Wildlife is most active at dawn and dusk, which is also when there is least light to photograph it. Every technical decision in wildlife photography follows from that conflict.
The three variables and their costs
Exposure depends on shutter speed, aperture and sensitivity, and in dim conditions each can be adjusted only at a price.
A slower shutter admits more light and blurs a moving animal, while a wider aperture admits more light and reduces the depth that stays sharp.
Raising sensitivity brightens the image at the cost of noise and reduced tonal separation, which shows most in shadow areas and smooth backgrounds.
Why long lenses make it harder
Telephoto lenses gather light over a narrow angle, and the fast versions are large, heavy and expensive because a wide aperture at long focal length requires a very large front element.
Longer focal lengths also magnify camera movement, so the shutter speed needed to avoid shake rises as the lens gets longer.
Stabilisation systems counter camera movement but not subject movement, so they help with a perched owl and not with a bird in flight.
Where sensor size enters
Larger sensors collect more light per pixel at a given resolution, which is why full-frame cameras generally produce cleaner images in dim conditions.
Smaller sensors give greater apparent reach for the same lens, which is genuinely useful for distant subjects and costs image quality when light falls.
Neither choice is universally correct, and the decision usually comes down to whether a photographer is more often limited by reach or by light.
What has actually improved
Sensor noise performance has advanced substantially over the past decade, and images at sensitivities once considered unusable are now routinely publishable.
Noise reduction applied after capture has improved further still, particularly methods trained to distinguish detail from random variation.
Autofocus in low light has improved in parallel, which matters because a noisy sharp image is usable and a clean unfocused one is not.
Working with the limitation
Many photographers deliberately use slow shutter speeds to render motion as blur, which suits birds in flight and running mammals and turns the constraint into a subject.
Silhouettes exploit the same conditions differently, exposing for the sky and reducing the animal to outline, which needs no shadow detail at all.
Both approaches accept that the light will not support a conventional image, which is more productive than pushing settings until the result is technically sound and visually flat.