A photograph shows the result rather than how it was obtained, which is why ethical practice in this field depends on norms rather than on anything visible.
Disturbance
The central concern and frequently underestimated.
Animals responding to human presence expend energy, abandon feeding, and in some cases abandon nests or young.
Which means an animal that appears to tolerate approach may be experiencing measurable stress, and behavioural signs of tolerance are unreliable indicators.
The costs are highest during breeding, during winter when energy reserves matter most, and for species already under pressure.
Nest photography
Particularly sensitive, and regulated in several jurisdictions.
Approaching a nest can cause abandonment, expose it to predators by creating a visible path, or draw attention to its location.
Which is why photographing certain species at the nest requires a licence in some countries, with conditions attached.
The general practice is to work from a distance with long lenses, or from established hides, and to leave if the birds show any disturbance.
Baiting
Using food to attract animals, which is contested rather than universally condemned.
The objections are that it alters natural behaviour, can habituate animals to humans with consequences for their safety, may cause aggregation that spreads disease, and can be nutritionally inappropriate.
Established practices — garden bird feeding, some managed hide operations — sit in a different position, and the line is genuinely debated.
Baiting predators specifically is widely regarded as unacceptable given the risk of habituation.
Captive and controlled subjects
Images of animals in game farms or captive facilities presented as wild.
Which is a disclosure issue rather than an animal welfare one directly, and it misleads audiences about how rare or approachable a species is.
Major competitions require disclosure and disqualify undisclosed captive images.
Location disclosure
Publishing precise locations of rare species can attract crowds or, in some cases, collectors and poachers.
Which is why many photographers withhold specific locations for sensitive species.
Geotagging in image metadata is easy to forget and can disclose a location unintentionally.
Manipulation
Post-processing is normal and the boundary is disclosure.
Adjusting exposure, contrast and colour is generally accepted.
Adding, removing or combining elements changes what the image documents, which matters for anything presented as a record.
Competition rules specify what is permitted and disqualifications for undisclosed manipulation occur regularly.
Drones
A newer issue with clear evidence behind the concern.
Studies have documented physiological and behavioural stress responses in wildlife to drone approach, sometimes without visible reaction.
Which has led to restrictions in many protected areas, and the regulations vary considerably by jurisdiction.
The practical standard
The welfare of the subject comes before the photograph, which is the principle every major code states.
Which in practice means longer lenses, more patience, accepting that some images will not be obtained, and leaving when an animal shows any sign of disturbance.
It also means the best images generally come from time spent rather than from proximity, which is fortunate.
Regulations
Legal requirements vary and are frequently stricter than photographers assume.
Disturbance of protected species is an offence in many jurisdictions regardless of intent, with specific provisions for breeding sites.
Licensing regimes exist for photographing certain species at the nest, and operating without one is an offence.
Which is worth checking before rather than after, since prosecutions occur.
Competitions and standards
Major competitions publish codes covering welfare, disclosure and manipulation.
Which has raised standards, since entrants must comply and disqualifications are publicised.
Submitting original files for verification is standard for finalists.
The audience effect
Images shape public understanding of what is normal and what is achievable.
Which means images obtained through close approach normalise close approach, and images of captive animals presented as wild misrepresent how accessible species are.
Photographers with large audiences have argued that this responsibility is a genuine part of the practice.
Practical fieldcraft
The techniques that produce good images without disturbance are learnable.
Understanding the animal's behaviour so that approach is unnecessary — knowing where it will be and waiting.
Using natural cover and existing structures.
Approaching indirectly rather than straight toward the animal, and stopping at any sign of alertness.
And returning repeatedly, since animals habituate to a predictable non-threatening presence over time.
Working from hides
Permanent and portable hides allow close observation without the animal responding to a person.
Which is the technique behind a large proportion of the best images, and it requires patience rather than equipment.
Sharing responsibly
Including how an image was obtained, when it is unusual, informs the audience and sets expectations.
Which is increasingly common practice and it costs nothing.
Which also builds the audience understanding that makes conservation arguments land, and that is arguably the field contribution beyond the individual images.
Several photographers work explicitly on this basis, treating the documentation as the point rather than the aesthetics.
Which is a reasonable position to arrive at and it is not the only defensible one.