Chicken unable to stand – keeps both legs straight

[aptenodytes]

Hi all, I'm new to the forum and have a couple of fairly serious chicken problems to ask about... They're separate questions so I'll ask them in separate posts.

One of our chickens, Polly, is unable to stand up today. She was fine yesterday, and last night when we closed up the coop she was on the perch with the rest of the flock. But this morning when we opened the door, she was lying on the ground beneath the perch, unable to stand up.
We thought she might have hurt herself jumping from the perch, but we can't see any injury. She seems to want to keep both legs outstretched, totally straight. I can't identify any difference between the legs, though if I try to stand her up, she generally falls to the right.
Trying to gently bend the leg joints, her knees seem fine. Both ankles are very stiff and straight, she doesn't want to let me bend them. But she doesn't wince or really react at all when I move either joint. There's no swelling.
Her feet seem fine, and she can put them flat on the ground with no problem when I hold her upright. She can take her weight on her legs, but just doesn't seem to want to bend them or be able to balance.

Other than the legs she seems fine. She occasionally tries getting to her feet by flapping her wings, but she falls over again.
If we keep her upright and offer her food, she eats eagerly.

Polly has always been a very small, light chicken. She's also always had a tendency to sort of fall asleep on her feet – she'll sometimes just stand there with her eyes shut. But she's also never shown any other signs of ill health, and certainly no problem standing or running around until this morning.

 

[azygous]

Has it been very hot where you live? This type of paralysis involving both legs can be a sign of heat sickness. Treat her with electrolytes with sugar in it. A cup of water with a teaspoon of sugar and a pinch of salt and baking soda. Or Gatoraid with sugar, not sugarless.

Another cause can be spoiled feed. Mold will produce such symptoms, as well as other toxins such as botulism. Also, petroleum distillates will produce such symptoms if it doesn't kill her outright. If she doesn't recover in a day following electrolytes, I would suspect toxin poisoning.

 

[aptenodytes]

Thanks for the reply @azygous

It's funny you mention the heat, because yes, it has been very hot for the last week (high 30s Celsius during the day) for the last week, but last night was actually the first cool (sub-20°C and breezy) night we've had, and today it's nowhere near as hot as it has been.

The good news is that in the hours since I posted this, Polly seems to have more or less fully recovered. She's now on her feet and eating. We've still got her inside for now, but she is more or less back to her usual self. She even flew up onto the windowsill and landed on her feet with no problem.
She doesn't seem to want to drink much, if anything – should I just let her drink if she wants to, or should I try giving her some sugar water with a syringe if she doesn't drink of her own accord?

 

[azygous]

She may still be experiencing effects of heat stroke, so make the sugar water electrolytes available as the only water source. That she' improving on her own does point to the heat as her issue. She needs her electrolyte balance restored even though the worst symptoms are improving.