Thomas Bregnballe

Evidence for density‐dependent survival in adult cormorants from a combined analysis of recoveries and resightings

1. The increasing population of cormorants (Phalacrocorax carbo sinensis) in Europe since 1970 has led to conflicts with fishery interests. Control of cormorant populations is a management issue in many countries and a predictive population model is needed. However, reliable estimates of survival are lacking as input for such a model 2. Capture-recapture estimates of survival of dispersive species like cormorants suffer from an unknown bias due to permanent emigration from the study area. However, a combined analysis of resightings and recovery of dead birds allows unbiased estimates of survival and emigration. 3. We use data on 11 000 cormorants colour-ringed as chicks in the Danish colony Vorsø 1977-97 to estimate adult survival and colony fidelity. Recent statistical models allowing simultaneous use of recovery and resighting data are employed. We compensate for variation in colour-ring quality, and study the effect of population size and winter severity on survival, as well as of breeding success on fidelity by including these factors as covariates in statistical models. 4. Annual adult survival fluctuated from year to year (0·74-0·95), with a mean of 0·88. A combination of population size in Europe and winter temperatures explained 52-64% of the year-to-year variation in survival. Differences in survival between sexes was less than 1%. Cormorants older than ≈ 12 years experienced lower survival, whereas second-year birds had survival similar to adults. Colony fidelity declined after 1990 from nearly 1 to ≈ 0·90, implying 10% permanent emigration per year. This change coincided with a decline in food availability. 5. Apparently, survival was more severely affected by winter severity when population size was high. This could be caused by saturation of high-quality wintering habitat, forcing some birds to winter in less good habitat where they would be more vulnerable to cold winters. There was thus evidence for density dependence in adult survival, at least in cold winters. 6. The high population growth rate sustained by European Ph. c. sinensis in the 1970s and 1980s can partly be accounted for by unusually high survival of immature and adult birds, probably caused by absence of hunting, low population density and high food availability.

Diagnosing a decline in return rate of 1‐year‐old cormorants: mortality, emigration or delayed return?

1. In long-lived birds with delayed recruitment, variation in prebreeding population parameters is difficult to study, although potentially important. Cormorants (Phalacrocorax carbo sinensis) often return to their natal colony when 1 year old, enabling direct study of first-year survival and emigration. Density-dependent regulation mechanisms may be strong at this stage of the life cycle. 2. In the Danish cormorant colony Vorsø, 11 000 chicks have been colour-ringed between 1977 and 1997. Returning birds have been resighted in the colony and dead recoveries have been collected. Since 1990, the proportion of a cohort observed in the colony as 1-year-olds (return rate) has declined from 0·40 to 0·10. Possible explanations include increased mortality, increased emigration and a later age of first return to the colony. Breeding success has also declined strongly as a consequence of low food availability. 3. We used capture-recapture analysis of recovery and resighting data to investigate variation in first-year survival, emigration and resighting probabilities. Survival fluctuated widely (0·42-0·75, mean 0·58); emigration increased in the 1990s from 0·05 to 0·15; resighting probability declined from 0·75 to 0·20 after 1990. 4. First-year survival was particularly low in 1993 and 1996. The causes of the year-to-year variation were not clear. Survival may have been affected by food availability during the post-fledging period. 5. The declining return rate was caused mainly by decreasing resighting probability of 1-year-olds, although increasing emigration also contributed. The biological mechanism was that increasing numbers of cormorants did not return to the natal colony as 1-year-olds. We suggest that this was a consequence of low physiological condition among newly fledged young, caused by low food availability in the 1990s. 6. We conclude that declining food availability has had several consequences for colony dynamics. First-year emigration and the age of first return to the colony have increased and, in the worst years, survival has decreased. If the decline in food availability was due to cormorant predation, this would constitute an example of density-dependent regulation of immature colony attendance.

