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HOME: www.hiltonpond.org

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THIS WEEK at HILTON POND
8-14 November 2005
Installment #292---Visitor #

(Back to Preceding Week; on to Next Week)

UPCOMING OPPORTUNITY:
Join us for another
Winter Hummingbird Expedition to Costa Rica
in February 2006





"STEALTH" FLIES ON BIRDS:
THE HIPPOBOSCIDS

Looking at birds through the window is a fun thing to do, and each year tens of
millions of Americans invest--incredibly enough--billions of dollars on feeders,
bird seed, and associated paraphernalia. People spend this money because they
admire birds for their colors, their songs, their ability to fly through the
air, and because they like to bring these feathered wonders in for closer view.
Folks can discover a great deal from simply observing all these backyard birds
with binoculars, camera, or spotting scope, but we can learn only so much from
such encounters; there are times when having a living bird in-the-hand reveals
far more. Such was the case recently at Hilton Pond Center when an Eastern
Towhee we captured gave us interesting insight into its relationship with a
stealthy little insect most birders never get a chance to see.

Early one morning, the towhee in question blundered into a mist net we deploy to
catch wild birds for banding. From our office window in the Center's old
farmhouse we saw the bird hit the net and immediately went outside to retrieve
it. The towhee was a young hatch-year male just completing his post-juvenal (AKA
"first pre-basic") molt; that is, he had traded nearly all his dingy juvenal
plumage for jet-black body feathers that are quite different from the rich brown
of towhee females. (See John James Audubon's comparative rendering above right
of male and female Eastern Towhees, formerly called "Rufous-sided.") We removed
the bird gently from the net and carried him inside to the banding table for a
closer look.



All text & photos © Hilton Pond Center

As we bent over our notebook and tools, an insect suddenly sprang from the
towhee's plumage and flew directly into our left eyebrow. From experience we
knew this must be a hippoboscid fly, so we made a quick grab for a small
collecting jar we keep on the desk. When we plunked the vial over our eye, the
fly flew in, and we slid on the lid to capture it. From side view (above) we
could see the quarter-inch-long insect was very flat from top to bottom with
long spindly legs, a configuration that reminds us of the compressed shape and
landing gear of the U.S. Air Force F-117 "stealth fighter" (below left). Could
it be the creators of the F-117 were part time entomologists who incorporated
the hippoboscid fly's unusual shape into their own aeronautical design?

F-117s depend on their radical configuration and engineering components to sneak
past enemy radar, while the shape of our newly captured fly serves a slightly
different function. Members of the Hippoboscidae (Louse Fly or Flat Fly Family)
are ectoparasites that dine on blood of birds and mammals. Some are wingless
adult forms called "keds" that infest deer, sheep, and other hoofed creatures
and have economic importance. Those that occur on birds, however, usually
have--like their hosts--a single pair of functional wings throughout their lives
and are "insignificant" to humans. (There are a few records of hippoboscids
biting people, but these are probably accidental occurrences when the parasite
was simply "testing the waters.") In any case, the flatness of flat flies allows
them to slide between feathers or fur to get at the epidermis of the host, where
they plunge a hypodermic-like mouthpart through the skin to suck up succulent
blood meals.

All text & photos © Hilton Pond Center

The mouthpart, called a "palp," is normally held flat against the fly's body and
is hidden by other mouthparts, but when our captive hippoboscid died its muscles
relaxed and the needle-shaped structure unfolded for viewing, as shown in the
photo above. (Also see lateral view near top of page.) Although the fly's death
was unexpected, it was opportune; taking close-up photos of a hyperactive
hippoboscid while it's alive is next to impossible.

