A significant differentiator between birding lenses today is the ability to apply  real-time image stabilization, or IS.  Different manufacturers have their own, proprietary implementations of IS, with different names, but they all seek to improve image sharpness by reducing the effects of camera/lens shake.
    The figure below illustrates the effect of image stabilization at slow shutter speeds.  Using a 560mm lens wide open at f/5.6, I shot this highly cooperative House Owl (species: Owlus plasticus) with IS turned on (left image) and with IS turned off (right image).  Both photos were taken hand-held, without a tripod and with nothing to support the lens but my arms.




At this size it’s difficult to see much difference, though if you look at the feathers on the bird’s breast, you can see that the image on the left looks sharper.  In the image below I’ve zoomed in on the bird’s left eye, and also applied some sharpening in Photoshop (the same amount to both images).  The IS version (left) clearly has less blur than the non-IS version (right).  If you look at the white eye ridge at the 2 o’clock position above and right of the eye in both versions, you can see that in the IS version (left) there is some fine detail that is largely obliterated in the non-IS version (right).
The lack of sharpness in the non-IS version of the image above is due to my inability to hold the camera and lens perfectly still while taking the shot, resulting in slight image movement while the shutter was open.  That slight movement resulted in the light rays from each point on the bird being scattered over a small area on the image sensor, rather than striking a single point on the sensor.  This smearing of detail information results in image blur, which even a sharpening filter in Photoshop can’t fix.
    Note that, for the non-IS image above, I resisted the temptation to exaggerate the problem by intentionally shaking the camera slightly while taking the shot.  Because I’ve spent many hundreds of hours shooting with this lens, hand-held, I’ve gotten quite good at holding it very, very steadily.  Even so, at 1/40 sec it’s very difficult to get tack-sharp images with any non-IS lens, when shooting hand-held.  Although I very rarely shoot at speeds as low as 1/40 sec, for crepuscular subjects (meaning: active often at dawn and dusk) like the one above, without flash there’s little choice but to shoot at low shutter speeds.  Note that for the photo above I opened my lens’ iris all the way to f/5.6 (the maximum aperture of this lens) and cranked the ISO up to a whopping 3200, and still wasn’t able to get my shutter speed up past 1/40 sec without underexposing my subject.  It’s in precisely these situations that IS becomes a real godsend (especially if you don’t have a tripod with you).
    It’s important to note that IS becomes more useful the more you zoom in on your subject.  That means that lenses with larger focal lengths have a correspondingly greater need for IS.  And as illustrated by Figure 3.5.2, above, it also means that the more aggressively you crop your image (i.e., the more “digital zooming” you apply in Photoshop), the more you’ll notice the effects of camera shake, and the more you’ll wish your lens (or camera) had IS.  The reason larger magnification factors are more susceptible to camera shake is due to what’s known as optical leveraging, as illustrated by the figure below.



The idea of optical leveraging is simple.  Imagine a see-saw in which one end of the see-saw is longer than the other (see the figure above).  Then if you move one end of the see saw by a fixed amount, say 12 inches, then the other end of the see-saw will move more than 12 inches, because that arm of the see-saw is longer.  In the figure above, the three see-saws differ in the length of their right-hand arms.  If we shake the left-hand end of all three see-saws by the same amount, the right-hand ends shake by different amounts, and the longer the right-hand arm of the see-saw, the larger the shake.  In an optical system, this corresponds to greater image blur, because if the scene is moving while the shutter is open, light rays from individual points on the bird won’t collect at individual pixels on the imaging sensor, but will instead be smeared out over a certain radius on the sensor.
    In practice, I find that for shutter speeds of 1/200 sec or faster, I typically don’t need IS.  That’s true both when I’m shooting at 400mm hand-held, and when I’m shooting at 800mm tripod-mounted.  Because of the effects of optical leveraging, even tripod-mounted setups can benefit from IS at low enough shutter speeds.  If the wind is blowing (or there’s an earthquake or nearby vocanic eruption), your need for IS will increase, especially for large lenses that offer a larger surface to catch the wind.  Also note that while flash photography in dimly lit environments can sometimes make IS unnecessary even at extremely slow shutter speeds (because the flash’s 1/50,000 sec pulse freezes the bird), this doesn’t work in well-lit scenes, since the ambient light contributes a non-negligible amount to the overall exposure, resulting in visible image blur.