PASTEURELLA MULTOCIDA FROM OUTBREAKS OF AVIAN CHOLERA IN WILD AND CAPTIVE BIRDS IN DENMARK

An outbreak of avian cholera was observed among wild birds in a few localities in Denmark in 2001. The highest mortalities were among breeding eiders (Somateria mollissima) and gulls (Larus spp.). Pulsed-field gel electrophoresis (PFGE) was conducted using ApaI and SmaI as restriction enzymes and restriction enzyme analysis (REA) using HpaII. The Pasteurella multocida subsp. multocida strain isolated from birds in this outbreak was indistinguishable from a strain that caused outbreaks in 1996 and 2003. Most isolates from domestic poultry had other PFGE patterns but some were indistinguishable from the outbreak strain. Among 68 isolates from wild birds, only one PFGE and one REA pattern were demonstrated, whereas among 23 isolates from domestic poultry, 14 different SmaI, 12 different ApaI, and 10 different HpaII patterns were found. The results suggest that a P. multocida strain has survived during several years among wild birds in Denmark.

Seasonal distribution and timing of migration of Cormorants Phalacrocorax carbo sinensis breeding in Denmark

The recent expansion of the Cormorant Phalacrocorax carbo population in Europe has led to management conflict throughout Europe, increasing the relevance of describing the migration pattern of each countrys breeding population. We use 2279 recoveries and 16 769 resightings of 4735 colour-ringed individuals to describe dispersal and timing of movements of Danish Cormorants. Most Cormorants dispersed from the colonies to coastal areas and freshwater lakes in Denmark, Sweden and northern Germany in July. Southward movements took place throughout July to December. Major departure from the post-breeding areas occurred from August to mid-October, with many birds staging at Dutch and Alpine lakes between September and October, and with arrival in the Mediterranean mainly from October to November. Spring migration was fast, occurring from mid-February to March. First-year birds migrated south faster and reached the wintering areas sooner than adults, but left these later and moved north more slowly than adults. ...

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.

Tools in waterfowl reserve management: effects of intermittent hunting adjacent to a shooting-free core area

We explored how waterfowl in a large Danish coastal wetland responded to one day of marsh shooting at intervals of three weeks (1998), two weeks (1999), and one week (2000 and 2001). Shooting took place around sunset on a section of salt marsh that usually is part of a large shooting-free refuge. The behavioural response of waterfowl was recorded, and all waterfowl were counted on the day of the hunt and on the first and second day after each hunt. The wildfowl responded to the first 1–7 shots by moving into the open water parts of the refuge (dabbling ducks) or to sites > 8 km away (geese and waders). When the salt marsh was flooded early in autumn 2001, wigeon Anas penelope, teal A. crecca and lapwing Vanellus vanellus restricted their response to movements to other non-hunted parts of the salt marsh. On the first day after the hunts, mallard A. platyrhynchos occurred in significantly lower numbers both on the adjacent shallow and on the salt marsh. Wigeon numbers on the shallows were not affected by shooting, though they returned in lower numbers to the salt marsh on the first day after the hunts in 1998–2000, but not after the hunts in 2001. A similar pattern was observed in teal numbers on the salt marsh. We conclude: 1) that shooting lead to short-term displacements of dabbling ducks, 2) that the response of wigeon and teal varied depending on prevailing conditions on the salt marsh, 3) that intermittent regulation of marsh shooting was a management tool that ensured that waterfowl continued to exploit the area shot over on days when no shooting took place, and 4) that the relatively weak responses were linked to the existence of an extensive refuge adjacent to the area with marsh shooting.

