these issues.     

青海湖鸬鹚繁殖成功率初报

2000年3—8月对青海湖西山鸬鹚岛鸬鹚(Phalacrocorax carbo)不同繁殖阶段的成功率做了初步观察。鸬鹚卵的窝卵数为3．47枚,孵化率为37．9％,雏鸟成活率为61．1％,繁殖成功率为23．2％。孵化期的前3d,鸬鹚巢的损失率高达36．1％;育雏期前10d雏鸟损失率为25％。这些数据可看作鸬鹚种群-青海湖湿地生态系统的一个重要组分——动态监测的起始数据之一,可在未来进一步的比较分析中得到应用。     

Vision and the foraging technique of Great Cormorants Phalacrocorax carbo: pursuit or close‐quarter foraging?

Predatory diving birds, such as cormorants (Phalacrocoracidae), have been generally regarded as visually guided pursuit foragers. However, due to their poor visual resolution underwater, it has recently been hypothesized that Great Cormorants do not in fact employ a pursuit-dive foraging technique. They appear capable of detecting typical prey only at short distances, and primarily use a foraging technique in which prey may be detected only at close quarters or flushed from a substratum or hiding place. In birds, visual field parameters, such as the position and extent of the region of binocular vision, and how these are altered by eye movements, appear to be determined primarily by feeding ecology. Therefore, to understand further the feeding technique of Great Cormorants we have determined retinal visual fields and eye movement amplitudes using an ophthalmoscopic reflex technique. We show that visual fields and eye movements in cormorants exhibit close similarity with those of other birds, such as herons (Ardeidae) and hornbills (Bucerotidae), which forage terrestrially typically using a close-quarter prey detection or flushing technique and/or which need to examine items held in the bill before ingestion. We argue that this visual field topography and associated eye movements is a general characteristic of birds whose foraging requires the detection of nearby mobile prey items from within a wide arc around the head, accurate capture of that prey using the bill, and visual examination of the caught prey held in the bill. This supports the idea that cormorants, although visually guided predators, are not primarily pursuit predators, and that their visual fields exhibit convergence towards a set of characteristics that meet the perceptual challenges of close-quarter prey detection or flush foraging in both aquatic and terrestrial environments.     

Sexual character intensity and its relationship to breeding timing, fecundity and mate choice in the great cormorant Phalacrocorax carbo lucidus

Reproductive success and the intensity of five separate seasonal sexual characters, expressed similarly in both sexes and likely to be dependent on age and/or individual condition, were documented at a colony of great cormorants at Lake Naivasha, Kenya (00°46' S, 36°22' E) in 1996. The characters assessed were head filoplumes, thigh patch feathers, colour of cheek, neck and upper breast plumage, colour of gular skin, and colour of suborbital skin. More intensely developed sexual characters at the time of pair formation were associated with significantly earlier breeding. Pairs that bred earlier fledged significantly more chicks than those breeding later. However, correlations between the number of chicks fledged and the intensity of the parents' sexual characters at pair formation were generally weak and not significant. An exception was the colour of the cheek, foreneck and upper breast plumage: males (but not females) with the darkest plumage in these areas fledged significantly more chicks than others. Correlations between paired male and female sexual character intensity were weak, and significant only in the case of suborbital skin colour. Other factors may have been more important than these characters in influencing mate selection.     

Maximizing dietary information retrievable from carcasses of Great Cormorants Phalacrocorax carbo using a combined morphological and molecular analytical approach

Avian carcasses can provide important information on the trophic ecology of birds. Usually, the number of carcasses available for examination is limited and therefore it is important to gain as much dietary information per specimen as possible. In piscivorous birds and raptors, the stomach has been the primary source of dietary information, whereas the gut (intestine) has so far been neglected as it usually contains only a few morphologically identifiable hard parts of prey. Molecular approaches have the potential to retrieve dietary information from the gut, although this has not yet been verified. As well as identifying the prey, it is important to estimate any secondary predation to avoid food web errors in dietary analyses. The assignment of accidentally consumed prey is notoriously difficult regardless of the prey identification approach used. In the present study, morphological and molecular analyses were, for the first time, combined to maximize the dietary information retrievable from the complete digestive tract of Great Cormorants Phalacrocorax carbo sinensis. Moreover, a novel approach based on predator–prey size ratios was applied to these piscivorous birds to minimize the number of samples that might contain secondarily predated prey. The stomach contents of the examined birds were found to provide the most dietary information when morphological and molecular analyses were used in combination. However, compared with the morphological approach, the molecular analysis increased the number of fish species detected by 39%. The molecular approach also permitted the identification of fish DNA in the Cormorant guts. Predator–prey size ratios derived from morphological analysis of fish hard parts can reduce the incidence of potential confounding influence of secondarily predated prey by 80%. Our findings demonstrate that a combination of morphological and molecular approaches maximizes the trophic information retrievable from bird carcasses.     

