Konter • INTERBREEDING OF AECHMOPHORUS GREBES 
715 
TABLE 2. Number 
intermediate (IG) grebes 
of Aechmophorus grebes observed and classified into Western (WG), Clark’s (CG), and 
in Utah. USA. 
Welland 
Total grebes present 
Total glebes assessed WG {%) 
Species identification 
CG {%) 
IG (%) 
Farmington Bay, Crystal Unit <100 
58 
39 (67.3) 
9 (15.5) 
10 (17.2) 
Farmington Bay. other areas ±55 
40 
10 (25.0) 
26 (65.0) 
4 (10.0) 
Ogden Bay 
14 
14 
6 (42.9) 
6 (42.9) 
2 (14.2) 
Willard Bay 
±100 
84 
52 (61.9) 
24 (28.6) 
8 (9.5) 
Mantua 
±40 
22 
12 (54.5) 
6 (27.3) 
4 (18.2) 
Yuba Lake 
20-30 
14 
9 (64.3) 
4 (28.6) 
1 (7-1) 
Minersville Reservoir 
142 
142 
87 (61.3) 
40 (28.2) 
15 (10.5) 
Panguitch Lake 
±50 
43 
35 (81.4) 
2 (4.6) 
6 (14.0) 
Lake Powell 
15 
15 
12 (80.0) 
1 (6.7) 
2 (13.3) 
Bottle Hollow Reservoir 
18 
18 
12 (66.7) 
0 (0.0) 
6 (33.3) 
Strawberry Lake 
±200 
174 
138 (79.3) 
11 (6.3) 
25 (14.4) 
Other sites visited 
±70 
36 
12 (33.3) 
14 (38.9) 
10 (27.8) 
Totals 
829 
660 
424 
143 
93 
Percent 
100 
79.6 
64.2 
21.7 
14.1 
of intermediate color and identifiable neither as 
clearly orange-yellow nor as dull yellow-green 
were qualified as yellow. Other trails less obvious 
in the field were not primarily considered. Little 
intermediacy and no overlap between both species 
;tre observable from April to October (Storer and 
Nucchterlein 1985). All grebes not entirely 
conforming to the above characteristics of West¬ 
ern or Clark’s grebes were grouped separately 
according to their diverging traits. 
Paii- composition of nesting grebes or those 
caring for chicks or displaying was recorded at each 
location. A x'-tesl w as used to examine if pairing by 
grebes was random or assortativc with VassarStats 
(http://faculty.vassar.edu/lowry/VassarStats.htnil). 
Assortative mating was defined as like-with like 
pairing to overcome the problem of defining species 
status (Randier 2008). All grebes deviating from the 
description of either Western or Clark's grebes of 
Storer and Nuechtcrlein (1985) were treated as 
intermediates for the test. Bill si/e dimorphism w'as 
used to discriminate between males and females 
(Storer and Nuechterlcin 1985). 
RESULTS 
Numbers and Species Composition oj Popula¬ 
tions .—Eighty percent or 600 individuals ot all 
grebes encountered were assessed ot which 424 
(64.2%) were classified as Western Grebes and 
143 (21.7%) as Clark's Grebes. Ninety-three birds 
(14.1%) did not entirely match the traits of either 
species of Aechmophorus grebes (Table 2). 
Western Grebes were in majority at nine 
locations. They were most numerous at Panguitch 
Lake, Lake Powell, and Strawberry Lake (each 
—80%). Only the Turpin Unit of Farmington Bay 
had a majority of Clark's Grebes. Not entirely 
conforming individuals represented between 7.1 
and 33.3% of I he Aechmophorus grebes at the 
different sites. Their numbers were most impor¬ 
tant at Strawberry Lake (n = 25) followed by 
Minersville Reservoir (n = 15). Bottle Hollow 
Reservoir had the most important population 
of intermediate grebes (33.3%) and no Clark's 
Grebes were recorded at that site (Table 2). 
Occurrence of Intermediates. —Ninety-three 
grebes deviated from the descriptions of either 
Western or Clark's grebes provided by Nuechter- 
lein and Storer (1985). The first subdivision 
(Table 3) aggregated grebes displaying minor 
differences to Western Grebes that were classified 
as debatable Western Grebes. They were 7.0% 
(46) of all grebes assessed. The second subdivi¬ 
sion (// = 43) included those grebes with clearly 
intermediate traits between both Aechmophorus 
species and classified as intermediates. They 
represented 6.5% of all grebes assessed. The third 
subdivision aggregated those showing minor 
deviations with respect to Clark's Grebes and 
classified as debatable Clark's Grebes. Four 
grebes (0.6%) varied only slightly from Clark's 
Grebes (Fig. ]). 
Mixed Pairings .—Two pairs of Clark's Grebes, 
five pairs of Western Grebes, and one pair 
