INCREASE IN BIRD ACTIVITY ELICITS QUESTIONS
March 28, 2010
noticeably more active in warmer weather, leading to the following questions.
around our yard are changing color. I know that the males will soon become
bright yellow. Does the color of the feathers change, or do the birds
grow new feathers?
A: Many species
of birds take on breeding plumage much brighter than their drab fall and
winter look. The answer to the question about feather colors is virtually
the same for all of them, although the timing of the process is variable
among species. The short answer is that the color of a feather is set
until the feather is lost. Feathers are lost and replaced through molting,
and for some species, like goldfinches, the new feathers that appear at
the beginning of the breeding season are a brighter color. This prenuptial
molt leads to the beautiful yellow plumage with the contrasting black
wing bars displayed by male goldfinches in the summer. Sometime after
nesting is complete, goldfinches molt again, replacing the colorful feathers
with the more basic colors we see the rest of the year.
that take on a new appearance with breeding plumage after a prenuptial
molt are male summer tanagers, which become bright red; indigo buntings,
which become bright blue; and various warblers, which become more brightly
colored. Once a feather is in place for any of them, it stays the same
color until molting occurs. Injured feathers can be replaced singly, but
most finches, tanagers, and warblers have a prebreeding molt that involves
almost all of the feathers that have color.
Q: I have
heard of a bird that puts a snake skin in its nest. Is this true, and
if so, what is the purpose?
A: The great
crested flycatcher is noted for placing a shed snake skin in its nest
and leaving it there until the young have fledged. Other birds, including
tufted titmice and blue grosbeaks, have been reported to do the same on
occasion. Flycatchers, which nest in tree holes (or bird boxes), will
often drape a snake skin on the outside of the nest cavity, as well as
weaving others into the nest itself.
was reported in the scientific literature as early as the late 1800s,
and ornithologists speculated that the snake skin is a deterrent to would-be
egg predators that might themselves become prey to snakes. But establishing
which predators might be deterred and whether the strategy really works
has been difficult. Nonetheless, Elizabeth Medlin and Tom Risch of Arkansas
State University conducted a convincing study that identifies a predatory
culprit and suggests that the use of a snake skin as a greeting card is
an effective deterrent.
of animals that encompass most of the geographic nesting range of the
great crested flycatcher are rat snakes and flying squirrels. Flying squirrels
will eat bird eggs, and rat snakes will prey on flying squirrels. A rat
snake would also eat a bird egg, but flying squirrels are probably a greater
threat because of their voracious mammalian appetites and greater agility
in getting from one nest cavity to another. Medlin and Risch performed
a field experiment by putting up 60 nest boxes and placing quail eggs
in each nest, as well as a simulated egg of modeling clay. The eggs served
as indicators of whether the nest had been preyed upon. For some predators
the clay egg registered tooth marks. The researchers placed a snake skin
in the nest in 20 of the boxes; they placed one in the nest and one outside
the nest in 20 more; 20 nest boxes had no snake skin.
were straightforward enough to support the contention that snake skins
deter flying squirrels from entering a bird nest and eating the eggs.
None of the 40 nests with snake skins were attacked. Of the nests without
snake skins, 20 percent had eggs eaten by flying squirrels. Despite the
nationwide interest in bird-watching that has persisted for well over
a century, many ecological mysteries about birds remain to be solved.
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