EPOW - Ecology Picture of the Week

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A Tale of Two Grebes

Credit & Copyright:  Tom Kogut

[Editor's note:  This week we present another guest contribution
from wildlife biologist and photographer Tom Kogut]
  

Explanation:  This week we are at Lind Coulee- an extension of the expansive Potholes Reservoir in central Washington- where both Clark's and Western Grebes occur. They are most easily distinguished by their facial patterns and bills; Clark's Grebe has white above the eye and possesses a slightly upturned, straw-colored bill, while Western Grebe has white below the eye only, and has a straight, duller, greenish-yellow bill.  Both species nest in proximity to each other and exhibit a unique courtship display that involves a repertoire of ritual head movements which culminate in a spectacular "water dance". They occur together throughout much the western United States wherever large water bodies are found, either natural or man-made.  Clark's Grebe was thought to be a light-phase color morph of Western Grebe until research in the 1970's confirmed that hybridization between the two species is uncommon (Ratti 1979). How, then, have these two species diverged and remained separate despite sharing the same habitat and many behavioral traits?     Clark's Grebe pair engaged in courtship.    The answer may lie with the foraging habitats of these two birds. Clark's Grebe typically forages in deeper water and exhibits more "springing dives" than the Western Grebe (Neuchterlein and Burton 1989), which is more often found in shallower waters. This may give Clark's Grebe access to larger and different fish species than Western Grebe, limiting competition for prey between the two species.     Western Grebe.  Note the dark around the eye and the straight, greenish-yellow bill.     There is some recent evidence, however, that interbreeding between these two closely related species may be more common than previously thought, and may actually be increasing in some areas. Long-term studies and monitoring will tell if this trend will continue, and if the mechanisms that have kept these two species distinct will continue to prevail.     Clark's Grebe.  Note the white around the eye, and the slightly upturned ("recurved") yellowish bill.       Information:       Konter, A. 2012. Visual assessment of interbreeding by Aechmophorus grebes. The Wilson Journal of Ornithology 124(4):713-720.       Neuchterlein, G.L. and D.H. Burton. 1989. Diving differences between Western and Clark's Grebes. The Auk 106: 467-470.       Ratti, J.T. 1979. Reproductive isolation and isolating mechanisms between sympatric dark- and light-phase Western Grebes. The Auk 96: 573-586.