Management-Induced Reproductive Failure and Breeding Dispersal in Double-Crested Cormorants on Lake Champlain

ABSTRACT We studied breeding dispersal of double-crested cormorants (Phalacrocorax auritus) associated with management practices that suppressed their reproduction on Lake Champlain in the northeastern United States. We implemented an experiment on one colony by spraying corn oil on cormorant eggs in portions of the colony and leaving other portions untreated. Gulls (Larus spp.) consumed cormorant eggs during the oiling process, but we reduced and then eliminated predation levels after the first year of the study. We used mark-recapture techniques within the experimental framework to measure rates of breeding dispersal for cormorants from the experimental colony and an unmanaged colony in Lake Champlain. Egg oiling increased the movement rate to the unmanaged colony by 3% during the year with no egg predation by gulls. When gulls depredated cormorant eggs at high rates during egg oiling, movement to the unmanaged colony increased by 20%. When cormorants are managed to reduce population sizes, methods that limit dispersal away from the managed colony may be most effective. Such methods would mitigate effects to nontarget populations and allow for a greater portion of the metapopulation to be managed.     

Effects of management on double-crested cormorant nesting colony fidelity

The increase of double-crested cormorant (Phalacrocorax auritus; hereafter, cormorant) populations during the last 2 decades has impacted many stakeholder groups. The negative effects of nesting cormorants on trees and other vegetation have motivated private organizations and government agencies to manage nesting colonies and reduce their impacts to private property and public resources. Management-induced reproductive failure has been shown to influence cormorant inter-annual nesting colony fidelity, but not complete abandonment from a nesting colony site. We attached very high frequency (VHF) transmitters and Global Positioning System (GPS) transmitters to nesting cormorants to monitor their movement response on a managed site (Young Island, VT [YI]) and an unmanaged site (Four Brothers Islands, NY [FB]). Additionally, we monitored these sites to determine the influence of management activities on subsequent-year colonization. On YI, management consisted of egg-oiling all cormorant nests (some nests had been oiled in previous years) and culling approximately 20% of adults. Annual dispersal rates did not differ between managed and unmanaged sites, but a nesting period interaction occurred with greater dispersal on the managed site following the incubation period. After 4 years of both egg oiling and culling, cormorant nesting on YI declined to zero. Simultaneously, cormorant numbers increased on the nearby unmanaged FB. We propose either the cumulative effect of partial or complete reproductive failure (8 yr) or simply the inclusion of adult culling (4 yr) caused the abandonment. From a colony-specific management perspective, the rapid decline was beneficial to the goal of restoring the vegetative community on YI. The effects of adult culling at nesting colonies, prior-year reproductive failure caused by egg oiling, or the combination of these factors may be required for complete and rapid nesting site abandonment. The use of culling adult breeders reduced nesting and likely limits the cost and logistics of control and allows more rapid initiation of mitigation measures and island habitat restoration. © 2011 The Wildlife Society.     

Nutrient transfer from sea to land: the case of gulls and cormorants in the Gulf of Maine

1. The structure of communities is influenced by the transport of resources across ecosystem boundaries. Seabirds are capable of introducing large amounts of marine-derived nutrients to land, thereby modifying resource availability to terrestrial species. 2. In this study we investigated the hypothesis that variation in nesting densities of great black-backed gulls Larus marinus and double-crested cormorants Phalacrocorax auritus would modify the effect of these species on soil nutrients and plant species composition on offshore islands in the Gulf of Maine, USA. 3. Our results showed a significant positive correlation between nest density and concentrations of ammonia and nitrate in soils, but no significant relationship between nest density and phosphate. Ammonia and phosphate concentrations were good predictors of plant species composition; there were more annual forbs than perennial grasses in the abandoned cormorant colony compared with the gull colonies. Extremely high concentrations of ammonia in the highest density colony (active cormorant) may have been the main factor inhibiting plant germination at this site. All of the plant species in gull and cormorant colonies showed enriched delta(15)N signatures, indicating substantial input of marine-derived nitrogen from seabirds. 4. Our study demonstrated that gulls and cormorants are effective vectors for the transport of marine nutrients to terrestrial ecosystems. However, transported nutrients occurred in particularly high concentrations in areas with nesting cormorants. Nesting densities and species-specific variation in resource transport should be considered when predicting the effects of seabirds and other biogenic vectors of allochthonous resources.     

