here is a list of possible indications, posted on uk gov website:
"Signs of bird flu
Birds infected with the most serious strain of bird flu, called highly pathogenic avian influenza (HPAI), usually show some (or all) of the following signs:
sudden death
swollen head
closed and runny eyes
lethargy and depression
lying down and unresponsiveness
lack of coordination
eating less than usual
lethargy
sudden increase or decrease in water consumption
head and body shaking
drooping of the wings
dragging of legs
twisting of the head and neck
swelling and blue discolouration of comb and wattles
haemorrhages and redness on shanks of the legs and under the skin of the neck
breathing difficulties such as gaping (mouth breathing), nasal snicking (coughing sound), sneezing, gurgling or rattling
fever or noticeable increase in body temperature
discoloured or loose watery droppings
stop or significant drop in egg production.
Some species such as ducks, geese and swans can carry the avian influenza virus and spread it without showing any signs of illness.
Birds infected with the less serious strain of bird flu, called low pathogenic avian influenza (LPAI) may not show clear signs of infection. They may have mild breathing problems.
These signs can indicate bird flu, but the avian influenza virus can only be confirmed through laboratory tests."
https://www.gov.uk/guidance/avian-influenza-bird-flu#latest-situation