Prospecting enhances breeding success of first-time breeders in the great cormorant, Phalacrocorax carbo sinensis

In many species of colonial seabirds, young birds visit colonies in the years before they start breeding. This prospecting behaviour may allow them to obtain information that could enhance their future breeding success. We examined the reproductive consequences of prospecting behaviour in the colonial great cormorant, and found support for this idea. New breeders that had been prospecting actively in the previous year obtained breeding sites of higher quality (i.e. closer to sites where conspecifics had fledged young in the previous year) and had higher breeding success than those that had been less active. Prospecting occurred mostly late in the breeding season, and coincided with the time when the majority of the eggs had hatched but before the chicks started fledging, that is, when breeding success in the colony reflected habitat suitability. These results are thus consistent with the use of conspecific reproductive performance as a cue for the quality of a breeding habitat as expected from the 'performance-based conspecific attraction hypothesis'. Copyright 1999 The Association for the Study of Animal Behaviour.     

Sexual ornament size and breeding performance in female and male European Shags Phalacrocorax aristotelis

Research into sexual selection and animal ornamentation has been largely directed at males in sexually dimorphic species. In contrast, there is relatively little empirical information on female ornamentation. Using the European Shag Phalacrocorax aristotelis , a species in which both sexes have an ornamental crest during pair formation, we investigated the evidence for (1) a correlation between crest size and breeding performance and (2) assortative pairing on crest size. Crest size appeared unrelated to age in either sex, though our sample of males did not include first-time breeders. The year of study was one in which overall breeding performance at the colony was poor. Females that did not subsequently produce a clutch had significantly smaller crests during the courtship period than those that did, suggesting a link between ornament size and current condition. This relationship was also significant in males. Among those birds that did produce eggs, there was a significant relationship between ornament size and laying date, again significant in both sexes, with early breeders having larger crests. Early breeding is generally associated with relatively high breeding performance in European Shags. Given that both males and females contribute substantially to parental care and that there will be intrasexual differences in quality, choosiness is likely to be favoured in both sexes, and individuals were found to be paired assortatively with respect to crest size. Our results suggest that ornament size is related to state in European Shags, and thus may be involved in mate choice in both sexes.     

Geographical and temporal variation in the diet of Bank Cormorants Phalacrocorax neglectus in South Africa

The Bank Cormorant Phalacrocorax neglectus is endemic to the Benguela upwelling ecosystem off southwest Africa and is classified as Endangered owing to a recent large reduction in its number. It is thought that food scarcity, including a decreased abundance of West Coast rock lobster Jasus lalandii, has been a major driver of the decrease, yet its diet in South Africa is poorly known. We collected 941 pellets regurgitated by Bank Cormorants, at 18 South African breeding colonies during 1975–1985, and 1 523 pellets at 17 colonies during 1995–2002. The species composition of the diet (% numbers) was significantly different between the two periods, with widespread decreases in proportions of rock lobster in the west and of octopus and cuttlefish Sepia spp. at most localities. These taxa were replaced in the diet by fish, including Gobiidae and Clinidae. The pelagic goby Sufflogobius bibarbatus, an important prey of Bank Cormorants in Namibia, was absent from pellets collected in 1975–1985 but common at northern localities from 1995–2002. Composition of the diet by frequency of occurrence was only determined for 1995–2002, when rock lobster was present in 67% of all samples collected, cuttlefish in 39%, and Clinidae in 32%. Data for 1975–1985 and 1995–2002 showed that carapace lengths of rock lobsters eaten by Bank Cormorants averaged 56 mm (range 22–82 mm) and 50 mm (range 22–75 mm), respectively, which compares to the minimum legal size of 75 mm for fisheries in South Africa. This energy- rich prey item was an important constituent of the diet in the winter breeding period.     

