Could they have gotten into bad food? Moldy feed? Compost? Rotting vegetation? You may want to consider botulism, which occurs more in warm weather. Symptoms include limp neck and paralysis of feet, legs, etc.
A few links: https://transact.nt.gov.au/ebiz/dbi...8166171BB86BF6A69256EFE004F670B/$file/792.pdf
http://www.merckmanuals.com/vet/pou...view_of_botulism_in_poultry.html?qt=&sc=&alt=
http://www.helium.com/items/1387367-poultry-diseases-botulism
A few links: https://transact.nt.gov.au/ebiz/dbi...8166171BB86BF6A69256EFE004F670B/$file/792.pdf
http://www.merckmanuals.com/vet/pou...view_of_botulism_in_poultry.html?qt=&sc=&alt=
http://www.helium.com/items/1387367-poultry-diseases-botulism