As often is the case with parasites, many avian hippoboscid flies apparently are
host-specific; i.e., different species of birds host different species of
hippoboscids. We once netted a Red-shouldered Hawk with a "huge" hippoboscid
crawling among its feathers; the parasite was almost half an inch long, while
the smaller one pictured here was half that size. We've never encountered a
hippoboscid on a hummingbird but have heard from other banders who have; they
comment that a hummer hippoboscid is amazingly tiny--certainly to be expected
since a large parasite might overwhelm a host hummingbird. Along similar lines,
in most cases a single bird of any species seems to carry only a few
hippoboscids; "excess" flies apparently disperse, even though there are records
of an individual bird plagued by a hundred or more flat flies.

All text & photos © Hilton Pond Center

In top view (above), our flat-bodied hippoboscid from the towhee looks much like
any typical member of the Diptera (Fly Order), with large compound eyes, short
bristle-like antennae, and two wings held flat against the body when at rest.
(The left wing of the fly above is torn, something that must have happened
during the handling process; otherwise, the fly probably couldn't have made it
from the towhee to our eyebrow.) In hippoboscids, wing veins are heavy but few,
and to us the abdomen seems quite short compared to the rest of the body,
although it may have shriveled somewhat in the dead specimen. The hippoboscid
foot (right) is also nicely adapted to its feather- or fur-clinging lifestyle,
with two prominent tarsal claws that provide a sure grip.

After avian hippoboscids mate--which happens only once in many species--the
female never lays eggs. Instead a single ovum hatches internally and just one
larva at a time develops within the mother's uterus, nurtured by secretions from
special glands. The growing larva molts and goes through a total of three instar
stages, after which it emerges from the mother fly and usually falls to the
ground to form a pupa; when it expupates, the fly's initial task is to find a
bird somewhere and take a long, warm drink of blood. Curiously, most hippoboscid
species produce only one generation per year, which means flat flies have one of
the lowest reproductive rates of any insect group. Again, this may be a
mechanism that prevents a closely related hippoboscid family from wiping out its
host. Hippoboscids--especially females--are apparently rather long-lived; by
inhabiting a warm-blooded bird with lots of insulating feathers, a
temperate-zone hippoboscid is not affected by seasonal cold weather that is the
bane of many small arthropods.

Based on wing venation and head configuration, the hippoboscid from our Eastern
Towhee is apparently a species of Ornithomya, likely the relatively common O.
fringillina (formerly O. anchineuria). Interestingly, this fly species has been
implicated as a possible vector in the spread of West Nile Virus (WNV).
Epidemiologists have reported infestations of O. anchineuria and Icosta
americana (another common hippoboscid) on owls and other birds that died and
tested positive for WNV. The hippoboscids also tested positive for WNV, but it
is not known whether the virus was in the blood that flies had ingested from the
host or if the virus had actually infected the flies themselves.

All text & photos © Hilton Pond Center

Testing hippoboscid blood isn't easy, since it is indeed hard to separate the
blood meal in its gut from the insect's own blood. In the ventral view of the
hippoboscid above, the green matter at the leg bases and joints is blood, or
hemolymph. Insect blood contains no red pigment; when you swat a mosquito and it
splats red, that's YOUR blood, not the skeeter's. Devoid of hemoglobin, an
insect's blood does not carry gases, which instead are transported throughout
its body by a branched system of tiny tubes called trachae. Like human blood,
insect blood carries nutrients and hormones to the insect's cells and waste
products away from cells. The greenish color of hippoboscid blood in the photo
above apparently comes from plant pigments eaten by the fly's avian host.

One other interesting aspect of hippoboscid life history deals with "phoresy," a
symbiotic relationship in which a non-parasitic organism hitches a ride on
another. When a hippoboscid from a bird is examined microscopically, we often
find tiny chewing lice (ischnocerans) attached to the fly's abdomen. These
wingless lice don't dine on the fly but are merely using this larger winged
parasite as a way to get to a bird--which the lice DO bite. Chewing lice
typically have narrow bodies that allow them to hide between a bird's feather
shafts and avoid being dislodged when the bird takes flight or preens.

