With wildlife, 300mm is considered the minimum focal length you want (if we're really talking wildlife out in the wild, and not backyard semi-tame beasties that aren't shy of humans). 400mm is typically considered a minimum for birds. So, it does in some respects depend on what wildlife you're most typically stalking, in terms of how large and how shy they are. Smaller, shyer animals require longer lenses to fill a frame. And paramount over all this talk of glass, remember that fieldcraft is probably going to be far more important. Your ability to get closer to wildlife without disturbing them is a learned skill and some folks have more talent (or drive to practice) to gain this skill than others (go google "hide" and "ghillie suit"). So again, what focal lengths you'll need rely a great deal on your fieldcraft capabilities. Most of the stunning wildlife images you see were obtained with a lot of sweat, time, patience, and expense--don't expect that picking up a 70-300mm lens is all you need to do to get wildlife shots. But you probably can't expect to get them without one. :)

As a Nikon D3000 shooter, your choices of lenses over 300mm will mostly boil down to either Nikon or Sigma (supposing you have a four-figure budget). If your budget is below $1000, chances are good you can only afford a 70-300mm zoom, and all you have to worry about is whether or not you want to go Nikon or 3rd party. Your main issue, of course, will be finding a lens with AF-S so you'll have autofocus.

Most of these lenses do not let you shoot Macro as well, despite a number of 3rd-party 70-300 lenses being labelled as "macro" lenses. While you can get relatively close, they don't do the true 1:1 magnification of a dedicated macro lens, and if you plan on shooting very small subjects, like bugs or small flowers, chances are good you'll prefer a dedicated macro lens instead.

So, the main lens specs you'll probably be concerned with (aside from cost) are:

• Focal length. As I stated above, 300mm on a crop body is good for larger or less-shy wildlife. But if you're thinking about small birds that aren't in the backyard, then 400mm or longer is more likely to be what you want. And the cost leap will be considerable. Say, from $600 to $1000 (third-party) or $2000 (OEM).

• Image quality. Sharpness, contrast, and distortion will probably be the main concerns. With longer lenses, you typically don't worry much about vignetting or C/A. Zooms may be slightly softer than primes, but not always, if you can afford the four-figure price tags.

• Maximum aperture. Wildlife moves fast, and often in low light. But this is going to be the most expensive thing you can go for. f/4 300mm lenses are astronomical. You don't want to see the prices for f/4 at 400mm. Or f/2.8. Think five figures. You may not have a choice of going with f/5.6 or even f/6.3 with third-party lenses for this. But because wildlife tends to be active in the golden hours, and because the longer focal lengths require faster shutter speeds to mitigate camera shake blur, a larger maximum aperture will help widen the possibilities. The 1/focal_length rule of thumb doesn't really restrict you much at 50mm, but at 500mm, it's something completely different.

• Autofocus speed. No point in having a great, sharp supertelephoto to shoot flitting birds if it can't autofocus fast enough to grab the shot before the bird flies off. However, given that you have a D3000--you're going to have to get an AF-S lens anyway just to have autofocus. But pay attention to this in reviews.

• Stabilization. If you plan to handhold while you stalk critters (once you go past 400mm, chances are good you have to shoot with a monopod or tripod, simply from the weight), stabilization can help a lot with the shutter speed restrictions the longer focal length will impose upon you. VR, for longer lenses, is worth it.

• Focus lock. Some of the longer lenses also have a "focus lock" that allows you to tell the lens to ignore a certain portion of the autofocus range in order to speed up focus locking. This can be invaluable if you tend to shoot things that move fast and are always beyond your reach (i.e., birds in flight).

• Size/Weight. The longer or faster the lens, the bigger and heavier it gets. Some folks claim they can handhold a Sigma 150-500 OS, some folks cannot. But handholding telephotos and supertelephotos does require a certain amount of technique and care that shorter lenses do not. There are special holds you can use (e.g., the machine gun hold, where you place your left hand on your right shoulder, and rest the lens on your left elbow), as well as a plethora of other ways (beanbags, monopods, tripods, resting on fence rails, your knee), etc. to help with stabilization.

You may also want to see the following lensrentals.com videos about supertelephoto lenses to get a sense of what the really big guns are like:

• Comparing Nikon Supertelephoto Lenses
• Comparing Canon Supertelephoto Lenses
• Supporting Supertelephoto Lenses