Tele prime lenses, tele zoom lenses, or ultra zoom lenses, with a reach of about 400mm (full frame equivalent), or longer, are "long lenses" that can generally be called favorable and appropriate, for shooting still images and videos of birds and wildlife. For photographing small wary birds, 600mm (full frame equivalent), or longer, is generally recommended, if you want to fill up a substantial part of the frame with the main subject.

The quality of image stabilization has gradually improved among "long lenses", released during the latest two decades. The odds of successfully getting both sharp photographs and video recordings often depeds on image stabilization. Some mirrorless cameras have IBIS (In-Body Image Stabilization) inside the camera, that may sometimes be able to cooperate with, or replace, the image stabilization of a compatible "long lens", depending on the situation.

A set-up with a modern image stabilizated "long lens", paired with a suitable modern camera, is very good at neutralizing most normal hand held camera shake/vibration blur, letting the photographer use a slower shutterspeed, corresponding to between 4 to 8 stops of light, unless the subject moves. Panning image stabilization may be used when tracking and shooting subjects that move in a predictable/guessable manner, blurring the background with motion blur, while maintaining sharpness of a chosen steadily moving part of the subject in focus. Older image stabilized "long lenses", usually, compensate for less camera shake/vibration, letting the photographer use a slower shutterspeed, corresponding to about 2 to 3 stops of light. "Long lenses" without any image stabilization can still be used, but these lenses, generally, have a difficult time competing with image stabilized "long lenses" on the market, unless you have IBIS on the camera, unless you mount, and/or rest the lens, on something, like a good tripod, or something similar, to steady the lens and camera. Using a remote control to trigger the shutter may also help to reduce camera shake.

Improved autofocus, especially bird/eye tracking AI autofocus in mirrorless cameras, combined with improvements in general image quality, faster shooting framerate, more buffer memory, focus peaking, pre capture function, better memory cards and various other improvements also help to make it easier to capture special moments, for photographers willing to buy modern cameras and other modern equipment. Photographing sharp images of birds in flight has become much less difficult, compared to a decade ago, if you use suitable modern equipment. Recently a camera (Sony A9 III) with a global shutter was introduced. A global shutter is something that many photographers want. I hope and expect there will be more cameras using global shutters in the future.

If your sneaking/hiding skills are excellent and you have good patience/perseverance, and/or your living subjects are not very wary of you, you may be able get great photos using a lens with a shorter focal length. If you aim for an environment/landscape/group style photo that includes living bird/wildlife subjects, but it doesn't require you to fill the frame with the subject like a tight close-up, you may, perhaps, also do fine without any "long lenses" for that kind of photo.

Using a 300mm (full frame equivalent), or shorter, tele lens to photograph birds and wildlife, is possible. However, it may test your sneaking/hiding skills, by trial and error. You will be training/honing your skills, to avoid scaring/disturbing the birds and wildlife, making you expend more effort and you may miss some potential photo opportunities, compared to using a longer lens. Some photographers deliberately use a 300mm lens, instead of a longer lens, to train their sneaking skills on common local birds, and/or other common local wildlife subjects, that are readily available, to be better prepared for special occasions, when going further away from home to encounter rare birds and other rare wildlife.

Practicing your sneaking/hiding skills may help you to avoid becoming a "lazy" photographer. However, when capturing close-up portraits, and/or documenting specific natural behaviours, while trying to minimize disturbing your living subjects, a longer lens of 400mm, or longer, is generally convenient and advantageous. If the subject is very wary of you, you may still benefit from using your best sneaking/hiding skills even at 400mm, or longer, focal length. Any experience you may have gained, from practicing by using a shorter tele lens to photograph less wary subjects, will occasionally still be very useful when shooting with a longer focal length.

Using a "long lens" is, usually, recommended for safety reasons, when photographing dangerous wild animals. It may allow you to keep a more respectful distance and thereby avoiding stepping into the immediate danger zone, which is preferable, unless you are an adrenaline junkie, and/or a professional photographer, aiming for unique high risk "money shots" at a closer distance, and/or if there is a lot of heat haze, or rain, or something else in the way, on a particular occasion, demoting "long lenses" down to a backup choise.

Focal length is not the only important aspect when choosing a long lens. You may also consider other factors, for example: Camera compatibility, price, size, apertures, bokeh, sharpness, total weight, weight distribution, focus (manual/autofocus), optical stabilization options (vibration reduction, optical image stabilization, panoration stabilization), chromatic aberration reduction, weather sealing, closest closeup distance, sound noise, ergonomics, choosing either a prime a lens, or internal/external zoom lens etc.

