Yes, tracking small birds is very difficult.  That is
why you see fewer small bird flight images than large
bird flight images.  Smaller birds fly faster than
larger birds (a google search should find a plot that
proves this--I have seen such plots).  Of course there
are exceptions like a falcon diving.

The problem of tracking a fast moving object is the
angular velocity.  For a given flight velocity, like
1 meter per second, if the bird is close by, the angle
changes fast.  At greater distance, the angular rate
is less.  So the trick is photographing at greater
distances, which means you need a longer focal length lens.
That is why bird photographers tend to use super telephoto
lenses.

Roger
Some bird flight images:
http://www.clarkvision.com/galleries/gallery.bird

Birds will often put on the brakes or even hover before landing.
Particularly if the perch is moving or awkward (eg. a tree when
it is very windy).

A bird searching for food or trying to catch an insect on the wing will
sometimes hover or fly quite slowly. I've seen sparrows follow a moth
down to the ground with almost no forward movement and fantails will
make short hopping flights trying to stir up bugs in fallen leaves.

A bird flying repeatedly to a known landing site would allow you to
anticipate better and perhaps catch a good shot. One obvious example is
a nest though you'd probably want more than 200mm to avoid disturbance.

Just a couple of ideas that may be worthless in your circumstances.

A few flying birds:

http://www.pbase.com/mapleglen/onthewing

Rob.

It's tough, no doubt about it.  Here are some tips, based more on my
theories than actual successes since I also struggle with small birds
in-flight and fail most of the time ...

Faster aperture speeds up AF ... I think a one stop faster lens
focusses twice as fast, other things being equal.

Some camera bodies have much faster AF systems than others ... for
Canon users the 1D Mark II and MII N are the fastest, there's a 4 Mpix
Nikon that's really fast and I think the D2x is really fast as well.
If you ever shoot a 1D Mark II and then pick up a 10D or 20D you'll
really notice how slow the AF is on the consumer grade bodies.

A lens with a longer min focus distance will AF faster than one with
closer min focus distance because the lens doesn't have to hunt as
long.  An example is the Canon 400 f/5.6 L, which won't focus very
close so is faster than other Canon lenses that focus closer.

Some brands have fast focus lenses (Canon's USM for example) and slow
focus lenses (non-USM lenses) ... Nikon has the same deal though I
don't know their terminology.  Get the fast focus ones to better your
chances.  As an example the Canon 100-400 IS has USM and focusses
noticeably faster than the Nikon 80-400 VR, which lacks the Nikon
equivalent of USM.

If you know the path of the bird try to pre-focus as close as possible
to the line you think it will take so the lens doesn't have to hunt as
much.

You can either go 'one shot' mode and lock a particular distance or 'AI
Servo' mode (Canon terminology, sorry) and track the bird continuously.
Each mode works in certain situations so learn both and switch between
them as needed.

Try to set up with a clear background since the lens will lock focus
against a clear background much faster than against a "busy"
background.  For example, try for clear sky behind the bird instead of
a distant line of trees.

My main camera for birds lets me use one AF point or expand this by one
point or expand it by more points or expand it all 45 points ... the
more points the less precision but the greater chance of catching focus
on a small target ... play with this if your camera allows you to
expand the AF points.

If the lens has a limiter switch to reduce the AF distance then set it
to the tightest range possible.  For example on my favorite lens I can
AF from 30 ft to infinity, 15 ft to infinity or 15-30 ft ... I'm always
changing this to try to get the right range (if I had $10 for every
time I forgot to change it back and missed a later shot I could buy a
new car though :).

Rob Davison's comments were good, as were his shots ...

Here are some shots of mine to illustrate some of these ideas (or not).

http://members.aol.com/bhilton665/desert/rufous_U8507.htm and a couple
of other pages on the same site ... this guy was hovering near a
feeder, similar to what Rob discusses ... I was able to get him with a
very long lens (500 f/4 with 1.4x t/c) by setting the focus range to
15-30 ft because it would lock on him fast ... I also tried a faster
lens, a 70-200 f/2.8 L, and could get really close to the birds, but
couldn't AF as fast as with the longer lens because there is no narrow
range on that lens, ie, it was 4 ft to infinity (or whatever it is)
instead of 4 - 8 ft, so if I missed focus a bit it would 'hunt' and I'd
miss the shot as the bird flew off.  So here a longer, slower lens was
actually better for the job than a shorter, faster lens because I could
set a narrow distance range ...

http://members.aol.com/hiltonfotography/pribilofs/puff_flight.htm
http://members.aol.com/hiltonfotography/pribilofs/puff_fish.htm
These guys are really tough to shoot in flight, I got them with 45
point mode and a fast focussing lens and body.  I was with 5 other
people, who had Nikon D100's and Canon 10D's (been a couple of years),
and they were not able to catch ANY of these guys in focus.  With my
gear I was able to nail them about 20-30% of the time.

http://members.aol.com/bhilton665/tanzania_2006/barbet_T0449.htm (2nd
image)
http://members.aol.com/bhilton665/tanzania_rainy_2006/birds_8.htm (2nd,
3rd images)
More hovering ... in both cases I focussed on something else, with the
bee-eaters I was locked on the guy on the branch ... these birds spot a
bee or fly, zip off to snag it mid-air and return, sometimes hovering,
so I was set up for him (and lucky the eyes are on the same plane).
The Whydah bird is dancing in mid-air to court the female and I had the
500 with a 2x converter, so pretty slow AF, so I focussed on the female
instead and the male was in focus when on the same plane as her.

So get a fast lens, fast camera, learn when to use the different modes
and have some fun.

Bill

I've had some luck keeping both eyes open; tracking the bird with the
naked eye, and centering him in the viewfinder with the other.  It's
hard to do and requres practice, but does seem to work under some
conditions.   Try it, but don't blame me if you wind up with a headache
or worse (permanently cross-eyed!) ;)

-Gerg