The Short Answer: Melon Is Flock-Safe
Yes — watermelon is safe for chickens, flesh, seeds and rind alike, and in hot weather it earns its place as one of the most useful treats you can carry out to the run. The flesh is mostly water with a little natural sugar, the small soft seeds are ground in the gizzard like any other small seed a hen picks up, and the rind gives the flock something firm to work at long after the sweet centre has gone. No part of a fresh, sound watermelon is a problem for a healthy hen.
What the Melon Actually Gives Them
Hydration is the headline. Hens drink steadily, but on a genuinely hot afternoon a flock working through a melon half takes in fluid and feed at the same time, and birds panting in the shade will often eat melon when they will not touch their pellets. The flesh also carries a modest spread of vitamins, and the pecking itself matters: a melon half is a good stretch of occupation for a flock in a run, which is time not spent feather-pecking each other out of boredom.
Serving It So Every Hen Gets Some
Cut the melon into halves or thick quarters and set the pieces cut-side up on the ground, on a board or in a low tray. In a flock with a strict pecking order, put out one piece per handful of hens and space them well apart, so the shy birds at the bottom of the order feed at their own piece instead of waiting for scraps from the bold ones. If you cube the flesh instead, scatter it widely over clean ground for the same reason. Rinse a dusty rind before serving, and keep melon off bare mud, where it picks up droppings and grit as they work.
How Much, and How Often
Treat watermelon as the garnish, not the meal. A quarter of a medium melon is plenty for a small backyard flock in one sitting, offered a couple of times a week through a hot spell. Their layers' feed remains the diet — it carries the protein and calcium that eggs are made from — and the first melon day may produce looser, watery droppings simply because of the water load. That settles on its own once the melon stops; droppings that stay watery for days afterwards are a separate matter worth investigating.
Clearing Away: the One Real Rule
Melon spoils fast in heat. Whatever is left sitting in the run by evening is drawing flies and wasps and starting to ferment, so lift leftovers away within a few hours, and never serve melon that smells sour, looks slimy or has gone mouldy in storage. Skip the treat for a hen who is off her feed and under the vet's care, and check with the vet before extras resume for her. Frozen chunks of melon are a fair variation on the hardest afternoons — let them soften for a few minutes so beaks can get into them.
Writing 'melon on hot afternoons' into KinsLe's Care Teams as an assigned duty means the flock's summer treat still happens on the days you are away, given by whoever holds that duty.