The Wilson Journal of Ornithology 124(4):71 3-720, 2012 
VISUAL ASSESSMENT OF INTERBREEDING BY 
AECHMOPH OR US GREBES 
ANDRE KONTER 1 
ABSTRACT.—Population composition of Aechmophorus grebes was investigated in Utah and the occurrence of possible 
intermediates between Western ( Aeclwwplwrus occidentalis) and Clark's (A. clarkii ) grebes was assessed. Individuals with 
clearly intermediate traits represented an aggregated 6.5% (43 individuals) in the populations investigated while another 
7.6% (46 individuals) did not entirely conform to the description of Aedunophnrus grebes provided by Storer and 
Nuechterlein. The number of intermediates has increased in Utah in comparison to historical data. A similar survey in 
California and Oregon in 2009 also found increased percentages of intermediates. The results a priori contradict growing 
reinforcement of incompatibilities between both Aechmophorus grebes. Western and Clark's grebes in these major areas of 
sympairy appear to interbreed as frequently as in areas of relative allopatry. No evidence against assortative mating was 
found. Hybridization for Aechmophorus grebes may reflect adaptive mate choice rather than a mistake. Introgressive 
hybridization may be important and mask real rates of hybridization. Received 22 Jamum 2012. Accepted R May 2012. 
The American Ornithologists’ Union split the 
two North American Aechmophorus grebes in 
1985 into Western ( A. occidentalis) and Clark’s 
grebes (A. clarkii) (AOU 1985). More recent 
genetic investigations (Ahlquist et al. 1987, 
Bledsoe and Sheldon 1989, Guerra and Speed 
1996. Hebert et al. 2003, Savolainen et al, 2005. 
Ralnasingham and Hebert 2007) found low levels 
of differentiation between both grebe species and 
the methods of DNA bareoding applied to North 
American bird species suggest Western and 
Clark's grebes are candidates for combination 
(Kerr et al. 2007). Both grebe species arose from a 
common ancestral population (Storer and Nuech- 
terlein 1985) that divided into northern and 
southern subpopulations that today are again 
largely sympatric. Positive assortative maling in 
mixed populations of Western and Clark’s grebes 
was demonstrated by different studies (Ratli 1979; 
Nuechterlein 1981a. b: Lindvall and Low 1982; 
Nuechterlein and Storer 1982). Differences in 
advertising calls were identified as critical to their 
reproductive isolation (Nuechterlein 198lb). The 
taxonomic status of Aechmophorus grebes may 
not be entirely settled and 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. First genera¬ 
tion hybrids could be rare, but they arc viable and 
produce viable offspring (Storer and Nuechterlein 
1985, Nuechterlein and Buitron 1998). 
I investigated population composition and 
occurrence of intermediate Aechmophorus grebes 
in California and Oregon in 2009 based on 
1 Museum of Natural History, 25 rue Munster, Luxem¬ 
bourg. L-2150, Luxembourg; e-mail: pocliceps@pt.lu 
subdivision of divergent areas in the plumage of 
the face between Western and Clark’s grebes 
described by Storer and Nuechterlein (1985). I 
found increased numbers of intermediates and 
mixed pairings (Konter 2011). However, these 
geographically limited findings may not be 
representative for both species more generally 
and I repeated the study in Utah, another major 
area of sympatry. 
METHODS 
Study Sires— Major sites visited having grebes 
included three protected bays on the eastern shore 
of Great Salt Lake (GSL); the freshwater and 
brackish marshes of Farmington Bay within 
Turpin Unit, Crystal Unit, and two ponds close 
to the visitor center; Ogden Bay and Willard Bay, 
an impoundment on the floodplain of the GSL; 
and part of Bear River Bay. Also included were 
Mantua Reservoir, an impoundment at the top of 
Box Elder Canyon East of Brigham City, Yuba 
Liike south of Nephi, Minersville Reservoir west 
of Beaver. Panguitch Lake west of Panguitch, 
Lake Powell only at one site when crossing it on 
the way to Natural Bridges. Bottle Hollow 
Reservoir near Roosevelt, and seven areas of 
Strawberry Lake between Duchesne and Heber 
(Soldier Creek, nearby marina. Chicken Creek 
West, bay between Chicken Creek East and West, 
Chicken Creek East. Mud Creek, and nearby 
marina and campgrounds). 
Additional wetlands visited having a few 
Aechmophorus grebes that could be identified 
were: Benson Marina on Bear River; a wetland 
adjacent to Cache Junction and Hyrum Reservoir, 
all close to the city of Logan; Jensen Park pond in 
713 
