Random pictures of ducks.
RandomDuck API is a free Animals API that requires no API key, is served over HTTPS and does not support CORS, so it should be called from a server.
Details from random-d.uk.

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Random pictures of Shiba Inu, cats or birds
The RandomDuck API is provided by Random-d.uk (random-d.uk).
Yes. The RandomDuck API is listed in the Animals category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The RandomDuck API requires no authentication, so you can start making requests without signing up.
Not directly. The RandomDuck API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
Yes. The RandomDuck API is served over HTTPS, so it's safe to use in production.