Diet, movement, and dispersal patterns of double-crested cormorants in Lake Champlain
The double-crested cormorant population on Lake Champlain has increased geometrically since they first nested in the lake basin in 1981. By the late 1990s, their nesting colonies on islands in the lake were so dense that trees had been killed and nesting habitat for numerous other species was threatened. Likewise, anglers were concerned that the thousands of foraging cormorants were depleting fish populations. In 1999, personnel of the Vermont Department of Fish and Wildlife began applying corn oil to cormorant eggs to reduce reproduction and slow population growth in one of the large nesting colonies. Within 2 years, the number of nesting cormorants in this colony, Young Island, Vermont, declined by nearly 40%. However, a concurrent increase of nesting cormorants was observed on Four Brothers Islands on Lake Champlain, New York, providing the basis for our study, which compared home ranges, foraging behavior, and food habits of cormorants from these 2 large nesting colonies. We also assessed the experimental program to control reproduction on Young Island by oiling eggs.
We studied the movements, home ranges, and foraging behavior of cormorants by attaching radio transmitters to 102 birds during 3 field seasons, 2001-2003, and following foraging flocks, with radioed birds, as they flew circuits among nesting, foraging, and roosting sites. Home ranges and activity centers shifted and expanded as the breeding season progressed. There were differences between the two nesting colonies, during the breeding period, and among years, but the most consistent of these patterns was larger home ranges for cormorants from the Four Brothers nesting colony, an indication of less efficient foraging.
Examination of stomachs collected from 154 cormorants confirmed our observations about foraging efficiency. Yellow perch were the most abundant fish identified in stomachs of cormorants and by far the most prevalent by weight (72%). Cormorants from the Young Island nesting colony foraged more commonly on yellow perch, than did birds from Four Brothers Islands, and foraged closer to the colony. Cormorants from Four Brothers Islands consumed more rainbow smelt, which often were found near the surface in deep waters close to the nesting colony. Despite the occasional abundance of smelt near the nesting colony, cormorants from Four Brothers Islands regularly flew longer distances to feed on yellow perch, clearly the most preferred forage fish in Lake Champlain.
During 2001, we sampled fish populations in locations where cormorants were observed foraging, in an attempt to assess preference by cormorants for fish species and size classes. We employed gill nets and electrofishing gear. Although the diversity of fish captured was much greater than what was found in stomachs of cormorants, yellow perch represented 71% of the number of fish sampled, almost identical to the volume of yellow perch found in stomach contents. Cormorants appeared to be feeding on fish in relation to abundance of fish. However, cormorants appeared to abandon feeding sites even though perch were still abundant. Thus, in 2002, we concentrated our studies in one frequently used feeding site near Young Island and investigated the temporal and spatial variability of forage fish, as well as growth of macrophytes, throughout the nesting season. Results of this study will be presented as a graduate thesis.
In 2001, we implemented an experiment on Young Island to determine if egg oiling causes cormorants to abandon their nesting colony. We established a 10- x 10-m grid and applied oil on all eggs in half of the grid blocks. Cormorants from oiled nests were less likely to return to Young Island than those from non-oiled nests. In 2002 and 2003, we observed color-banded cormorants, which we had captured in previous years, as they settled onto their nesting colonies on Young and Four Brothers Islands. From an analysis of mark-recapture observation from the banded birds, we found evidence that cormorants from oiled nests on Young Island were more likely to shift their nesting site to Four Brothers Islands. On Young Island, returning cormorants first settled in grid blocks that were not oiled. Cormorants that arrived later claimed remaining nests in oiled grid blocks. Thus, while egg oiling has the potential to stifle reproduction in a nesting colony, there is also an effect of discouraging cormorants from nesting in the colony where eggs are being oiled. This relationship on Young Island was confounded, however, by predation on cormorant eggs by Herring Gulls during the egg-oiling treatments. The technique for applying corn oil to eggs was modified in 2002 and 2003 by spraying eggs at night to eliminate predation by gulls. Our mark-recapture analysis of color-banded birds observed in 2003 produced a lower coefficient for nest-site shifts to Four Brothers, suggesting that this modification was effective. As noted above, the diet of cormorants from Young Island differs from those on Four Brothers Islands. Management of cormorants on Young Island has the potential to shift where cormorants forage and what they consume. These results illustrate the importance of establishing regional management strategies that coordinate goals among all stakeholders, especially for nuisance wildlife populations such as cormorants.
