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Marine Biology

, Volume 147, Issue 1, pp 27–35 | Cite as

Habitat use and group size of pied cormorants (Phalacrocorax varius) in a seagrass ecosystem: possible effects of food abundance and predation risk

  • M. R. HeithausEmail author
Research Article

Abstract

Both food abundance and predation risk may influence habitat use decisions. However, studies of habitat use by birds in marine environments have focused only on food abundance. I investigated the possible influences of food abundance and predation risk from tiger sharks (Galeocerdo cuvier) on habitat use by pied cormorants (Phalacrocorax varius) over two spatial scales and on cormorant group size. Cormorants were usually solitary, but group size was highest in shallow habitats during months when shark density was low. Regardless of season, cormorant density within shallow habitats was higher over seagrass than sand, and cormorants were distributed between these two microhabitats proportional to prey density. Therefore, cormorants appear to respond to prey abundance at a relatively narrow spatial scale (i.e., tens of meters). At the habitat-patch scale (~1 km), the density of cormorants and their prey (teleosts) was higher in shallow habitats than in deep ones, but the density of cormorants was influenced by an interaction between water temperature (i.e., season) and habitat. There was decreased use of shallow habitats as water temperature, and the density of tiger sharks, increased. When shark density was low, cormorants were distributed across habitats roughly in proportion to the abundance of fish, suggesting that cormorants respond to food abundance at the scale of habitat patches. However, as shark abundance increased, the relative density of cormorants dropped in the dangerous shallow habitats such that there was a greater density of cormorants relative to their food in deep habitats when sharks were abundant. This suggests that pied cormorants trade-off food and risk by accepting lower energetic returns to forage in safer habitats. This study provides the first evidence that marine habitat selection by birds may be influenced by such a trade-off, and provides further evidence that tiger sharks are important in determining habitat use of their prey and mediating indirect interactions within Shark Bay.

Keywords

Predation Risk Small Spatial Scale Food Abundance Bottlenose Dolphin Prey Abundance 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

I thank the National Geographic Society Expeditions Council, NSERC Canada (grant A6869 to L.M. Dill) and the Monkey Mia dolphin resort for providing financial and logistical support for fieldwork. Simon Fraser University, the National Science Foundation, and Florida International University provided personal support. J. McLash, L. Barre, J. Burghardt, C. Genrich, S. Buchannan, F. Bretos, P. Greene, and H. Finn provided field assistance. A. Frid helped with statistical analyses. Finally, I thank A. Frid, D. Moore, E.R. Heithaus and L.M. Dill for comments. Transects were conducted under authority of Western Australia Department of Conservation and Land Management permits DI000044, NE001649, DI 000067, and NE001927. This is contribution 16 of the Shark Bay Ecosystem Research Project.

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  1. 1.Behavioural Ecology Research Group, Department of Biological SciencesSimon Fraser UniversityBurnabyCanada
  2. 2.Department of Biological Sciences, Marine ProgramFlorida International UniversityNorth MiamiUSA

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