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THE WILSON JOURNAL OF ORNITHOLOGY • VoL 123. No. 3. September 2011 
TABLE I. Nest dimensions of Least Bell’s Vireo, White-eyed Vireo, and a mixed species pair. 
Mixed pair 
Least Bell's Vireo“ 
White-eyed Vireo h 
Average 
n 
Range 
Average 
» 
Range 
Nest height, m c 
0.86 
1.00 ± 0.40 
271 
0.30-3.5 
0.94 
29 
0.29-2.1 
Inside diameter, mm d 
46 
49.9 ± 5.1 
270 
35-70 
50.5 
12 
44-64 
Nest depth, mm e 
52 
46.7 ± 5.4 
269 
30-70 
53.7 
12 
38-73 
Cup height, mm f 
85 
62.1 ± 9.5* 
49 
97.2 
12 
69-146 
“ Unpublished data from this study for the San Luis Rcy River. California. 
h Hopp et al. 1995. 
c Distance from ground to top rim of nest. 
At widest point measured fawn inside the rim. 
e Distance of lowest rim to bottom of inside the nest. 
Highest rim of the nest to outside bottom of the nest. 
8 Arizona subspecies IV. h. arilomeY. Kus et al. (2010). Harrison (1979) provides an average range of 71-98 mm. 
did not attempt to catch the male Least Bell’s 
Vireo to minimize disturbance. We banded the 
White-eyed Vireo with a single aluminum num¬ 
bered federal band. We annually conduct inten¬ 
sive band observations for Least Bell’s Vireos 
throughout southern California and should be able 
to detect these individuals if they return in future 
years. Wc also collected DNA samples from each 
individual. We collected a pin feather sample 
from each nestling and a blood sample from the 
female via a toenail clip. This genetic material is 
currently being analyzed for further evidence of 
this interspecific breeding occurrence. 
The nest was empty on 14 June and two 
fledglings were heard calling nearby. Three more 
visits were made to the territory to search for a 
second nesting attempt, but none was detected, 
nor was the female White-eyed Vireo observed. 
One individual (MAB) observed one of the 
banded fledglings with the male Least Bell’s 
Vireo on 27 June. The fledgling resembled a Least 
Bell's Vireo fledgling in appearance with a white 
underbelly, gray crown, back and primaries, and 
black eyes. 
The pair's pensile nest was suspended in an 
arroyo willow' from the crook of a Y-shaped 
horizontal branch. We obtained nest dimensions 
(Table l) for comparison with average measure¬ 
ments for nests of Least Bell’s Vireo and White¬ 
eyed Vireo. The nest was left undisturbed in the 
territory. 
DISCUSSION 
Interspecific breeding among vireos is rare. A 
Blue-headed Vireo ( V. solitarius) and Yellow- 
throated Vireo (V. flavifrons) hybrid has been 
recorded (James 1998). Additionally, Hauser 
(1959) provides documentation of a female 
Blue-headed Vireo and male Yellow-throated 
Vireo constructing a nest together, although the 
nest was not used. Additional hybrids have been 
implicated through genetic analysis or physical 
characteristics (McCarthy 2006, Chartier 2008). 
but documentation in the field of successful 
nesting of any two vireo species is lacking phot 
to our observations. 
A comparison of cytochrome b sequences using 
gel electrophoresis showed Bell’s Vireo and 
White-eyed Vireo to be sister taxa and more 
closely related to each other than to any other 
vireo species analyzed (Murray et al. 1994). Leasi 
Bell’s Vireos and White-eyed Vireos share similar 
breeding behaviors that may have facilitated this 
pairing and successful production of young. Least 
Bell's Vireos typically breed in dense, low, earh 
successional vegetation (Kus et al. 2010), and 
White-eyed Vireos arc common in low' trees and 
shrubs, dense secondary deciduous scrub, wood 
margins, and overgrown pastures (Hopp et al. 
1995). Both male and female Least Bell s Vireos 
and White-eved Vireos participate in nest-build¬ 
ing. incubation, and parental care. 
Least Bell’s Vireos typically lay three to four 
eggs per clutch, incubate for —14 days, and fledge 
young -10-12 days later (Kus et al. 2010). 
White-eyed Vireos typically lay four eggs P^ 
clutch, incubate for —14 days, and fledge young 
9-11 days later (Hopp et al. 1995). Harnson 
(1979) notes that Bell’s Vireo nests are indistin¬ 
guishable from White-eyed Vireo nests, while 
Kus et al. (2010) suggest Bell's Vireo nests may 
be smaller, more finely constructed, and have a 
shorter outside height than White-eyed Vireo 
nests. The height of the mixed pair's nest 
was within the average range for those recorded 
for Least Bell’s Vireo and White-eyed Vireos 