Water retention in the plumage of diving great cormorants Phalacrocorax carbo sinensis

Cormorants are assumed to have a "partially wettable" plumage as a mechanism to reduce buoyancy while swimming underwater. This assumption is mainly based on 3 observations: 1) the volume of air in the plumage of submerged carcasses is small compared to other water birds, 2) cormorants assume a "wing drying" posture when they exit the water, and 3) the feather structure of the plumage. An alternative mechanism to reduce buoyancy is to release air out of the plumage by ptilomotion without allowing water to penetrate. How wet cormorants actually get is an open issue that has important implications for the energy budget of these warm blooded aquatic predators. Here we report empirical measurements on the amount of water retained in the plumage of live great cormorant Phlacrocorax carbo sinensis during voluntary swimming and diving in an experimental design that simulates a foraging diving bout. The amount of water retained in the plumage increased as a function of time spent in water. However the birds limited their dive bouts to less than 18 minutes so that the added mass of retained water did not exceed 6% of their body mass. This maximal level of water retention is estimated to reduce the buoyancy of the dry bird by 18%. This maximal level is also similar to measurements of water retention of carcasses and suggests that measurements preformed on carcasses yield only the upper level of water absorption while live birds slow down water penetration, allowing longer periods of foraging.     

Numbers and distribution of Northern Lapwings Vanellus vanellus breeding in England and Wales in 1998

This paper presents the results of the BTO/RSPB survey of breeding Northern Lapwings Vanellus vanellus in England and Wales carried out in 1998. Numbers of breeding Lapwings and land-use types were recorded by volunteers in 1312 randomly selected tetrads using identical methods to those adopted in a previous survey in 1987. The results suggest a population of 62 923 breeding pairs (95% confidence intervals 55 268 to 74 499), representing a 49% decline in 11 years. The declines were greatest in southwest England and Wales, resulting in a marked contraction in range. Both arable and pastoral farmland was found to be important for Lapwings, highlighting their vulnerability to changes in a range of agricultural systems. Recent major changes in land management practices, including the switch from spring to autumn sowing and the conversion of rough to intensively managed, improved grassland is likely to have reduced both the area and the suitability of these habitats for Lapwings. Grassland within arable areas is known to provide important chick rearing habitat, and spring tillage with adjacent grassland was significantly more likely to hold Lapwings than spring tillage that did not abut grass fields. The prevalence of grass and spring tillage fields in close proximity has declined in recent years with the loss of traditional mixed farming and the polarization of pastoral systems in the west of Britain and arable in the east. The implications of these results for the conservation of Lapwings are discussed and it is recommended that, in the light of the scale of the decline recorded, Lapwings should be afforded much higher conservation priority in Britain than they are at present. The range contraction has resulted in high numbers being concentrated on reserves, highlighting the importance of sensitive management of these areas for this species. However, wider countryside measures should also be considered, such as specific options within agri-environment schemes, designed to enhance farmland habitats for Lapwings.     

Fitness Consequences of Timing of Migration and Breeding in Cormorants

In most bird species timing of breeding affects reproductive success whereby early breeding is favoured. In migratory species migration time, especially arrival at the breeding grounds, and breeding time are expected to be correlated. Consequently, migration time should also have fitness consequences. However, in contrast to breeding time, evidence for fitness consequences of migration time is much more limited. Climate change has been shown to negatively affect the synchrony between trophic levels thereby leading to directional selection on timing but again direct evidence in avian migration time is scarce. We here analysed fitness consequences of migration and breeding time in great cormorants and tested whether climate change has led to increased selection on timing using a long-term data set from a breeding colony on the island of Vorsø (Denmark). Reproductive success, measured as number of fledglings, correlated with breeding time and arrival time at the colony and declined during the season. This seasonal decline became steeper during the study period for both migration and breeding time and was positively correlated to winter/spring climate, i.e. selection was stronger after warmer winters/springs. However, the increasing selection pressure on timing seems to be unrelated to climate change as the climatic variables that were related to selection strength did not increase during the study period. There is indirect evidence that phenology or abundances of preferred prey species have changed which could have altered selection on timing of migration and breeding.     

Seed germination and seedling survival in a colony of the common cormorant,Phalacrocorax carbo

Germination and seedling survival of some woody plants were examined in a colony of the common cormorant at Unoyama, central Japan. The germination ofQuercus serrata andPinus densiflora was less successful in the colony than outside the colony. The percentage of germination success was negatively correlated with the amount of cormorant faeces scattered on the ground and also with the soil water conent. Most of the germinated seedlings inside of the colony died with symptoms such as necrosis spreading from the edge of leaves. Saplings ofQ. serrata also tended to die more in the colony than outside the colony. The survival rate ofQuercus glauca seedlings with scattered faeces on their leaves and/or on the the surrounding soil was significantly lower than the rate of those free of faeces. These results suggest that in the cormorant colony, germination of seeds and seedling survival are greatly inhibited due to both the direct and indirect effects of cormorant faeces.