The Wilson Journal of Ornithology 123 ( 1 ): 132 - 136 , 2011 
INTERBREEDING OF AECHMOPHORUS GREBES 
ANDRE KONTER 1 
ABSTRACT.—I analyzed the occurrence of intermediates between Western ( Aechmophorus occidentalis) and Clark’s 
(A. clarkii ) grebes, of mixed pairings, and of species composition in populations of Aechmophorus grebes in California and 
Oregon, USA. Western Grebes comprised 69% of the aggregated total of grebes identified while intermediates represented 
~3.5% (41-46 individuals) in the populations investigated. I conclude that numbers of intermediates between purebred 
parental individuals have increased. Higher percentages of mixed pairings were observed at Lake Almanor; an aggregated 
7.9% of nesting pairs were not composed of two purebred grebes of the same species. Statistically, mating remained 
strongly assortative. Received 1 June 2010. Accepted 22 September 2010. 
The American Ornithologists’ Union split the 
two North American Aechmophorus grebes in 
1985 into Western (A. occidentalis) and Clark’s 
(A. clarkii) grebes (AOU 1985). Both arose from a 
common ancestral population that divided into 
northern and southern subpopulations; differences 
between both developed during a period of 
geographical isolation (Storer and Nuechterlein 
1985). More recently the ranges of both have 
become largely sympatric and differences in their 
advertising calls were identified as critical to their 
reproductive isolation (Nuechterlein 1981b). 
Genetic investigations suggest the taxonomy of 
Aechmophorus grebes might not be entirely 
settled (Ahlquist et al. 1987, Bledsoe and Sheldon 
1989, Guerra and Speed 1996, Hebert et al. 2003, 
Savolainen et al. 2005, Ratnasingham and Hebert 
2007). However, genetically low levels of differ¬ 
entiation cannot be used alone to establish species 
boundaries as they reflect the time of divergence 
of lineages (Ahlquist et al. 1987). A crucial 
question to be answered in the field is whether 
barriers to random mating between Western and 
Clark’s grebes are increasing or vanishing. 
My objectives were to investigate: (1) mixed 
pairings of Aechmophorus grebes, (2) the occur- 
lence of intermediates, and (3) the proportions of 
Clark’s and Western grebes at different locations 
in northern California and southern Oregon, USA. 
METHODS 
Study Sites. —Sites visited during the study and 
having grebes were Upper Klamath Lake, Lake 
Ewauna and Drews Reservoir, all in Oregon, Tule 
Lake National Wildlife Refuge (NWR), Lake 
Almanor, Lake Shastina, East Park Reservoir, all 
in California, and Goose Lake, straddling both 
1 Museum of Natural History, 25, rue Munster, Luxem- 
ourg, L-2150 Luxembourg; e-mail: podiceps@pt.lu 
states. Water surfaces, elevations, and geograph¬ 
ical coordinates of study sites varied (Table 1). 
An additional survey was made along the Pacific 
Coast, south of Fort Ross, California when I 
unexpectedly encountered a group of Aechmo¬ 
phorus grebes at sea on 7 August 2009. 
Timing and Recording of Data— Study sites 
were visited for 12 days between 25 July and 5 
August 2009. Three different areas were screened 
for grebes at Upper Klamath Lake: Moore Park 
and the Marina, Wocus Bay, and Eagle Ridge. 
Two screens were made at Lake Ewauna from 
Timber Mill Shores. Few grebes were observed at 
Goose Lake due to low water levels and the site 
was not further explored. Tule Lake NWR was 
visited by driving along the shore on two access 
roads. Each time grebes were encountered, I 
stopped to identify them. Two grebe colonies 
were present in the northwest part of Lake 
Almanor. Observations there occurred from the 
shore. East Park Reservoir was screened at four 
different access points. Low water levels at Lake 
Shastina permitted access by walking to a small 
island from where data were recorded. 
Aechmophorus grebes present were counted 
with the help of 10 X 25 Zeiss binoculars. All 
grebes sufficiently close for species identification 
were scanned using a Konica Minolta Dynax 7D 
camera with a Sigma AF 800 mm auto focus lens 
mounted on a tripod. After identification, the 
species (Western, Clark’s, or intermediate grebes) 
was registered by an assistant. The composition of 
all pairs encountered was also recorded. Photos 
were immediately taken of each intermediate, of 
each grebe with a doubt about a pure-bred species 
status, of mixed pairs, and of pairs comprising 
individuals with uncertain species status. Species 
status for nesting pairs at Lake Almanor was 
assessed per nest platform. Species status was also 
recorded for displaying pairs. Species distribution 
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