I recently skipped rocks across a small frozen lake, it made such an
interesting sound, almost like a bird chirping.  What is the physical
mechanism behind this phenomenon?  My theory is this; the sound is
produced by the surface ice vibrating at its resonant frequency like a
drumhead.  Could someone please confirm or correct me?

Short answer: it's possible, but I doubt it. The mass of the ice
should make the natural frequency of the ice sheet very low, so the
'resonant' sound would be more of  a "thunk" then a chirp.

Were there any structures nearby (specifically any metal or chain-link
fences)? If may have heard an interaction between the sound of the
rock hitting the ice, and the reflected sound bouncing/diffracting off
a nearby object.

There was no man-made object anywhere near the lake.  I am 99% certain
the sound was from the ice.  the other strange thing is that the
chirps would persist until the rock came to a complete stop.  even a
rock spinning but otherwise not moving on the surface of the ice would
make a chirping sound.  it was quite something.  the size of the lake
was not large, perhaps 200 yards in diameter.

For those in the Seattle area, this happened at round lake and the
other larger lake near it (forget the name) on Tiger Mountain in
Issaquah.  Go up there and try it yourself, it's amazing!

I experienced exactly the same thing on a very small loch (about 30
metres diameter)in Scotland on Christmas Day. Sounded a bit like a
metal sheet being wobbled but more high pitched. There certainly
weren't any man-made structures around. Even once the stupid dog had
jumped through the ice after a rock, we could throw another rock and
it would still resonate.

I've got to agree it is a wonderful noise. I did it once, also on a
Scottish loch. I was 'skipping' chestnuts found on the ground beside
the loch and what added tremendously to the fun was the ducks trying
to reach the nuts, they were literally doing back-flipson the ice. It
was hysterical. Try it!

we've occassionally heard this on our lakes (Orcas Island, WA), but
only when the ice is quite thick (at least 5-6 inches). It couldn't
possibly be sound reflecting off other objects!  It's really a
phenomena; one of the rare pleasures of winter . . .

ts

Lovely comments from others.

It?s just a rather pleasant resonance -- bit like railway lines
singing when you are about to be mashed by 500 tons of train. But for
a really cool sound on ice, try outdoor curling. Not only do you get
the sibilant singing but also deep rumbling and the occasional
sickening 3D crack darting across the loch. Oh happy times.

Best

My late father-in-law, who was Canadian, once told me that he loved
the chirping sound caused by a hockey stick striking the ice of a
frozen lake. He said it reminded him of whales' songs.

The hockey stick sound is mentioned here:

http://lists.nau.edu/cgi-bin/wa?A2=ind9609&L=phys-l&F=&S=&P=616

"There was no man-made object anywhere near the lake.  I am 99% certain
the sound was from the ice. "

It's definitely not simple resonant behavior, not in the sense that
you described it ('drumhead'). It could be dispersive behavior alluded
to in the comment above. Dispersion refers to the fact that different
types of sound waves can exist in solids, each wave moves in different
directions at different speeds. When several of these waves are
created at the same time, the interact with each other, and can create
chirping effects.

(this is a simplified explanation)

If the ice was coupled with another object, say corrugated pipe
(drainage or culvert), the ice may have excited it, causing the chirp.

I am a telecommunications technician (Ex-British army), and also a
live sound-reinforcement engineer (soundman & recording engineer for
bands etc.)Yesterday, My wife and I were walking across St.Albans Bay
(Lake Champlain, VT.,)when we experienced the same phenomenon,
although the conditions and sounds were slightly different, (there was
no stone-throwing, curling stones or hockey sticks.) the sounds we
were hearing were very deep, almost sub-sonic "whale-song" sounds.
Obviously, as someone who is very interested in the propogation of
sound through different media, this fascinated me. After a little
confusion, (we couldn't immediately see a reason for the noises, and
being out on the Ice, at least 100ft from the shore, we were very
certain that the noises were coming from the ice itself, although
there seemed to be no discernable movement of the ice,) we finally saw
the cause of these strange sounds - the vehicles travelling on the
ice! the bigger the vehicle, the louder the sound, (regardless of the
vehicle's distance from us!) some of these vehicles were over a mile
from us. We spent 2 beautiful sun-filled hours walking all around the
bay, all the while making the observation that the volume of the
sounds were very consistant, as were the frequencies, (approximately
15 - 35 hertz, with some mid-high frequency harmonics, {BIG birds
chirping!?}) during this time, we asked some ice-fishermen roughly how
thick the ice was, the standard reply being 12-18 inches. I am
assuming from what I learnt on the ice, and from reading the other
questions on this site, that the characteristics of this phenomenon
are as follows:
1) The Baseline (I don't mean Bass-line, although the Baseline freqs.
were in the Bass range!) frequencies were a product of the size of the
body of water and the thickness of the ice. (The "Drum-Shell" &
"Drum-Head" effect.{I must assume that the sounds DO travel through
the UN-frozen water too!})
2) the frequencies AND strengths of the various harmonics of the
Baseline frequency seem to me to be dependent on many factors, for
instance the shape, complexity and slope gradient of the shore-line.
(A combination of the "Dispersion" effect, and basic Acoustics {think
room shape & size x surface textures & densities = reverberation/echo
quality of said room.})
3) the volume of the Baseline frequency in relation to the volume of
the harmonics seems to be a function of the "amount" of flex of the
ice at differing locations, that is to say, how big the vehicle is,
(the drum-stick!), and where it is positioned on the lake, (where the
drum-stick hits the drum-head.) In other words, ( as Musicians would
say, "The Dynamics of the drum-kit AND the Drummer.")
4)ALL of the above is dependant on ambient temperature, barometric
pressure, humidity and vantage-point.
and that's about all I've got!!! (I am also assuming that the lake has
near infinite "Baffle" properties( pretty close to a sealed "Drum" )
which would result in a slight accentuation of the HIGH frequencies,
(unlike I.B. speaker enclosures, the media we are talking about are
water and ice, the densities of which propogate differently to
"thin-air.")

P.s. I don't think that it could be JUST dispersion effect - Holistics
prevents this, assuming that the ice is touching other media, for this
reason it can't be simple resonance either!!( the water would surely
dampen the natural high frequencies.) I think that NEITHER theory, on
it's own, could explain this phenomenon exclusively. I think that the
"shear-tension" of the ice probably plays a small part too, ( bend a
thin piece of wood - it creaks, bend a thick peice of wood, it
groans.)