Wait. I think I hear a Summer Tanager calling from that shrub over there.
No, it sounds more like a Great Crested Flycatcher. But, in a shrub?? No
I think I'm hearing a Rufous-sided Towhee!? Is this a strange flock? Hold
it, I see a bird that's small and olive, with some yellow below... . It's
a White-eyed Vireo singing away!
Despite being a secretive and modest bird, the White-eyed Vireo has many
fascinating aspects of its behavior. For example, the White-eyed Vireo is
a superb vocal mimic, incorporating the calls of many species in its song.
Exactly why it does this remains a controversial topic and an ornithological
mystery.
Although subdued in color, the White-eyed Vireo is handsome. The grayish
olive upperparts are set off by yellow sides, and the luminescent white
irises contrast with bright yellow goggles. They sport neat white and yellow
wing bars and tasteful blue-gray legs. Our favorite part of the vireo's
appearance is rarely observed by birders in the field; the White-eyed Vireo
is one of the few birds that has a jet black mouth lining.
White-eyed Vireos breed commonly from the eastern United States through
northern Mexico. Most U.S. populations migrate, whereas birds in the south
coastal plains through the gulf coast of Mexico remain resident. Migratory
White-eyed Vireos can be found throughout the Caribbean Basin during the
winter, but probably most White-eyed vireos winter on the Yucatan Peninsula,
where it is one of the most abundant species.
In the north, White-eyeds are common in scrubby habitats. One can only imagine
how well they did after the eastern forests were cleared for farms, then
abandoned to second growth. Perhaps because of continued forest regeneration,
White-eyeds may be declining in abundance in some areas - but they remain
a common bird of southern shrublands.
The White-eyed Vireo is one in a complex of 9 vireo species found in Mexico,
Central America and the West Indies. The White-eyeds closest competitor
is the resident "Mangrove" Vireo - an abundant bird of secondary
scrub on the Yucatan Peninsula. For the most part, these two species coexist
peacefully except when it comes to feeding on the fruits of the Gumbo Limbo
tree (Bursera simaruba). At this tree White-eyeds exert their dominance.
The two species separate, to a degree, by habitat. White-eyed Vireos are
most common in dry forest. When they are found in second growth or agricultural
lands, they are often associated with small patches of trees. However, rather
than forest or forest patches, per se, it is possible that White-eyed Vireo
distribution is tied to that of its beloved Gumbo Limbo tree. In more humid
tropical areas, such as Chiapas and Veracruz, White-eyeds are found primarily
in scrub, riparian woods and coffee plantations.
White-eyeds are among the most consistently territorial migratory birds
during the winter. Both sexes defend small territories from October - April.
Also unusual feature is their habit of singing throughout the winter months.
In addition, White-eyeds give a staccato chatter-burst which sounds more
like a wren then anything else.
White-eyed Vireos are planters of the tropical dry forest. Gumbo Limbo trees
are one of the most common forest trees. To a great degree, this tree relies
on resprouting from roots and suckers to reoccupy sites after hurricanes
or fires. However, to disperse into new areas, Gumbo Limbo trees need to
have their fruit consumed and the seeds regurgitated by birds. Most of the
Gumbo Limbo fruits are planted by White-eyed Vireos.
Each morning White-eyes visit their Gumbo Limbo trees and search the thousands
of fruits for the few that are ripe on a given morning. The search requires
keen eyesight since the greenish- or rose-colored capsules remain closed
and must be squeezed and popped open. Only capsules with ripe red fruit
inside will open and the only clue to ripeness is slight lines where the
three segments of the capsule cover converge. No wonder most frugivorous
birds pass by the Gumbo Limbo tree until late in the 6-month fruiting season
(March and April) when the remaining fruits ripen almost all at once.
In the temperate zone, White-eyed Vireos are abundant insectivores in early
successional habitats. Like other vireos, White-eyeds have short, stout
bills with a small hook. The fibers of their jaw muscles are arranged for
maximum force of bill closure, and the bill can be opened very wide at the
tip. Furthermore, vireos often use their feet when wrestling prey. These
features allow vireos to feed on large caterpillars and insects with hard
carapaces, as well as to husk tropical fruits.
This focus on eating fewer, larger insects probably explains the fundamental
behavioral difference between warblers and vireos with which most birders
are familiar. Vireos move deliberately, with a low frequency of long hops
and flights compared to warblers. Even this small vireo (11-12 g - about
the size of a Hooded Warbler), can be readily distinguished from a warbler
by its more sluggish movements.
In sum, the modest and deliberate White-eyed Vireo is a bird of many voices,
a prodigious consumer of insects, and a planter of tropical forest - traits
worthy of the SMBC bird of the month distinction.