Konter • INTERBREEDING OF AECIIMOPHORUS GREBES 
717 
WG DVO IG KG CG 
FIG. 1. Subdivision of Aechmophorus grebes identified 
into Western Grebes (WG), debatable Western Grebes ( ' 
WG), intermediates (IG), debatable Clark's Grebes ( * CGI. 
and Clark's Grebes (CG) in Utah. USA. 
Grebes, two Clark’s Grebe families, an interme¬ 
diate female with two chicks was paired to a 
Western Grebe male, a debatable Western Grebe 
male, and a Western Grebe female cared for two 
chicks. 
Additional pairs were recorded either display¬ 
ing or swimming. An intermediate male displayed 
with a female Western Grebe at Strawberry l.ake 
and at Lake Powell, and three Clark's Grebe pairs 
and two Western Grebe pairs displayed at Farm¬ 
ington Bay, Turpin Unit. A debatable female 
Western Grebe was with a male Western Grebe at 
Minersville Reservoir, and an intermediate male 
appeared to be paired with a female Clark’s Grebe 
at Willard Bay. Two Clark's Grebe pairs were 
recorded at Benson Marina and at Cache Junction, 
one Western Grebe pair was at Lake Powell, and 
one Clark’s and two Western Grebe pairs were at 
Willard Bay. No clearly identifiable pairs were 
present elsewhere (Table 4). 
Observed and expected frequencies of pair 
composition varied (Table 5). The x : Goodness 
of fit test provided no evidence contradicting like- 
with-Uke or assortative mating, whether consid¬ 
ering only grebes with platforms and pairs having 
ollspring or including displaying pairs and grebes 
in twos that seemed paired. 
DISCUSSION 
I found no evidence contradicting assortative 
mating in the two Aechmophorus species. How¬ 
ever. 6.5% of 660 grebes assessed had apparent 
intermediate traits and another 7.6% were either 
debatable Western or Clark's grebes. These 
represented 14.1% of the population assessed. 
This percentage could be biased towards interme¬ 
diates as they could be over-represented outside 
active breeding colonies. They could disperse to 
non-breeding areas relatively more rapidly than 
Western and Clark's grebes as sexual selection 
may put them at a disadvantage (Nuechterlein 
1981a). A comparison with historical occurrences 
of intermediates in Utah and other major areas of 
sympatry suggests present day numbers have 
increased. Intermediates were few' in 1963 (Storer 
1965) at Bear River Migratory Bird Refuge, Utah, 
and they represented only 0.7% of the population 
in 1975 in = 3.376; Ratti 1979). They were also 
<1% in >8.000 observations in California, 
Nevada. Oregon, and Utah (Ratti 1981). My data 
from northern California and southern Oregon in 
2009 confirmed assortative mating, but percent¬ 
ages of intermediates were increasing; 3.3% of the 
grebes clearly had intermediate traits and another 
0.4% did not entirely conform to either species 
(/; = 1,293) (Konter 201 l). This percentage is not 
comparable to the 7.6% reported for Utah as the 
methodology used in 2009 included a major part 
of debatable individuals in the counts of either 
Western or Clark's grebes; if used in Utah, only 
1.5% of the grebes were classified as debatable 
whereas the remaining 6.1% were counted as 
either Western or Clark’s grebes. 
It is generally debatable to what extent 
individuals displaying intermediate traits in the 
field are hybrids between two closely related 
species or represent phenotypic variation. Studies 
of hybridization in other species based on visual 
identification, whether constructing a hybrid 
index (Hoffmann et al. 1978, Bell 1997) or not 
(Rasmussen 1991. Baker and Boy Ian 1999). relied 
TABLE 4. Pair composition differentiating among Western (WG), Clark's (CG). debatable Western (± WG). 
debatable Clark's (± CG). and intermediate (IG) Aechmophorus grebes. Utah. USA. 
Total of WG pairs 
Total of CG pairs 
Mixed pair composition 
Pairs with platform/nest 
10 
5 
1 IG X WG 
Families 
21 
2 
2 WG X IG. 3 ± WG X WG. I WG X ± WG 
Gther pairs 
5 
8 
2 IG X WG. 1 WG X ± WG. 1 IG X CG 