Outbreak of Pasteurellosis among Wintering and Breeding Common Eiders Somateria mollissima in Denmark

In 1996, an epizootic occurred among wintering and breeding common eiders Somateria mollissima in southwest Kattegat, Denmark, causing the death of at least 900 birds during late winter, and of a total of 3,146 females in five local breeding colonies, corresponding to 35–95% of the females present within the single colonies. The cause of death was related to a bacterial infection by Pasteurella multocida isolated from all examined eiders collected on wintering and breeding grounds. This is the first documented incidence of pasteurellosis in Scandinavia. Based on knowledge of the phenology and winter distribution of eiders, the temporal occurrence of the disease suggests that apparently healthy birds acted as carriers of the disease bringing it from the wintering grounds to the breeding colonies.

Site fidelity of wintering cormorants Phalacrocorax carbo sinensis in Europe

The breeding population of cormorants Phalacrocorax carbo sinensis has expanded more than twenty-fold in continental Europe since 1970, and these large piscivorous birds cause conflicts with human fishery interests in large parts of Europe, including areas outside their breeding range. Culling aimed at reducing local population size takes place in some wintering areas, particularly in France and Switzerland. The effectiveness of such management actions is unknown and depends on how high cormorant site fidelity is. We use winter resightings from the period 1980–2000 of 24,000 cormorants colour-ringed in Denmark and The Netherlands to estimate site fidelity. Year-to-year fidelity to single sites (typically lakes) was 85–90%. For cormorants that changed site mean distances moved from year to year were typically between 10 and 50 km, depending on geographical location. However, long-distance movements (maximum observed was 1,891 km) also occurred regularly. For larger parts of the winter range, exemplified by France, about 3% of all birds left the area from year to year. A mathematical model showed that the observed levels of movement among wintering areas were sufficiently high to sustain a wintering population in France at levels of culling which would drive it to extinction in the absence of immigration. Although cormorants are not nomadic outside the breeding season, as previously believed, site fidelity is sufficiently low and movement among wintering areas sufficiently high that it must be taken into account in management programmes.

Net-entrapment of great cormorants Phalacrocorax carbo sinensis in relation to individual age and population size

Abstract Population control of great cormorants Phalacrocorax carbo sinensis has been suggested as a tool to mitigate fish losses by cormorants foraging in pound nets, gill nets and fykes. The objective of our study was to quantify the difference between first-year birds and older birds in the risk of drowning in fishing gear, and to explore the influence of population size on the proportion of individuals drowning. An index of the proportion of first-year birds and older birds that drowned was obtained for cormorants ringed in the Danish Vorsø colony using resightings of colour-ringed individuals and ring recoveries of individuals found dead. First-year birds were approximately 10 times more likely to drown than older birds. We used ring recoveries of cormorants ringed in Denmark and found dead in Denmark, North Germany and South Sweden (i.e. in the main post-breeding area) to reveal changes over a 25-year period in the proportion drowned among those recovered. Among first-year birds the proportion drowned declined from 66% in 1978–1984 to 24% in 2000–2002, and among older birds the proportion declined from 46% in 1978–1984 to 26% in 1999–2002. During 1978–2000, breeding numbers in Denmark increased from 1,400 to 42,500 nests, and the proportion of cormorants drowned among those recovered was significantly negatively correlated with population size. This suggests that the proportion of the population foraging in nets declined as the population increased. The damage caused by cormorants foraging in pound nets is, therefore, unlikely to decrease in proportion to reductions in population size reached through population control.

Current and Potential Threats to Nordic Duck Populations — A Horizon Scanning Exercise

We review the current and future threats to duck populations that breed, stage, moult and/or winter in the Nordic countries. Migratory duck species are sensitive indicators of their changing environment, and their societal value confirms the need to translate signals from changes in their distribution, status and abundance into a better understanding of changes occurring in their wetland environments. We used expert opinion to highlight 25 major areas of anthropogenic change (and touch briefly on potential mitigation measures through nature restoration and reserve management projects) that we consider key issues likely to influence Nordic duck populations now and in the near future to stimulate debate, discussion and further research. We believe such reviews are essential in contributing to development of successful management policy as well as stimulating specific research to support the maintenance of duck species in favourable future conservation status in the face of multiple population pressures and drivers.