Usually, you may need to "sacrifice" some level of image quality, and/or low light capabilities, as a compromise, if your budget constraints, and/or other practical demands, are not compatible with expensive/heavy/large long lens options, that otherwise may provide a slightly higher potential for the very best image quality. For example, even if you are a strong person, a heavy high quality long lens is often not practical to carry, and/or handhold, for extended periods of time, forcing you to use a tripod, or something similar, while your muscles may still get sore from carrying the heavy lens and other equipment. A light long lens, with modern image stabilization, may let you experience more enjoyment with less fatigue to yourself, making it easier to travel, go on hikes and shoot hand held for extended periods of time.

Some photographers own several different "long lenses". They may choose to take only one, or a few, appropriate lenses with them during a photoshoot/hike. Wildlife photographers may leave some of their lenses at home/basecamp/car/hotell/safe, according to the situation. Sometimes, it may also be a good idea to rent an appropriate "long lens" for a special occasion.

Aside from long lenses that originally have a reach of 400mm (full frame equivalent), or more, it is also possible to sometimes achive above 400mm (full frame equivalent) by adding a teleconverter (usually 1.4x, or 2.0x), to a slightly shorter tele lens (for example a 300mm lens), and/or using a camera that has a crop factor (usually 1.5x, 1.6x, or 2.0x), and/or has a high resolution image sensor that produce images that can be cropped, to multiply the orginal focal lenght to achieve the desired full frame equivalent focal length, to compose an image. However, long lenses with a reach of 400mm (full frame equivalent), or more, are often (but not always) also compatible with teleconverters. Teleconverters are, usually, best suited to be combined with very sharp lenses, such as most prime lenses and some compatible high quality zoom lenses, especially if the subject is relatively slow/static in the frame, and/or moves in a predictable way.

Trying to use a teleconverter on a long lens of low quality, usually, doesn't yeld enough significant improvement to make it worth using it, compared to cropping and upscaling in post processing. This is because using a teleconverter may add various other drawbacks, compared to using the same long lens without the teleconverter. Such drawbacks include high risk of dust getting in your camera, taking time and effort to adding/removing the teleconverter, less sharpness, less ease of framing the subject, f-stop limitations, worsened image stabilization problems, auto focus problems, chromatic abberations, vinjetting, DOF choices gets limited. (However, a specific depth of field range may be either a disadvantage, or an advantage, depending on what you prefer.)

If you want to use a tele converter, or not, is kind of like a balancing act, because there are both advantages and disadvanages to using a tele converter, depending on your circumstances. Occasionally, you may also want to add an extension tube (or a stack of several extension tubes), if you want to photograph subjects at a close distance, closer than the minimal focusing distance of your lens, an/or add a macro converter (closeup filter).

"Long lenses" with zooming capability have gradually become better on the market. Depending upon the situation, it may be possible to get image quality close to, or similar to, a comparable prime lens, using a modern zoom lens. There may still be benifits using a prime lens, but zoom lenses also have advantages. It depends on your preferences what you prefer. If you choose a zoom lens, it is a good idea to test it at a few different settings at different focal lengths. Zoom lenses may have some issues with softness near the extreme long end of their focal range, but the sharpness may improve if you zoom back slightly, and/or use a slightly narrower aperture.

Afocal photography through a spotting scope made for bird watching, or through a telescope made for astronomy viewing, can be called digiscoping (if a digital camera is used), or phonescoping (if a smartphone is used). In a pinch, digiscoping and phonescoping can also be done through a barrel of a pair of binoculars. Digiscoping was popular during a few years, from when the concept was first introduced in 1999. Since then, technological development regarding relatively affordable long lenses, with image stabilization (both for panning and multidirectional vibration reduction), dedicated to photography and videography, has made digiscoping generally less favourable among photographers and videographers. However, digiscoping and phonescoping may still be an option depending on your circumstances, especially if you already have a smartphone with a suitable camera, and/or own (or can borrow) a spotting scope etc. If you are a birder and mainly want some pictures to ID the birds, but isn't too hung up on nitpicking image quality, then digiscoping, and/or phonescoping, may suit your conditions.

An alternative way of photographing birds and wildlife, or sports, is by setting up stationary cameras in appropriate places. The stationary cameras are either triggered remotely (by remote control/smartphone/laptop/tablet etc.), or they can be triggered automatically by using motion/sound/heat sensors, and/or timers etc. These stationary cameras may sometimes make use of night vision settings (for example trail cameras and surveillance cameras), to capture pictures and videoclips, or alternatively they may use prepared flashlight set-ups, to capture pictures in otherwise less than ideal light conditions.

Another alternative way of photographing birds and wildlife, or sports, is by using a camera (usually an action camera) mounted on remote controlled flying drones, or on remote controlled model cars/planes/boats/helicopters, or on real size vehicles, or on living animals/humans etc.