The 1992 epizootic of Newcastle disease in double-crested cormorants in North America

In the summer of 1992, morbidity and mortality in juvenile double-crested cormorants (Phalacrocorax auritus; DCC) attributable to Newcastle disease virus (NDV) was observed for the first time in seven northern USA states and one Canadian province, and recurred in three western Canadian provinces. Based on clinical signs and laboratory diagnostic findings, DCC mortality from NDV occurred in 59 of the 63 nesting colonies and two of three non-colony sites investigated. An estimate of in excess of 20,000 DCC died, with mortality rates ranging from < 1 to 37% in Great Lakes colonies to 20 to 92% in Minnesota (USA) and North and South Dakota (USA) colonies. Sick juvenile white pelicans (Pelecanus erythrorhynchos) exhibiting signs similar to sick cormorants, and dead pelicans were observed in Minnesota and North Dakota. Mortality rates in pelican colonies were as high as in the adjacent cormorant colonies, but no cause for the mortality of an estimated 5,000 pelicans was determined. No evidence of NDV was found in other species nesting in proximity to affected cormorants. Although the source of the NDV infection is unknown in cormorants, the simultaneous onset of the epizootics in juvenile birds over a wide geographic area implies that the virus was acquired by adults prior to migration and was carried back to nest sites, exposing susceptible nestlings. The possible transmission of this virus from free-ranging wild birds to domestic poultry is a concern. Based on repeated epizootics in cormorants since 1990, NDV seems to be established in DCC.     

Migration and wintering sites of Pelagic Cormorants determined by satellite telemetry

ABSTRACT Factors affecting winter survival may be key determinants of status and population trends of seabirds, but connections between breeding sites and wintering areas of most populations are poorly known. Pelagic Cormorants (Phalacrocorax pelagicus; N= 6) surgically implanted with satellite transmitters migrated from a breeding colony on Middleton Island, northern Gulf of Alaska, to wintering sites in southeast Alaska and northern British Columbia. Winter locations averaged 920 km (range = 600–1190 km) from the breeding site. Migration flights in fall and spring lasted ≤5 d in four instances. After reaching wintering areas, cormorants settled in narrowly circumscribed inshore locations (～10‐km radius) and remained there throughout the nonbreeding period (September– March). Two juveniles tagged at the breeding colony as fledglings remained at their wintering sites for the duration of the tracking interval (14 and 22 mo, respectively). Most cormorants used multiple sites within their winter ranges for roosting and foraging. Band recoveries show that Pelagic Cormorants in southern British Columbia and Washington disperse locally in winter, rather than migrating like the cormorants in our study. Radio‐tagging and monitoring cormorants and other seabirds from known breeding sites are vital for understanding migratory connectivity and improving conservation strategies for local populations.     

The impact of nesting cormorants on plant and arthropod diversity

Seabirds can strongly affect several major factors correlated with species diversity by concentrating marine nutrients on their nesting islands and by physically disturbing island vegetation. In this study, we investigated the effects of nesting cormorants on the abundance, species richness, and composition of plants and arthropods (Coleoptera, Heteroptera, Araneae, and Chironomidae) on islands in Stockholm archipelago, Sweden. Nesting cormorants negatively affected plant species richness and vegetation cover and that changed plant species composition. The effect of nesting cormorants on island arthropods varied between feeding groups and sampling methods. Most orders did not change in abundance or species richness but some, such as coleopterans and spiders changed in species composition. Herbivorous coleopterans were generally negatively affected by cormorants whereas fungivorous species and scavengers were generally positively affected. In structural equation modeling we found that the effect of cormorants was sometimes direct, such as on scavengers, but many effects on island consumers were mediated by changes in vegetation caused by cormorant presence. Overall, arthropod communities were highly dissimilar between cormorant and reference islands, and we therefore conclude that nesting cormorants not only affect the diversity of their nesting islands but also of the archipelago as a whole. The total diversity in the archipelago may increase through regional increased habitat heterogeneity and by adding species which are favored by seabirds (e.g. scavenging and fungivorous coleopterans).     

Response of double-crested cormorants to a large-scale egg oiling experiment on Lake Huron

We report on a management experiment examining the effects of large-scale egg oiling on double-crested cormorant nest abundance and measures of seasonal cormorant density (bird-days/km²) from 2000 to 2005. We employed the staircase design to distinguish transient responses to management treatments from site and year effects that generally contribute to variation in populations. The response to egg oiling in Georgian Bay was as expected with a decline in nest abundance attributable to egg oiling. In the North Channel, nest abundance did not decline because of egg oiling but increased, reflecting either retention of nesting adults or recruitment to colonies. This surprising outcome may stem from fish escapement from pen rearing facilities in the vicinity of the oiling experiment in the North Channel. We observed no effect of egg oiling on the July–August seasonal density of cormorants. The strongest effect size was associated with site effects followed by year effects for nest abundance and seasonal density. The effect size of egg oiling on variation in nest abundance did not exceed 5% for any year in both the North Channel and Georgian Bay. Fish pen culture appears to affect coastal distribution of cormorants. © 2011 The Wildlife Society.     

