I had a juvenile black-headed grosbeak hit a window yesterday. I heard
a thumping and commotion on my front porch, and when I went outside
there was a mystery "crime scene" in which the grosbeak was near death
on a part of the porch away from the window, and a Doug squirrel was
hanging off the railing staring at it. After the bird had expired, I
was inspecting its wings and body feathers and two Hippoboscid flies
jumped out and onto its breast. I have handled many many birds during
banding and putting up museum skins and have never encountered these
flies before! They looked like sheep keds (now those are gross) with
wings. When disturbed, they flew off the body, but quickly landed again
and scuttled beneath the feathers. I just found it fascinating that
flying parasites live on birds. They have a very interesting life
history (see below). I should have probably have collected the flies,
but I didn't.

"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."

p.s. I do have UV stickers--a bird strike is very unusual at my house.
I suspect a Cooper's hawk may have taken a dash at the feeder.

Here's a little website with photos of the flies.
http://www.hiltonpond.org/ThisWeek051108.html

Angela Percival
Olympia, WA
Angela at Stillwatersci dot com