The direct answer for a canary
It is not safe for a canary to eat food made for other kinds of pets beyond the odd accidental peck. A canary is a small finch with a diet refined over centuries of cage keeping: fine seeds it can hull with a delicate beak, leafy greens, and carefully timed extras. Food designed for a dog, cat, rabbit or large parrot answers none of that. The ingredients may look harmless because they are made of grains and meats, but the proportions, particle sizes and added nutrients are calculated for bodies nothing like a canary's.
Why the canary's diet is so particular
A canary weighs very little and eats tiny amounts, so every beakful carries more weight in its diet than the same bite would for a bigger animal. Dog and cat foods concentrate animal protein, fat and salt at levels suited to carnivores, and their vitamin additions are scaled for animals that eat bowlfuls, not teaspoons. Parrot mixes, which owners often assume are close enough, are built from large oily seeds a canary's beak handles poorly and its body does not need, and a canary filling up on sunflower simply displaces the fine seed blend that actually sustains it. Rabbit and guinea pig pellets bring yet another profile, heavy in fibre and fortified for mammals that graze all day.
The realistic risks of regular access
A canary that routinely snacks from other pets' bowls tends to drift out of condition in recognisable ways: it grows heavy and less active, its song shortens or stops outside the normal moult, and droppings loosen. Wet pet foods standing in bowls add bacterial risk, and a canary walking through a cat's dish picks contamination up on its feet and feathers as well as in its beak. There is also a simple physical hazard, because food pieces sized for larger animals can lodge or be swallowed awkwardly by a bird built for millet-sized seeds. None of this comes from one stolen crumb; it comes from access left unmanaged week after week.
If a canary has eaten another pet's food
Remove the canary gently back to its own cage, refresh its water, and offer its normal seed so its gut settles on familiar food. Over the following day, look for the changes that count in a finch: droppings that stay watery, a bird that stops singing and sits with feathers loosened, laboured breathing, or food interest disappearing altogether. Singing is a genuinely useful gauge in a male canary, since a healthy, comfortable bird sings and a unwell one goes quiet early. Contact an avian veterinarian if those signs appear, or immediately if the food eaten was wet, raw, mouldy or had been standing a long time.
Feeding a canary well in a multi-pet home
Keep the canary's cage and feeding station away from where dogs and cats are fed, and lift other bowls after mealtimes rather than leaving them down for grazing that the canary might join during flight time. The canary's own menu, a quality canary seed blend as the base, small daily greens, cuttlebone available for calcium and beak care, and egg food offered at breeding and moulting times rather than constantly, covers its needs far better than variety borrowed from other species. Store each pet's food separately and serve with its own scoop. For the moments when a canary has eaten something odd and you want a second head on it straight away, KinsLe's Kinny can help you sort what you saw into what matters and what does not.