Wildlife comes in all shapes and sizes. Wildlife has a wide range of tolerance for human presence. And wildlife photographers have a wide range of get-close skills (and motivation). Adding to the complication, wildlife images can be framed in a variety of ways ranging from tight head shots to full body portraits to wide environmental portraits.
I'll quickly get the environmental wildlife portrait lens recommendations out of the way - your general purpose lens should adequately handle this task.
For the more popular, more tightly framed wildlife portraits, the above mentioned issues combine to create a challenge in determining the best focal length for a wildlife lens. A kit of lenses in various focal lengths is the best option, but for one-lens kits, wildlife photographers usually need as much focal length as they can carry or can afford. It is usually easier to get farther away from wildlife than it is to get closer. Get too close and your subject is off to the next county. The farther away you can photograph your subject from, the more likely you will have time to get multiple natural shots of your wild subject. Longer focal lengths also make it easier to create a strong background blur - that makes your subject pop from the image.
I don't recall ever hearing a bird photographer complain about having too much focal length. Nor anyone photographing potentially dangerous game. The bull elk in rut featured in the sample picture above was captured with a 500mm lens.
The cost of going beyond 400mm (and retaining good image quality) is high, but a longer-than-400mm lens is usually desired for wildlife photography. APS-C/1.6x FOVCF DSLRs have a 1.6x narrower angle of view, making a 400mm lens frame like a 640mm lens. This of course is only an advantage if the pixel density of the sensor is higher than the full frame DSLR being compared to as the full frame image can be easily cropped to the 1.6x framing.
As I write this, most of Canon's current DSLR cameras with APS-C format sensors indeed have higher pixel density sensors than the full frame options. The downside is that APS-C cameras have lesser image quality including lesser image quality at the pixel level. While some opt for an APS-C body for their focal-length-limited wildlife needs, I have opted for getting closer and/or cropping. The choice is yours of course.
Wildlife, especially mammals, is often active early and late in the day - when light levels are at their lowest. Wildlife photography from a lighting perspective is usually best done during the golden hour at sunrise and sunset. This again can mean low light levels.
Wider apertures are great to have in low light - for blurring the background and especially for helping to stop subject motion. The stillness of the subject of course can vary greatly - even for the same subject. One of my favorite wildlife subjects, White-tailed Deer, will stand completely still while surveying for danger. But they can also run and leap. Some birds will stand still for a long time, but will move a lot when feeding. And some subjects remind me of the hyperactive squirrel "Hammy" in "Over the Hedge". Wide apertures will make capturing low light motion easier, but they are only usable if enough of your subject remains in the shallow depth of field remaining.
If your subject is not moving or is moving slowly, image stabilization can be a huge asset for low light wildlife photography. A tripod or other support can alternatively be used if this works with your photography scenario.
Wildlife does not mind rain or snow. Some lenses (and cameras) do. A weather-sealed camera and lens kit rules, but you will want to have rain/snow protection along with you if your gear is not sealed (and perhaps even if it is).
Wildlife also does not mind cold weather. I don't hear of too many issues about the cold affecting lenses, but you definitely want to avoid condensation when returning to a warm environment. Cameras can have issues with cold LCDs not working until warmed again, but keeping your batteries (many of them) warm is a primary issue when shooting in the cold.
I will start the wildlife lens recommendations list with the best of the best. Their prices reflect this. Your results will also reflect this. If you are shooting professionally - or you really care about the image quality of your wildlife photos, these are the lenses you need to be using. These are the investments you need to make. Buy or rent them.
Lenses at this quality level have historically held their value very well. You can likely recover much of your investment back when you are done using them. As prices increase, healthy profits can sometimes be made.
Read farther down the list for more economically friendly recommendations.
The Ultimate Canon wildlife Lenses - My Recommendations
Impressive Overall Performance
The 600 L IS II gets my highest recommendation for wildlife photography - when you can shoot from a tripod, monopod, or other support - or when you are only handholding for short periods of time. This is one of the larger, heavier and most costly lenses available, but it has a great long focal length, it has excellent image quality, it accurately focuses very fast and it is built for the abuse that can occur in the field.
Note that I have removed the Canon EF 800mm f/5.6 L IS USM Lens from my recommendations list. While this lens remains an excellent choice, the 600mm f/4 L IS II with the 1.4x extender mounted behind it has slightly better image quality, is slightly less expensive, is slightly lighter, has a slightly longer focal length, has a much shorter MFD and higher MM (0.21x vs. 0.14x - a big deal for small birds and animals) and of course has the option of being a 600mm f/4 lens. The 800 L is modestly smaller with the hood in place. I sold my 800 L after testing the 600 L IS II.
Impressive Performing Lens
The 500 f/4 L IS II is my wildlife lens of choice for handholding, for carrying for long periods of time and for airline travel. It is not as long in focal length as the 600mm f/4 L IS II, but the 500mm f/4 L IS II is otherwise a match in performance and build quality - and it is smaller, lighter and less expensive. The longer focal length 600mm lens will create a stronger background blur at the same aperture with similar subject framing. Like the other big white Canon L IS II lenses, this lens performs very well with extenders mounted behind it.
Incredible Image Quality, Extremely Useful Focal Length Range, Image Stabilization, Overall Impressive Lens
The 200-400 L is a fantastic lens choice for wildlife photography. While it does not have an aperture as wide as some of the prime lenses in this focal length range, it adds great versatility with its zoom focal length range. With the built-in extender, this 200-400mm native focal length range lens extends to 560mm (add external extenders for even more focal length). The 200-400 L's image quality is surpassed by few lenses.