Dr. Barry OConnor of the University of Michigan further informs us that
"Hippoboscids are also hosts to hyperparasitic mites in the Epidermoptidae. Some
of these mites have a bizarre life-cycle modification where immatures and males
are parasites on the avian host like other epidermoptids, but adult females are
hyperparasites on hippoboscids. [Hyperparasites are, simply stated, parasitic on
parasites.] Females are white and rounded (they swell quite a bit on fly
hemolymph) and are typically seen near the end of the abdomen. They use highly
modified front legs to embed in the fly cuticle."

With this information in mind, we used a 10X macro lens to check the abdomen of
our dead hippoboscid fly for mites and found a barely macroscopic yellow object
(above left) that Dr. OConnor says definitely is NOT a epidermoptid mite. It
might, however, be a syringophilid mite--they're long & yellow and about that
size--but live inside feather quills; syringophilid associations with
hippoboscids are not well-documented. (On the other hand, the yellow object
might just be a piece of tiny fly trash, but 10X magnification is the best we
could do without a microscope.)

Like we said, looking at birds through your window is entertaining and
informative, but not until you get a bird in the hand and a ectoparasitic fly in
your eyebrow does it open up a whole 'nother aspect of avian ecology--from
hyperparasites to phoretic chewing lice to hippoboscid stealth flies on Eastern
Towhee hosts.

All text & photos © Hilton Pond Center

Thanks to Barry OConnor (Univ. Of Michigan), Peter Adler (Clemson Univ.), Jeff
Boettner (Univ. Of Massachusetts), Russ Rogers, and Paul Beuk for sharing
hippoboscid info



--------------------------------------------------------------------------------

Comments or questions about this week's installment?
Please send an E-mail message to INFO.

Be sure to scroll down for an account of all birds banded or recaptured during
the week, plus other nature notes of interest.

--------------------------------------------------------------------------------

"This Week at Hilton Pond" is written & photographed
by Bill Hilton Jr., executive director of
Hilton Pond Center for Piedmont Natural History.

You may wish to consult our Index of all nature topics covered since February
2000. You can also use the on-line Search Engine at the bottom of this page.

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Oct 15 to Mar 15:
Please report
your sightings of
Vagrant & Winter
Hummingbirds


BIRDS BANDED THIS WEEK
at HILTON POND CENTER
8-14 November 2005

SPECIES BANDED THIS WEEK:
Golden-crowned Kinglet--1 *
Yellow-rumped Warbler--5
Chipping Sparrow--1
Dark-eyed Junco--1
Northern Cardinal--6
White-throated Sparrow--2

* = New species for 2005

--------------------------------------------------------------------------------

WEEKLY BANDING TOTAL
6 species
16 individuals

--------------------------------------------------------------------------------

YEARLY BANDING TOTAL (2005)
59 species
1,261 individuals

--------------------------------------------------------------------------------

BANDING GRAND TOTAL
(since 28 June 1982)
124 species
46,568 individuals

--------------------------------------------------------------------------------

NOTABLE RECAPTURES THIS WEEK
(with original banding date, sex, and current age)
American Goldfinch (1)
04/24/04--after second year female

--------------------------------------------------------------------------------

This Week at Hilton Pond
is part of the



OTHER SIGHTINGS OF INTEREST
--The drought continues at Hilton Pond Center and across the Carolina Piedmont,
with another week gone by without rain. On top of that, on two days this week we
set record highs--on 11 Nov the mercury reached 82 degrees and eclipsed the
previous high of 80 set in 1975.

--Several phone callers to the Center this week have lamented the absence of
birds at their backyard feeders. In our experience it's not unusual to have such
a lull in early November, after the summer birds have gone and before the winter
birds settle arrive in good numbers. The phenomenon seems especially noticeable
in years such as this when temperatures are unusually warm and the birds are out
gobbling down nuts, berries, and still-active insects. Why visit a seed-laden
feeder when all these natural foods are available?

All text & photos © Hilton Pond Center









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