    Although I highly recommend buying a lens (or camera) that has some form of IS built into it, it’s worth noting here that there are situations where IS can be an impediment.  First, some IS implementations will actually increase image blur when used on a tripod.  The newest lenses from Canon have fixed this problem, because they can detect when the camera is tripod-mounted and will adjust the operation of the IS accordingly.  Similarly, for birds in flight, Canon and Nikon stabilizers have an optional panning mode (“mode 2” on Canon lenses) that you can manually enable, since standard (non-panning) IS tends to work poorly when panning, and may actually cause more image blur.
    Second, because IS can slow down the initial focus acquisition by the autofocus system, for restless birds like warblers it’s typically best to turn IS off at high enough shutter speeds (say, 1/200 or 1/300 sec or faster).  Otherwise, you may find that just as soon as the camera has acquired focus, the bird has moved to a new location and you need to re-acquire focus.  With IS turned off in these situations, you may gain a fraction of a second on the bird, which can be just enough to take the shot.
    As mentioned previously, different companies have their own implementation of IS.  Nikon calls theirs VR (“vibration reduction”), Sigma calls theirs OS (“optical stabilization”), and Tamron calls theirs VC (“vibration compensation”), while Canons refers to theirs as IS.  More important than the names are the differences in the way these systems work.  The biggest difference is the one between in-the-lens IS and in-the-camera IS.  In the case of in-the-lens IS, the lens contains a small, highly sensitive gyroscope, which can detect tiny movements in the lens housing.  When movements are detected, the lens counteracts these by rapidly moving an extra group of optical elements in the opposite direction, so as to keep the image stationary.  As with all types of IS, there’s a limit as to how much compensation can be applied, so that large amounts of camera shake will still cause image blur.
    In the case of in-the-camera IS, there are two popular options: digital IS, or mechanical IS.  Mechanical, in-the-camera IS again uses a gyroscope to detect small movements in the camera, but then compensates for these by moving the imaging sensor.  Digital IS, on the other hand, doesn’t move anything in the camera or lens, but simply detects image shake using digital image analysis circuitry and then shifts the image (digitally, via built-in software in the camera) in real-time.  Digital IS is mainly used in video cameras. 
    In DSLR’s, mechanical, in-the-camera IS is touted as a cheaper alternative to in-the-lens IS, because you only pay for the IS once—when buying the camera.  Once you’ve got a camera with in-body IS, you can use IS at any time, no matter what lens is attached.  With in-the-lens IS, you only get IS if the lens you’re using has IS built-in, so you end up paying for IS many times—once for each lens that you buy.  Conventional wisdom says, however, that in-the-lens IS should be more effective than in-body IS, and this claim is somewhat supported by the fact that the two biggest camera makers (Canon and Nikon) use only in-the-lens IS, though these companies obviously have a vested interest in lens-based IS, since they can make more money that way.  Rigorous comparisons between in-lens and in-body IS are very hard to find, and existing comparisons on the internet are somewhat contradictory.  While lens-based IS has been around for at least 10 years, in-body IS is still under active development by various camera manufacturers, so the final verdict will probably not be in for a few years.
    A few quick, additional notes on IS are in order.  First, while IS can be very effective at eliminating or reducing the effects of “camera shake”, it does nothing whatsoever to compensate for “bird shake”—that is, IS can’t reduce motion blur due to movement of the subject, only movement of the camera. 
    Second, the effectiveness of a particular IS system at counteracting camera shake is typically measured in “stops” of IS.  Although we’ll defer a complete discussion of stops of exposure to Chapter 6, we’ll just mention here that a stop of IS is equivalent to a doubling or halving of the shutter speed.  Thus, if you can normally get tack-sharp images at 1/200 sec with your particular lens and shooting technique, then with a 3-stop IS system you should (in theory) be able to get similarly sharp images at 1/25 sec, since 1/200 × 2³ = 1/25.  Canon’s newest crop of IS-enabled super-telephoto lenses feature a 4-stop image stabilizer.
    Finally, note that in-the-lens IS tends to drain your camera’s battery fairly quickly.  These are mechanical systems which require lots of electricity—much more than is required to open a shutter, for example.  If you spend hours with your finger on the shutter release button (meaning that the IS gyroscope is kept spinning the whole time) while waiting for those rare occurrences where an owl or eagle or other stationary subject strikes an interesting or unusual pose, you can expect your camera’s battery level to drop quite a bit (though how much it drops is highly dependent on the type of battery in your camera, since some cameras have much better batteries than others).