Avian Predation on Steelhead is Consistent with Compensatory Mortality

Numerous factors such as predation, disease, injury, and environmental conditions (e.g., river flows, hydropower operations) can influence survival rates of fish. Although mortality due to predation is commonly assumed to be additive and result in a directly proportional reduction on survival rates, compensatory processes may work to counteract or negate the effects of predation mortality on survival rates. We applied a random effects model to a long-term, mark-recapture-recovery data set on anadromous steelhead (Oncorhynchus mykiss) from the Snake River Basin in the northwestern United States to assess whether avian predation mortality constitutes an additive or compensatory source of mortality. Specifically, our assessment focused on predation mortality due to double-crested cormorants (Phalacrocorax auritus) and Caspian terns (Hydroprogne caspia) on colonies in the Columbia River estuary. In addition, we evaluated several candidate environmental indices to examine potential interactions between the effects of predation versus environmental conditions on steelhead survival rates. Average predation rates were 3.3% for the double-crested cormorant colony and 17.0% for the Caspian tern colony. For both colonies, the estimated correlation between the predation rate and survival rate of steelhead was near zero, indicating that mortality due to avian predation is compensatory. Models that included variables for river flow, juvenile migration timing, and an index of forage biomass in the ocean accounted for 56–59% of the variation in steelhead survival, whereas avian predation rates accounted for <1% of the variation. Management efforts to reduce the abundance of the bird colonies are unlikely to improve the survival or conservation status of steelhead; however, results indicate that steelhead survival could be improved by hydropower management decisions that increase river flows and reduce juvenile migration delays. © 2020 The Wildlife Society.     

Pattern of natural 15N abundance in lakeside forest ecosystem affected by cormorant-derived nitrogen

Waterbirds are one of the most important groups of organisms inhabiting the land–water interface, especially with regard to mediating the transport of materials from the aquatic to the terrestrial environment. The great cormorant (Phalacrocorax carbo) is a colonial piscivorous bird that transports nutrients from fresh water to forest. We measured cormorant-derived nitrogen at two nesting colonies on the Isaki Peninsula and Chikubu Island at Lake Biwa, Japan, and analyzed the long-term effects of cormorant colonization on the forest nitrogen cycle, and the mechanisms of nitrogen retention. Three sites were examined in each colony: a currently occupied area, a previously occupied but now abandoned area, and a control area never colonized by cormorants. High nitrogen stable isotope ratios of cormorant excreta, the forest floor, mineral soil, and living plants showed cormorant-derived nitrogen in both occupied and abandoned areas. The relationship between δ¹⁵N and N content showed that the high δ¹⁵N of the excreta and N turnover in the soil were important at the occupied sites, whereas high δ¹⁵N of litter was important at the abandoned sites. Physiological changes of various organisms are also important for the N decomposition process. In conclusion, cormorant-derived nitrogen remains in the forest ecosystem as a result of two cormorant activities: heavy deposition of excreta and collection of nitrogen-rich nest material. Colony stage (occupied, abandoned, or never inhabited) and historical change of N decomposition process of an area can be identified from the relationship between δ¹⁵N and N content.     

Using species distribution models to define nesting habitat of the eastern metapopulation of double‐crested cormorants

When organisms with similar phenotypes have conflicting management and conservation initiatives, approaches are needed to differentiate among subpopulations or discrete groups. For example, the eastern metapopulation of the double‐crested cormorant (Phalacrocorax auritus) has a migratory phenotype that is culled because they are viewed as a threat to commercial and natural resources, whereas resident birds are targeted for conservation. Understanding the distinct breeding habitats of resident versus migratory cormorants would aid in identification and management decisions. Here, we use species distribution models (SDM: Maxent) of cormorant nesting habitat to examine the eastern P. auritus metapopulation and the predicted breeding sites of its phenotypes. We then estimate the phenotypic identity of breeding colonies of cormorants where management plans are being developed. We transferred SDMs trained on data from resident bird colonies in Florida and migratory bird colonies in Minnesota to South Carolina in an effort to identify the phenotype of breeding cormorants there based on the local landscape characteristics. Nesting habitat characteristics of cormorant colonies in South Carolina more closely resembled those of the Florida phenotype than those of birds of the Minnesota phenotype. The presence of the resident phenotype in summer suggests that migratory and resident cormorants will co‐occur in South Carolina in winter. Thus, there is an opportunity for separate management strategies for the two phenotypes in that state. We found differences in nesting habitat characteristics that could be used to refine management strategies and reduce human conflicts with abundant winter migrants and, at the same time, conserve less common colonies of resident cormorants. The models we use here show potential for advancing the study of geographically overlapping phenotypes with differing conservation and management priorities.     

