Pennsylvania Game News - July 2020 - 63

low as 5 feet off the ground, and no higher
than 15 feet, yellow-throated vireos build
their nests in the crowns of the tallest trees,
20 to 50 feet up, their nesting habits difficult for researchers to observe.
It takes the pair about eight days
to construct what ornithologist Arthur
Cleveland Bent, in his Life Histories of
North American Wagtails, Shrikes, Vireos,
and Their Allies, described back in 1950
as "the handsomest nests of any of the
vireos, ... the whole body of the nest is
almost completely covered with small
bits of variously colored tree lichens, all
held securely in place by numerous fine
strands of spider silk, the deep cup, with
its thick walls and incurving rim above it,
is neatly lined with fine grass tops..." and
attached to a forked limb.
Then Bent refers to the three to five,
creamy or pinkish white eggs with a
scattering of dark reddish to brown and
black spots on their larger ends as "the
handsomest and most heavily marked of
any of the eggs of the vireos."
Both parents incubate those eggs,
although the female covers them at night
and half the day while the male alternates
with his mate during the day. They vigorously protest, chase and sometimes attack
any jays or crows that venture near their
nest, but the only predators ever recorded
on yellow-throated vireos and their
offspring are long-eared owls, Cooper's
hawks and blue jays.
Like other vireos, they contend with
brown-headed cowbird parasitism and
usually accept those eggs with their
own and feed the nestling cowbirds that
compete with their remaining nestlings.
Researchers have found that in nests
without cowbirds, there are at least three
vireo young, but in those with cowbirds,
there is less than one vireo nestling.
The yellow-throated vireo eggs hatch
JULY 2020

into naked young in 13 days and take a
another 13 days to develop into feathered
fledglings that have been tended and
fed by both parents the usual insect and
spider fare, such as caterpillars, butterflies, moths, stinkbugs, scale insects, leaf
hoppers, beetles, flies and bees, most of
which they forage from the interior parts
of leaves and branches in the middle and
upper levels of the forest canopy.
The parents continue feeding the
fledglings for at least two more weeks,
but the second week, each goes its separate ways with half the young, although
they may reunite at a later period for a
short time.
For instance, Mark, last July 24, found
a family group of six yellow-throated
vireos in an area of young black locusts
between the spruce grove and the pond.
But eventually they part, beginning
their slow migration south in late August.
Our latest record is from Oct. 17, but
more normally they leave around the
middle of September. During migration,
they use open woodlands and brushy
woodland understory and edges and are
not observed as often in the fall as they are
in the spring, even though both sexes still
display their bright yellow throats.
Some researchers have evidence to
suggest that yellow-throated vireos might
use a more-eastern route during fall
migration than in spring, and migrate in
a southeasterly direction.
That might explain why they are rarely
seen in Pennsylvania during fall migration, and then no more than one or two
birds in any one place.
Usually, they forage singly with mixedspecies flocks of wood-warblers, chickadees, titmice and other small songbirds.
We can only hope that the yellowthroated vireo and all its relatives continue
to thrive in their nesting grounds.
63

Pennsylvania Game News - July 2020

Table of Contents for the Digital Edition of Pennsylvania Game News - July 2020