LaPergola el al. • NEST AND EGGS OF THE COZUMEL VIREO 
747 
based on (1) the presence of adult Cozumel Vireos 
emitting alarm or scold calls close to each nest 
during the first days we observed them empty, and 
(2) the highly developed state of nestlings (prima¬ 
ries and secondaries completely lacking sheaths) on 
the last day observed in nests #1 and #2. 
JBL and JGMH candled the eggs in nest #3 
on 8 July 2009 using a headlamp. Neither egg 
showed signs of development, but we returned 
both to the nest. One egg had broken before we 
checked the nest on 15 July 2009; we collected (he 
remaining intact egg but it broke in transit. 
DISCUSSION 
Our observations represent, to our knowledge, 
the first account of the nest, eggs, and breeding 
phenology of the Cozumel Vireo. Sample sizes 
are limited, but available data indicate that several 
aspects of the species' breeding biology are 
consistent with those of other 'eye-ring’ vireos. 
including other members of the same superspecies 
iAOU 1998) within the Vireo subgenus; White¬ 
eyed Vireo ( V. griseus) and Mangrove Vireo (V. 
pa Ileus). 
Cozumel Vireos, by suspending nests from 
forked branches of a small tree and by using plant 
fibers and spider silk for exterior walls and grass 
fibers for lining, are typical of most Vireonidae 
(Baicich and Harrison 1997). Incorporation of 
more varied construction materials by White-eyed 
Vireos. including use or rootlets and hair in the 
lining (Hopp et al. 1995), probably reflects more 
diverse contexts for nest-building across that 
sister species' wide geographic range. The only 
described nest of the Mangrove Vireo. from a 
highland area in Belize, differed by having a 
lining of pine needles (Figueroa and Albanese 
-903). a resource not available on Cozumel. By 
including prominent dead leaves in exterior walls. 
Cozumel Vireo nests especially resemble those of 
the island-endemic San Andres Vireo (V. car- 
ihaeus) (Barlow and Nash 1985). although lacking 
die dead leaves that hang from the bottom of San 
Andres nests. 
Measures of nest dimensions permit compari¬ 
sons with species for which data are available 
'Table I). Values from the three Cozumel Vireo 
nests encompass those for Mangrove Vireo with 
regard to four parameters, but front internal nest 
depth and narrowest internal diameter were larger 
tor the slightly heavier Cozumel species. Most 
Cozumel Vireo nest dimensions were greater than 
those for the considerably smaller San Andres 
Vireo, whereas nests of the Thick-billed Vireo (V. 
crussirostris), with body size roughly similar to 
that of Cozumel Vireo, appear to have smaller 
interior dimensions but larger exteriors. 
Precise data concerning canopy height on 
Cozumel are not available, but the rather low 
height of Cozumel Vireo nests (~ 1.5 m above 
ground) may reflect use of small shrubs and 
saplings (2-3 m tall) and not larger trees (up to 
15 m) that are present in our study area, as in most 
of Cozumel's forests (JBL. JEMG. and RLC. pers. 
obs.). The tendency to place nests relatively low' 
in vegetation is fairly consistent across other 'eye¬ 
ring' vireos (e.g.. < I m nest height in White¬ 
eyed Vireo; Hopp et al. 1995), including those on 
islands (Table I). although higher nest placement 
(mean height = 4.9 in) by Puerto Rican Vireos (V. 
latimeri) in montane habitat (Tossas 2008) 
suggests flexibility within the subgenus. Detection 
bias favoring low nests may complicate some 
comparisons. 
Cozumel Vireo clutch size appears to reflect 
broad latitudinal trends (Martin 1996), but its eggs 
may be comparatively large. Cozumel Vireos 
produce smaller dutches, at two to three eggs, 
than most temperate-zone congeners; for example, 
clutch size in White-eyed Vireos averages four 
eggs (Hopp el al. 1995). Cozumel Vireo clutch 
size approximates that of congeners breeding at 
similar latitudes (Thick-billed and Mangrove 
vireos), but exceeds that of the more southerly 
San Andres Vireo (Table I). The Cozumel Vireo 
egg we measured was longer than that of the 
similar-size Thick-billed Vireo and larger overall 
than eggs of the smaller Mangrove. San Andres, 
and White-eyed vitcos (Table 1; Hopp et al. 
1995). but more data are needed to definitively 
characterize egg dimensions of V. bairdi. 
Our limited observations regarding breeding 
success provide a preliminary basis for compar¬ 
isons. Hatching success (67%) in nest #1, which 
yielded our most detailed information, was lower 
than that of eggs in unparasitized Puerto Rican 
Vireo nests (86%; Woodworth 1997). Our sample 
suggested relatively high fledging success (67% 
of nests producing at least one fledgling). Other 
insular Vireo spp. may have higher rates of nest 
failure; for example. 63% of Puerto Rican Vireo 
nests were depredated (Tossas 2008). 
The observed nesting period of the Cozumel 
Vireo is somewhat shorter than other insular 'eye¬ 
ring' vireos, but the length of its incubation and 
nestling stages are more typical (Table 1). Limited 
