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.     

Identification of 24 polymorphic microsatellite markers for the double-crested cormorant (Phalacrocorax auritus)

Twenty-four polymorphic microsatellite markers were developed for the double-crested cormorant (Phalacrocorax auritus). The number of alleles ranged from two to 13 and observed heterozygosities ranged from 0.032 to 0.871. The use of these loci should enable researchers and biologists to learn more about the population structure and ecology of this species.     

Cardiac responses to first ever submergence in double-crested cormorant chicks (Phalacrocorax auritus)

Heart rates were recorded from double-crested cormorant chicks during their first ever and subsequent voluntary head submergences and dives, as well as during longer dives made after the chicks were accustomed to diving. Despite variation between chicks, the cardiac response to first ever and subsequent voluntary submergence (head submergences and dives) was similar to the response observed in adult cormorants. Upon submersion the heart rate fell rapidly when pre-submersion heart rate was high (325–350 beats min⁻¹). The heart rate established within the first second of voluntary submergence was between 230 and 285 beats min⁻¹, well above resting heart rate (143 beats min⁻¹). The same initial cardiac response occurred during longer dives performed after the chicks were accustomed to diving. In these dives the heart rate remained at the level established on submersion, unlike the response observed in shallow diving adult cormorants in which the heart rate declined throughout the dive. The heart rate was also monitored in a separate group of chicks in which the first exposure to water was during whole body forced submergence. Again, the observed response was similar to the adult response, although the cardiac response of chicks to forced submergence was more extreme than to voluntary submergence. Our results do not support the hypothesis that learning (by conditioning or habituation) is involved in the cardiac adjustments to voluntary submergence. It is suggested that the initial cardiac adjustments are reflex in nature and this reflex is fully developed by the first submergence event. Although the nature of this reflex pathway is obscure, cessation of breathing before submersion and the close linkage between breathing and heart rate might provide a plausible mechanism.     

海鸬鹚繁殖习性的初步观察

1995年7月至1998年8月,在山东长岛自然保护区的车由岛进行了海鸬鹚繁殖习性的观察,初步掌握了海鸬鹚的择偶,营巢,产卵,孵化,育雏等繁殖习性,探讨了影响海颅鹚捕食的因素,并报道了海鸬鹚在山东省的繁殖区。     

Foraging and breeding performance of Japanese cormorants in relation to prey type

Seabirds are high trophic predators in marine ecosystems and are sensitive to change in food supply and thus seabirds can be used as monitors of the marine environment. In order to study the foraging responses of Japanese cormorants Phalacrocorax filamentosus breeding at Teuri Island, Hokkaido to changes in fish availability, the diet was assessed from the regurgitations of parents and chicks, and diving behavior was measured by using time-depth recorders. Breeding performance (brood size, chick growth, breeding success) was monitored using conventional methods to study their breeding responses. Japanese cormorants changed the diet and foraging behavior over four summers. The birds fed mainly on epipelagic schooling fish when they were available and on demersal fish when pelagic fish availability was low. They tended to dive deeper and longer in a year when they fed mainly on demersal fish than the other years, reflecting the change in the depth distribution of prey fish. Chick growth rate did not differ among years, but fledging success was lower in the years of demersal fish as their meal delivery rate was low. When epipelagic schooling fish were considered scare, parents maintained chick growth by reducing brood size. High variability and unpredictability in pelagic fish abundance are key factors affecting the foraging and breeding performance of Japanese cormorants, which could potentially be used to monitor fish resources.     

Diet spectrum of great cormorants (<Emphasis Type="Italic">Phalacrocorax carbo sinensis</Emphasis> L.) at the Donji Miholjac carp fishponds in eastern Croatia

This paper reports the results of a diet analysis from the digestive tracts of 203 great cormorants shot at the Donji Miholjac fishponds in eastern Croatia, in the period 2000–2002. Eight fish species were determined. The dominant species was common carp (Cyprinus carpio) with a relative frequency of 73.4%, followed by grass carp (Ctenopharingodon idella) (11.6%), bighead carp (Aristichthys nobilis) (7.2%), pikeperch (Sander lucioperca) (2.0%), wels catfish (Silurus glanis) (1.7%), pike (Esox lucius) (1.5%), topmouth gudgeon (Pseudorasbora parva) (1.5%) and Prussian carp (Carassius auratus gibelio) (1.1%). The diet spectrum of the great cormorants from the Donji Miholjac fishponds was in accordance with the structure of the fish population in the fishponds. The average stomach weight per bird was 244 g. When birds shot with an empty stomach were excluded, the average stomach weight increased to 286 g. The length of consumed fish ranged from 40–335 mm, with 47% of the fish belonging to the length category 100–149 mm. The study revealed no significant relationship between the weight of the consumed fish and the body weight of male and female cormorants.     

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.     

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.     

A comparison of three methods to investigate the diet of breeding double-crested cormorants (Phalacrocorax auritus) in the Beaver Archipelago, northern Lake Michigan

In order to understand the role of waterbirds in aquatic food webs it is important to first get an accurate depiction of their diet. Three methods of dietary assessment (pellets, regurgitate and stomach contents) are compared here for breeding double-crested cormorants (Phalacrocorax auritus) of the Beaver Archipelago, northern Lake Michigan. By numerical frequency (percent number), each method yielded different depictions of the diet. However, in terms of presence and absence (percent frequency) of possible prey types, stomach content data did agree with both pellets and regurgitate data. However, differences were noted between regurgitate and pellets. In terms of biomass measured (percent biomass) in regurgitate and stomachs, data gathered agreed. In essence, pellets underestimate the importance of alewife (Alosa pseudoharengus) and overestimate the importance of crayfish (Orconectes sp.) in the diet when compared to both regurgitate and stomach analysis. The non-lethal method of regurgitate collection and analysis appears most practical in assessing cormorant diet in this system. In combination with information on avian foraging ecology and prey populations, these data may be used to investigate the relationships among cormorants and their prey, and lead to a better understanding of Great Lake food web dynamics.