Publications & Presentations
Update of cormorant research. Lake Champlain Ecosystem Team Meeting. 19 September 2001. Adam Duerr, David Capen, Terri Donovan.
Population Studies of Double-crested Cormorants on Lake Champlain. Cooperator’s Meeting, Vermont Cooperative Fish and Wildlife Research Unit. 12 July 2002. Adam Duerr.
Use of Radio Telemetry in Wildlife Ecology--Cormorants on Lake Champlain. Community College of Vermont. 19 March 2002. Adam Duerr.
Cormorant Conundrum. Invited public lecture, Lake Champlain Basin Science Center. April 2002. David Capen (Taped by Public Access TV and replayed numerous times).
The role of changing fish availability to double-crested cormorant foraging. School of Natural Resources 19th Annual Graduate Research Symposium, Burlington, VT. 18 October, 2002. Marc Eisehower.
Update of population status and research on Double-crested Cormorants in Lake Champlain. Great Lakes Area Working Group for Colonial Waterbirds. Shackleton Point, NY. 22 October 2002. Adam Duerr, David Capen.
Double-crested Cormorants on Lake Champlain: Why are they in the news? Invited Lecture, Ornithology Class, University of Vermont. 12 March 2003. Adam Duerr.
Studies of Double-crested Cormorants on Lake Champlain. Wildlife Division Meeting, Vermont Department of Fish and Wildlife. 15 January 2003. Adam Duerr, Terri Donovan.
The role of changing fish availability to double-crested cormorant foraging. Vermont Department of Fish and Wildlife Research Symposium, Seyon Ranch, Groton, VT. 15 January, 2003. Marc Eisenhower.
Behavior and Population Dynamics of Double-crested Cormorants: Integrating the Influence of Management through Decision Analysis. Dissertation Proposal Seminar, School of Natural Resources, UVM. 31 March 2003. Adam Duerr.
Egg oiling and nest colony fidelity of Double-crested Cormorants in Lake Champlain. Northeast Fish and Wildlife Conference, Newport RI. 15 April 2003. Adam Duerr (David Capen and Terri Donovan listed as coauthors).
Fish availability and changes in forage site fidelity by Double-crested cormorants. Northeast Fish and Wildlife Conference, Newport, RI, 15 April 2003. Marc Eisenhower (Donna Parrish listed as coauthor).
Individual growth and diets of yellow perch in Lake Champlain in relation to cormorant predation. Percis III Symposium, Madison WI, 22 July 2003. Donna Parrish (Bernie Pientka and Marc Eisenhower listed as coauthors).
Metapopulation Dynamics of Double-Crested Cormorants on Lake Champlain: Responses of Cormorants to Management. Annual Conference of The Wildlife Society, Burlington VT. 8 September 2003. Adam Duerr (David Capen and Terri Donovan listed as coauthors).
Metapopulation Dynamics of Double-crested Cormorants on Lake Champlain. Great Lakes Area Working Group for Colonial Waterbirds, Kingston ON. 8 October 2003. Adam Duerr, David Capen.
Growth and Management of the Double-crested Cormorant Population in Lake Champlain. Annual Meeting, Vermont Water Resources and Lake Studies Center, Lake Champlain Sea Grant. 10 October 2003. David Capen.
Growth and Management of the Double-crested Cormorant Population in Lake Champlain. High Peaks Audubon Society, Newcomb, NY. 8 November 2003. David Capen, Adam Duerr.
Double-crested Cormorant Research on Lake Champlain. Invited seminar, New York Department of Environmental Conservation, Albany NY, 19 February 2004. Adam Duerr.