Konter • AECHMOPHORUS GREBES 
135 
tions investigated. Earlier indications for interme¬ 
diates were 2.8% at Upper Klamath Lake in 1977 
and less than 1% for Oregon and California 
combined (Ratti 1979). Nuechterlein (1981a) 
identified only 18 intermediates at Upper Klamath 
Lake and Tide Lake NWR together over two 
seasons in 1978 and 1979; a period when the 
population of Tule Lake NWR was much larger 
than today. The present number of 13-14 
intermediates should be relatively higher. Other 
major areas of sympatry between Western and 
Clark’s grebes also reported lower figures of 
intermediates. For instance, at Bear River Migra¬ 
tor}'Bird Refuge, Utah, intermediates were few in 
1963 (Storer 1965) and they represented 0.7% of 
the population in 1975 (n = 3,376; Ratti 1979). 
They also comprised <1% in over 8,000 obser¬ 
vations in California, Nevada, Oregon, and Utah 
(Ratti 1981). 
We should expect Aechmophorus grebes to 
hybridize to a lesser extent in regions of sympatry 
than in regions of relative allopatry (based on 
Randier 2006), such as prairie Canada with only 
1-4% of Clark’s Grebes. Intermediates represent¬ 
ed <4% of the breeding population at Delta 
Marsh, Manitoba, around 1980 and in the early 
1990s (Nuechterlein and Buitron 1998) and, in 
2008, they comprised ~3% on different lakes in 
Manitoba and Saskatchewan (Konter 2009). 
These numbers are in the range of those found 
‘ n this study and a priori indicate comparable 
levels of hybridization in sympatric and allopatric 
populations. It is also noteworthy that Mexican 
populations, generally dominated by Clark’s 
Grebes, seem to show a completely different picture 
a nd, in surveys by Feerer (1977), intermediates 
represented 51 %{n = 54) at Laguna Tuxpan with a 
total population of 105 and 30% (n = 15) at Laguna 
Cuitzeo with 50 individuals. Unfortunately, no 
recent data exist for comparison. 
Speciation is regarded as essentially complete 
‘f- during sympatry, interbreeding is reduced to a 
level that prevents genetic swamping by the 
Parent species (Mayr 1951, Bigelow 1965, Ratti 
W9). We may question whether the high 
numbers of pairs not composed of two purebred 
grebes of the same species and the increased 
Percentages of intermediates found here are 
sufficient to prevent both Aechmophorus species 
from merging. We also have to consider that the 
numbers of all-generation intermediates recorded 
ln the field may be biased by their detection rate 
(Randier 2004) and. that by introgressive hybrid¬ 
ization (Anderson and Hubricht 1938), they are 
absorbed into one of the parental species. Later 
generation hybrids are likely cryptic and similar to 
the dominant parental type. The resulting asym¬ 
metric introgression could perhaps best be assess¬ 
ed by analyzing shifts in the composition of the 
combined Aechmophorus populations, although 
causes other than hybridization may alter the 
relative abundance of both species. Complete 
counts have only been introduced more recently in 
California and Oregon in the context of declining 
global grebe populations following breeding and 
wintering habitat loss or degradation (e.g., 
consequences of oil spills, eutrophication, and 
water level fluctuations at breeding sites) and 
conflict with leisure activities. The data do not 
permit assessing historical developments in this 
respect as species composition was only analyzed 
to a limited extent. 
Aechmophorus grebes at the current levels of 
intermediates and assortative mating still provide 
sufficient confidence for conferring species status 
to both. Developments should be closely moni¬ 
tored as mate choice may break down when the 
population consists almost entirely ot highly 
recombined hybrids (Barton and Hewitt 1985). 
ACKNOWLEDGMENTS 
I am grateful to my wife Maria for her persistent 
assistance during fieldwork, to J. J. Vlug for comments on 
the first draft of this paper, and to referees of the Wilson 
Journal of Ornithology for welcome suggestions. 
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