The Longest Focal Length f/2.8 Lens
The 400 L IS II is the longest f/2.8 focal length lens available - except for Sigma's enormous and incredibly-high-priced Sigma 200-500mm f/2.8 EX DG IF Power Zoom Lens. If your wildlife photography is occurring in low light conditions, this may be the right lens for you. With the f/2.8 max aperture, this lens retains a reasonably wide aperture with extenders mounted. The image quality from this lens is very impressive as is the rest of this lens' performance.
Simply Awesome Lens
It is not as long in focal length as the 400mm f/2.8 L IS II, but the 300mm f/2.8 L IS II is at least a match in performance - and it is much smaller, lighter and less expensive. The 300mm focal length is going to be short for a lot of wildlife needs, but the image quality from this lens remains impressive with extenders mounted behind it. Add a 1.4x to get a 420mm f/4 IS lens or add a 2.0x to get an impressively compact and light 600mm f/5.6 IS lens that will autofocus on all Canon EOS DSLR cameras.
This lens is built for professional abuse including use in inclement weather.
Extends the Focal Length of Compatible Lenses
Multiply the focal length or focal length range of your current lens by 1.4 - and narrow the aperture by 1 stop. Those are the primary effects of mounting the 1.4x extender behind your extender-compatible lens. You will also see a small amount of image quality degradation and slightly decreased autofocus speed. The better the lens you start with, the better the with-extender results will be.
As a rule, but subject to change, the Canon lenses compatible with the Canon EF 1.4x III Extender include fixed focal length L lenses with focal lengths of 135mm and longer, zoom L lenses with at least 70mm of focal length on their wide end (the Canon EF 70-300mm f/4-5.6 L IS USM Lens excluded) and Canon TS-E Tilt-Shift lenses (though these are not included on Canon's official compatibility chart).
Significantly Extends the Focal Length of Compatible Lenses
Multiply the focal length or focal length range of your current lens by 2 - and narrow the aperture by 2 stops. Those are the effects of mounting the 2x extender behind your extender-compatible lens. Note that the 2-stops-narrower aperture means f/8 for an f/4 base lens. Many Canon DSLR cameras require an f/5.6 or wider max aperture to enable autofocusing.
The amount of image quality degradation and decreased autofocus speed is more noticeable with the 2x than with the 1.4x. Again, the better the lens you start with, the better the with-extender results will be - and I only recommend using the 2x behind the best Canon lenses.
Extender 2x compatibility is the same as Extender 1.4x compatibility.
More Affordable/Next-Best Canon wildlife Lenses - My Recommendations
Very Good Image Quality, Great Build Quality, Fast AF, Image Stabilization, Excellent Focal Length Range
The 100-400 L IS has the longest focal length available in a Canon zoom lens and is the only native 400mm f/5.6 lens with image stabilization. Both of these features are valuable for wildlife photography. The 100-400 L IS design is showing some age, the push-pull zoom mechanism is not preferred by all and this is not a small or light lens when compared to many of the shorter focal length telephoto zoom lenses, but this lens remains a top choice for wildlife photography.
Great Image Quality, Great Build Quality, Fast AF, Good Focal Length Range
The 70-300 L IS has become one of my favorite telephoto zoom lenses and I often use it for wildlife photography - especially when carrying a longer focal length lens in addition. APS-C format DSLR camera owners will find the 300mm focal length much better for a primary wildlife lens than those with full frame DSLRs. Fast Ring-USM AF, weather sealing and pro-grade build quality are additional attractions for the wildlife photographer.
Great Image Quality, Fast AF, Great Build Quality
The 400 f/5.6 L gives you a long focal length in a thin, light lens with fast autofocus and great image quality. Lacking image stabilization is the biggest drawback this lens has for the wildlife photographer. A tripod, monopod or other support will likely be needed when the light level is not ideal.
The 400 f/5.6 has slightly better image quality and is smaller/lighter than the 100-400 L, but the 100-400 L has IS and the versatility of the zoom range.
Great Image Quality, Fast AF, Great Build Quality
The 300mm f/4 L IS lens, especially with a 1.4x extender behind it (making it a 420mm f/5.6 IS lens), is a very popular choice for wildlife photography. This relatively small/thin/light lens deliver great image quality - even with the 1.4x mounted behind it.
The Ultimate Zoom Lens
The 70-200 f/2.8 L II IS Lens is an extremely impressive zoom lens, but most will find the 200mm focal length short for general wildlife use. The answer is to add extenders. With an extender mounted behind it, this lens still competes in image quality with some of Canon's longer zoom lenses. And it has the versatility of the f/2.8 max aperture.
Impressive Image Quality, Great Build Quality, Fast AF, Image Stabilization, Compact & Light, Great Value
This is a very impressive lens, though most will want to add a 1.4x extender to get the reach they want for wildlife.
Excellent Value, Very Good Image Quality, Pro-Grade Build Quality, Fast AF
If you can't afford any of the other wildlife lenses listed on this page, this is the lens for you. You should not expect to tightly frame small/distant subjects with a 200mm focal length, but this lens has fast and accurate autofocus along with very nice image quality. Add a 1.4x extender to increase the max focal length to 280mm. The downside is that the extender costs nearly as much as the lens.
This is my entry-level wildlife and all-around telephoto lens recommendation. You get a lot for your money.
The list above is not an exhaustive list of lenses that can be used for wildlife photography, but these are my top picks. Also visit the Canon Lenses page for other recommendations.