134 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 1, March 2011 
TABLE 2. Adult Clark’s (CG), Western (WG), and intermediate grebes (IG, ±CG, ±WG) in surveys and proportions 
of pure and intermediate grebes by survey site. 
Location 
CG 
WG 
Intermediates 
IG ±CG 
±WG 
Total population 
% CG-WG 
% IG, ±CG, ±WG 
East Park Reservoir 
60 
25 
4 
1 
1 
94 a 
66-27% 
4.4-6.6% 
Drews Reservoir 
4 
2 
1 
~30 
67-33% 
c 
Upper Klamath Lake 
143 
123 
12 
1 
~350 a 
51 —44% 
4.3-47% 
Lake Ewauna 
48 
21 
90 
70-30% 
0 
Tule Lake NWR 
3 
158 
1 
~180 a 
2-97% 
0.6% 
Lake Shastina 
52 
59 
1 
225 
46-53% 
0.9% 
Lake Almanor 
77 
462 
23 
1 
2,112 
14-82% 
4.1-4.3% 
Totals 
387 
850 
41 
2 
3 
3,081 
31-69% 
3.3-37% 
Pacific Coast b 
19 
64 
170 
23-77% 
Total population counted in areas surveyed. 
Birds were too far out for identifying intermediates. 
L Few grebes could be correctly identified at Drews Reservoir. 
The composition of observed pairings with 
known partners was strongly assortative (% 2 = 
236.49, df = 5, P < 0.0001) at Lake Almanor. 
This did not change if the nine intermediates with 
unknown partners were included, assuming an 
expected distribution of the partner’s species 
status (x 2 = 230.13, df = 5, P < 0.0001, 
Table 3). No historical data about mixed pairings 
could be found for Lake Almanor. All pairs 
tending young at Tule Lake NWR were composed 
of purebred Western Grebes (n = 34). 
DISCUSSION 
My study confirms strongly assortative mating 
between the two Aechmophorus species although, 
at Lake Almanor, the percentage of nesting pairs 
not composed of two purebred grebes of the same 
species (7.9%) largely exceeds those found in 
earlier studies in California and Oregon. For 
instance, Ratti (1979) recorded one mixed pair 
(2.8%, n = 36) in 1977 at Upper Klamath Lake 
and none at Tule Lake NWR (n = 139). 
Nuechterlein (1981a) observed only one mixed 
pair at Upper Klamath Lake in 1979, too. He 
found mixed pairs represented 3% of male-female 
courtship displays at Tule Lake NWR, but only 
1.1% in breeding pairs (n = 91) in 1979. Ratti 
(1979), in over 600 independent observations of 
pairs in California and Oregon, found that mixed 
pairs represented 1.2% of pre-nesting pairs and 
0.25% (n = 766) of pairs with young. Lindvall 
and Low (1982), in other areas of sympatry, 
observed 0.6% (n = 161; 1974), Ratti (1979) 
1.9% (n = 719; 1975) and 0.8% of mixed pairs 
(n = 506; 1976) at Bear River Migratory Bird 
Refuge. Ratti (1979) calculated that 1.2% of 1,185 
pairs observed in 1975-1977 for all of Utah 
represented mixed pairs. 
The current study found increased numbers of 
intermediates representing ~3.5% in the popula- 
l , ° bsei 7 ed and ex P ecte< l frequencies of pairings (nesting and displaying birds) among Western (« = 462). 
ar s n ), and intermediate grebes (n - 24), not considering the nine nesting intermediates with unknown partner 
and assuming a random d.stribution of their partners (i.e., 7 Western. 1 Clark’s, and 1 intermediate) reflecting the 
composition of the population. 
Pair composition 
WGxWG 
WGxCG 
WGxIG 
CGxCG 
CGxIG 
IGxIG 
X 2 Goodness of fit test 
Without 
nine intermediates with unknown partners 
With random distribution of the nine intermediates' partners 
Observed 
frequency 
Expected 
frequency 
Percentage 
deviation 
Observed 
frequency 
Expected 
frequency 
Percentage 
deviation 
225 
186 
+21.1 
225 
192 
+17.3 
6 
62 
-90.3 
6 
64 
-90.6 
6 
19 
-69.0 
13 
20 
-34.9 
34 
5 
+566.1 
34 
5 
-545.0 
3 
3 
-6.9 
4 
3 
+20.1 
2 
1 
+315.4 
3 
1 
+503.4 
x 2 = 
= 236.49, df = 5, P < 0.0001 
X 2 = 230.13, 
df = 5, P < 0.0001 