The advertising display of double-crested cormorants varies with microhabitat and time of the season in a tree-nesting colony

Male double-crested cormorants (Phalacrocorax auritus) perform advertising displays at potential nest locations at spring breeding grounds to attract females. Yet, there is no research on how the frequency of this behaviour changes over time or its association with habitat features. Studies of this behavioural display may provide insight into how birds choose areas for nesting, an important issue given the environmental and societal impacts of dense colonies of cormorants. Advertising behaviour was observed in trees throughout the 2014 nesting season, at six different sampling stations, using both scan sampling (for temporal changes in display frequency) and focal sampling (for temporal changes in four microhabitat variables: tree health, height in tree, nest density and presence of a nest). Advertising data (scans N = 484 birds; focal samples N = 827 birds) were divided into pre-incubation, incubation and chicks-present categories using a breeding chronology dataset. The number of cormorants advertising varied temporally and spatially. Using scan data in a marginal model, we found that the number of cormorants advertising was highest at the beginning of the season (peaking at week 3) and lowest once chicks were present. The GLM (focal data) showed that most cormorants advertised without nests especially past the second week of the season. The model also indicated that cormorants advertised in low nest density trees from pre- to mid-incubation. Contrary to our predictions based on colony expansion over the season, we did not observe an interaction of tree health*time or station*time, which suggests that the advertising display revealed a nest site selection process that was not visible at the population level.     

Long term effects of cormorant predation on fish communities and fishery in a freshwater lake

Cormorant impact upon natural fish populations has long been debated but little studied because of the requirements of sound data that are often hard to fill. In this study I have monitored fish community composition abundance before and after a cormorant colony was established in a high productive lake, Ymsen, of south-central Sweden. Data on fish abundance before cormorant establishment enabled me to control Ibr changes in fish densities prior to cormorant colonisation. To control for possible changes in fish populations caused by factors other than cormorant predation (i.e. large-scale regional changes due to climate) data were compared with a control lake, Garnsviken, with no cormorants. Since Lake Ymsen also harbour an Important commercial fishery, cormorant impact upon fishery yields was evaluated. The most important fish species in the diet of the cormorants were ruffe (75% by number), roach (11%) and perch (10%). Except for perch, commercially important fish made up a very small fraction of the cormorant diet. Eel, the most important fish for the fishery, was absent in the cormorant diet, pikeperch constituted 0.2% and pike 1,5%, Estimated fish outtake by the cormorants was 12,8 kg ha^-1 yr^-1 compared to 8.6 kg ha^-1 yr^-1 for the fishery. Despite considerable fish withdrawal by the cormorants, fish populations did not seem to change in numbers or biomass. The present study indicates that cormorant impact upon fish populations in Lake Ymsen was small and probably in no case has led to declines of neither commercial nor of non-commercial fish species. Still, the number of breeding cormorants in Lake Ymsen, in relation to foraging area, is among the highest known for Swedish lakes.     

Effects of the common cormorant,phalacrocorax carbo,on evergreen forests in two nest sites at Lake Biwa,Japan

The effects of the habitation of the common cormorant on vegetation were evaluated in two colonies in Lake Biwa, central Japan. The state of the vegetation of each colony was classified arbitrarily into the following three types according to the appearance of the tree, shrub and herb layers; dense tree layer (Type 1), sparse tree layer and herb layer (Type 2), and sparse tree layer with dense herb layer (Type 3). The density of cormorant nesting was almost similar among the three types, except for part of Type 2. Trees, especially large ones on which cormorants nested, were found to be heavily damaged, and there were few seedlings and saplings found under the canopy trees.Chamaecyparis obtusa andCryptomeria japonica were more heavily damaged than other evergreen and deciduous trees. In the herb layer, only a few species such asReynoutria japonica orPhytolacca americana were abundant. These results suggest that the habitation of cormorants could simplify the structure and the species composition of forests.     

