Konter • AECHMOPHORUS GREBES 
133 
TABLE 1. Characteristics of study sites in California and Oregon, USA. 
Geographical coordinates 
Name Water surface (ha) 
Upper Klamath Lake 
±25,000 
Lake Ewauna 
±100 
Tule Lake NWR 
±16,000 
Goose Lake 
±50,000 
Drews Reservoir 
±800 
Lake Shastina 
±740 
Lake Almanor 
±11,000 
East Park Reservoir 
±700 
Elevation (m asl) 
N 
1,260 
42° 18' 839 
1,245 
42° 13' 170 
1,220 
41° 55' 956 
1,435 
41° 98' 910 
1,495 
42° 07' 208 
845 
41° 30' 854 
1,395 
40° 15' 334 
365 
39° 19' 311 
E 
121° 32' 675 
121° 46' 457 
121° 32' 675 
120° 41' 085 
120° 37' 145 
122° 23' 346 
121° 14' 055 
122° 29' 417 
and composition at each location was estimated 
based on identification from all samples. A x 2_test 
using the VassarStats web site for statistical 
computation was applied to test if pairing by the 
grebes was random or assortative with respect to 
species. 
Identification of Grebe Species (Western vs. 
Clark’s ).—Species identification of exclusively 
adults followed the descriptions provided by 
Storerand Nuechterlein (1985) and their subdivi¬ 
sion of diverging areas of the face between 
Western and Clark’s grebes into lores, above 
eye, behind eye, and below eye (Storer and 
Nuechterlein 1985: 103, fig. 1 ) complemented 
by descriptions in Ratti (1981), Eichorst and 
Parkin (1991), and Konter (2009). Little interme¬ 
diacy and no overlap between both species is to be 
found in adults during the breeding season from 
April to October. 
Individuals were classified as Clark’s Grebes if 
they had an orange-yellow bill with a sharply 
defined black culmen, white lores and white 
feathers above, behind, and below the eye so the 
black crown ended clearly above the eye. 
Individuals were classified as Western Grebes if 
they had a dull yellow-green bill and the black of 
the crown extended to below the eyes. 
All grebes not entirely conforming to the 
descriptions of purebred Western or Clark s 
grebes were a priori classified as intermediates, 
unless divergence was minimal, in which case 
they were classified as ±Westem or ±Clark’s 
grebes. The term “intermediate” as applied here 
ts not limited to first generation hybrids, but may 
include backcrosses. 
RESULTS 
Numbers and species composition of popula¬ 
tions observed varied (Table 2). Not considering 
possible intermediates, I found an aggregated 
69% of Western Grebes, which represented the 
majority of the grebes at Tule Lake NWR, Lake 
Almanor and, to a lesser extent, Lake Shastina. 
Western Grebes were present over three times as 
often as Clark’s Grebes in the group seen along 
the Pacific Coast. 
Forty-one grebes were classified as intermedi¬ 
ates between Clark’s and Western grebes. Another 
five differed only slightly from purebred grebes 
and their status was unclear. Intermediates 
represented between 0.6% (Tule Lake NWR) 
and 4.4-6. 6 % (East Park Reservoir) of local 
populations, or an aggregated 33-3.1% of all 
grebes identified in this study (Table 2). 
Most grebes at Lake Almanor were nesting 
and no pairs tending young were found. Several 
pairs were building on rather recent platforms 
and others searched the colony lor a nesting 
space. Some individuals were still engaged in 
water courtship. The owners for 267 platforms 
could be identified: 246 were occupied by 
purebred pairs composed of either two Western 
(n = 197 , 80.1%) or two Clark’s grebes (n = 29, 
12.0%). Four pairs (1.5%) were mixed, com¬ 
posed of one Western and one Clark’s grebe. 
Nine of the observed intermediates (3.4%), were 
paired with partners that remained unidentified, 
another seven ( 2 . 6 %) were paired to purebred 
Western (n = 4) or Clark s grebes (n — 3), and 
one pair ( 0 . 4 %) was composed of two interme¬ 
diates. Thus, 7.9% of the aggregated nesting 
pairs were not composed of two purebred grebes 
of the same species. This percentage was 27.8% 
( n = 5 ) for 18 displaying pairs: one pair was 
composed of two intermediates, two additional 
intermediates displayed with two Western 
Grebes, and two pairs were composed of one 
Western and one Clark’s Grebe. 
