Jim Nestler wrote:

"Yellow-rumps eat suet at feeders (but not at my place). But what are
these little guys naturally eating around here ("here" being the Pacific
Northwest or Cascadia or whatever Tweeters decided to call it)? "

Jim, Whenever in doubt, go to the "Bible" of A.C. Bent's Life Histories.
In Part 1 of his 'Life Histories of North American Wood Warblers'
(1953), for the Pacific Audubon's Warbler (Dendroica auduboni
auduboni) he quotes:

"Food. - Professor Beal (1907) examined the stomachs of 383
Audubon's warblers in California from July to May, inclusive. The
food consisted of 85 percent of animal matter (insects and spiders)
and a little more than 15 percent of vegetable matter. The largest
item was Hymenoptera, 26 percent, consisting mostly of ants, with
some wasps, and a few parasitic species. Diptera accounted for 16
percent, including house flies, crane-flies, and gnats, many of which
must have been caught on the wing, as this warbler is a good
flycatcher. Bugs, Hemiptera, amounted to nearly 20 percent of the
food, including the black olive scale, other scales, plant-lice, stink
bugs, leaf-hoppers and tree-hoppers. "Plant lice (Aphididae) were
contained in 39 stomachs, and from the number eaten appear to be
favorite food. Several stomachs were entirely filled with them, and
the stomachs in which they were found contained an average of 71
percent in each." Caterpillars amounted to nearly 14 percent and
beetles more than 6 percent of the food; most of the beetles were
injurious species. Other insects and spiders made up about 2
percent.

"The vegetable food consisted of fruits, mostly wild and of no value,
less than 5 percent, and seeds, over 9 percent, mostly weed seeds
and seeds of the poison oak. These warblers have been known to
puncture grapes and they probably eat some late fruit, but they do
very little damage to cultivated fruits and berries. C.S. Sharp (1903)
observed a flock of 200 birds, mostly Audubon's warblers, greedily
eating the raisins in the tray shed of his packing house; they had to
be constantly driven away. Mrs. Amelia S. Allen says in her notes
that they collect in great flocks in the live oaks to feed on the oak
worms in the spring, and that they eat myrica berries in the fall.
John G. Tyler (1913) says: "Audubon warblers share with Say
Phoebes the habit of catching flies from a window, sometimes
becoming so engrossed in this occupation as to cling for several
seconds to the screen where a south-facing window offers a
bountiful supply of this kind of food."

Not exactly the "Northwest", but Jewett et al. (1953) Birds of
Washington, tells us:

"Insects furnish a large item in its food, and the Audubon is adept
in their capture on the wing, making frequent erratic sallies into the
air like a flycatcher. But between times, instead of resting as a fly-
catcher would do, it continues active, always searching for susten-
ance. Occasionally an individual attaches itself to a group of other
small birds, as Nashville warblers or chestnut-backed chickadees.
In the spring of 1920 it was found at Copalis feeding on willow buds."


Curson et al. "Warblers of the Americas" (1994) notes for Audubon's
warblers that it "Feeds mainly on insects (much more so than 'Myrtle'),
but takes some fruit and berries in winter; sometimes visits feeders.
Feeds at all levels (including the ground) but especially high up,
gleaning and flycatching."

I have watched the wintering birds quite a bit down in the Willamette
Valley of Oregon. One of the best places to find flocks of this species
is to find where a farmer has dumped a pile of peppermint pulp onto
a field and hasn't yet spread it out. The piles sit and compost all winter
and the 'butter-butts' hunt around them with a vengance. I've always
presumed that they were after insects & spiders attracted to the heat
of the composting mint, but they might be after weed seeds as well?

Hope this helps.

Jon. Anderson
Olympia, Washington
festuca at olywa.net