Evaluation of Aerial Cluster Sampling of Double-Crested Cormorants on Aquaculture Ponds in Mississippi

Abstract Concern over increasing numbers of double-crested cormorants (Phalacrocorax auritus) and their impacts on channel catfish (Ictalurus punctatus) aquaculture has resulted in increased need for quantitative information to develop and evaluate depredation management efforts. We evaluated aerial surveys in a stratified cluster sampling (SCS) design to estimate and monitor abundance of cormorants on catfish aquaculture ponds in the Yazoo River Basin of Mississippi, USA (hereafter Yazoo Basin). Twice monthly abundance estimates and coefficient of variation during winter averaged 8,128 (n = 29, SE = 1,233) and 33% (n = 29, SE = 0.02), respectively. Counts of cormorants on catfish aquaculture ponds between survey years were correlated (r = 0.87, n = 28). The correlation between diurnal counts of cormorants on ponds and cormorant night roost counts was 0.64 in 2000–2001 and 0.58 in 2003–2004 (n = 20 in both years). A priori estimates of sample size indicated an average increase in sampling effort of 39% during peak periods of cormorant use would be necessary to detect a ±15% change in cormorant abundance on aquaculture ponds at α = 0.05 and β = 0.80. The sampling design we used has the potential to be an effective tool for providing quantitative information on cormorant abundance on catfish aquaculture ponds in the Yazoo Basin. However, increased sampling effort would be necessary to obtain desired levels of precision. The SCS design we evaluated represents only one of many possible survey methods, and we recommend additional evaluation of this method and related survey methods.     

Space utilization by a cormorant (Phalacrocorax carbo L.) colony in a multi-wetland complex in relation to feeding strategies

In this study, we investigated the response of inland breeding cormorants Phalacrocorax carbo to a complex spatial configuration of feeding habitats in relation to social and individual feeding strategies. The numbers of feeding trips outside the colony site (Lake Grand-Lieu, western France), where only solitary fishing is used by cormorants, and the number of birds fishing on the lake where social fishing predominates were investigated during the breeding season and compared with the fledging period. From the investigation of feeding trip traffic, we identified three major habitats used by cormorants in the vicinity of the colony site (< 25 km around the colony site) that accounted for 94.1 of the IN flights and 92.0% of the OUT flights (n = 1745 arrivals and 2404 departures respectively), and notably one area that accounted for 58% of total flights although it is the furthest away. No fundamental change in the relative significance of these feeding grounds for solitary fishing cormorants was found throughout the breeding season, even in a between-years comparison (1996-2001), in contrast to what has often been found elsewhere. Although the peak of foraging activity in the surrounding habitats and also within the lake waters largely coincided with the time when the majority of young had fledged, the index of cormorant numbers (ratio between bird numbers at a given time and that for a baseline date) on the lake remained at a high level until late August compared to movements outside the lake, as a result of regular social fishing (84.9 +/- 4.0% of fishing numbers). From these findings, we discuss factors governing the selection of feeding grounds throughout the breeding season in relation to energy considerations, feeding strategies and food resources.     

Cormorant (Phalacrocorax carbo [L]) populations and patterns of abundance at breeding and feeding sites in Northern Ireland,with particular reference to Lough Neagh

Breeding cormorants, Phalacrocorax carbo, in Northern Ireland remain seasonally dependent on a coastal environment where they can be censused with accuracy, but numbers in other habitats and at other times of the year are less certain. This study establishes the long term and regional patterns of abundance at breeding and feeding localities, which might in turn be related to diet.Birds were regularly observed and counted at a variety of feeding sites, and some aspects of their breeding success and fledgling diet evaluated at the largest N.Ireland breeding colony.Numbers of breeding birds increased dramatically over a period of eight years but recently show signs of declining. There is likely to be a dynamic relationship between populations of a tapeworm (Ligula intestinalis L.), the numbers of roach (Rutilus rutilus [L.]) and cormorant populations feeding at Lough Neagh, the largest lake in the British Isles. While roach are not a major component in the diet of fledglings, the large numbers of cormorants feeding at the perimeter of L. Neagh suggest that changes in roach populations will affect birds most acutely during or following the winter. We suggest that this might result in a reduction in the proportion of the population volunteering to breed in the subsequent season, but require further data. The longer term effects of one fish species on cormorant populations are unlikely to be critical, since these birds are highly opportunist.In other habitats cormorant numbers are either very stable (estuary) or variable (river) depending on the seasonal and annual availability of their prey. There is no evidence for a systematic seasonal shift in habitat, as suggested by other studies.     

Environmental determinants of Great Cormorant (Phalacrocorax carbo) distribution in small man-made waterbodies – a case study of gravel pits in southwest France

In the Midi-Pyrénées region (southwest France), the increasing number of gravel pits has allowed the wintering of numerous species of waterbirds such as Great Cormorants (Phalacrocorax carbo). The debate about cormorant predation on fish stock has been sufficiently strong to have resulted in reductions in cormorant numbers by control shooting. In this context, cormorants were studied during winters 1996/1997 and 1997/1998 at two gravel pit sites in the Garonne floodplain. Human disturbances and fish densities were found to be the main parameters determining the abundance of fishing cormorants. This work will help to prompt further research and the development of a management strategy for this species.     

Piscivorous Bird Use of Aquaculture and Natural Water Bodies in Mississippi

Double-crested cormorants (Phalacrocorax auritus) and great egrets (Ardea alba) have an extensive history of human-wildlife conflict with the aquaculture industry of western Mississippi, USA, due to their depredation of cultured catfish (Ictalurus spp.). Although aquaculture is abundant, western Mississippi also contains naturally occurring water bodies that offer alternative forage opportunities to these species. How cormorants or egrets distribute themselves among these 2 foraging options is unknown, but it has been generally assumed each species uses aquaculture disproportionately more because of the high density of available prey. To test this assumption, we surveyed these species on aquaculture and naturally occurring water bodies using aerial surveys from October through April of 2015–2016, 2016–2017, and 2017–2018. We modeled the proportion of each species on aquaculture as a function of year, date, and weather-related variables using quasi-binomial generalized linear models. Egrets used aquaculture consistently more than what was proportionally available to them and use was not influenced by any of the variables we measured. Proportional use of aquaculture by cormorants was lowest during October through January but steadily increased through April, indicating a distribution shift toward aquaculture in the months immediately prior to their migration. The highest proportional use of aquaculture by cormorants occurred in 2016, a year when lethal control measures were not allowed against cormorants. Conversely, the least proportion of cormorants on aquaculture was in 2015 when cormorants could be lethally controlled under authority of an Aquaculture Depredation Order. This trend highlights the potential influence of changes in mortality risk, caused by changes in policy regarding lethal take of cormorants, on cormorant distribution between foraging options. © 2020 The Wildlife Society.     

Nitrogen and phosphorus enrichment and balance in forests colonized by cormorants: Implications of the influence of soil adsorption

Although much concern has been directed at nitrogen (N) cycling in terrestrial ecosystems with bird colonies, little has been clarified on the processes of phosphorus (P) cycling itself, and few comparisons between P and N cycling in bird colonies have been made. On the Isaki Headland and Chikubu Island, which are located on or near the shore of Lake Biwa, Central Japan, a dramatic increase in the population of cormorants has occurred since the 1980s. There has been a concomitant increase in the transport of nutrients from the lake to the waterside ecosystems. We compared the pools and dynamics of N and P in the cormorant-colony forests in order to clarify the effects of differences in soil N and P dynamics on the N–P balance of these colony forests. The total N concentration in the forest floor at excrement-influenced sites was not significantly different from that at sites without such influence, in spite of the heavy load of cormorant-derived N. In contrast to N, forest floor P concentration at the sites with excrement influence was significantly higher compared to sites without such influence, resulting in the lower forest floor N/P ratio at the excrement-influenced sites even after colony abandonment. The site pattern of total N and P concentrations and N/P ratio for mineral soil was similar to that for the forest floor. It seems that the leaky character for N and the accumulative character for P are due to the high mobility of nitrate in soils and the tight absorption of inorganic P to clay minerals, respectively. The site pattern of N/P ratios observed for Chamaecyparis obtusa Sieb. et Zucc. leaves is consistent with that for the forest floor and/or mineral soil, suggesting that the soil geochemical property was reflected in the foliar N/P ratio. The chemistry of throughfall and soil solution was also changed due to deposition of cormorant excrement, and the changes continued for a few years after abandonment of the colony. The quantitative analyses for N and P suggested that the major part of N and P transported by cormorants was not retained in plant matter and the surface soil beneath the colony but instead leached into deeper soil layers. The influence of cormorant excrement on nutrient balance of the whole colony ecosystem is also discussed.