Bird flu in chickens most visibly shows up as swelling and purple discoloration of the comb, wattles, and the area around the eyes. In severe cases, chickens may die suddenly with no warning signs at all. The specific symptoms depend on whether the virus is a high-pathogenicity or low-pathogenicity strain, but there are clear visual and behavioral cues that something is wrong.
Visible Changes to the Head and Legs
The most recognizable sign of highly pathogenic avian influenza (HPAI) is what happens to a chicken’s head. The comb, wattles, and eyelids swell noticeably, sometimes dramatically. The skin in these areas turns from its normal red to a deep purple or bluish color, a result of poor circulation and blood vessel damage caused by the virus. Swelling around the eyes can become severe enough that the bird can barely see. The hocks (the joint partway up the leg) may also swell, and the shanks and feet can develop a red, bruised appearance from bleeding under the skin.
Feathers often look ruffled and unkempt because the bird stops preening. A healthy chicken keeps its plumage tight and smooth, so persistently puffed-up feathers are an early visual red flag even before more dramatic symptoms appear.
Respiratory and Digestive Symptoms
Infected chickens frequently develop nasal discharge, coughing, and sneezing. You may notice open-mouth breathing, which looks like a chicken sitting with its beak open and straining to get air. This is distinct from panting on a hot day because the bird will be doing it regardless of temperature, often while sitting still and looking lethargic.
Diarrhea is another common sign. The droppings may become watery or change color. Combined with a loss of appetite, this leads to rapid weight loss and dehydration.
Behavioral and Neurological Signs
One of the earliest things you’ll notice is a sudden drop in energy. Chickens that normally run to greet you or scratch around the yard will sit quietly, sometimes with their heads tucked. They stop eating and drinking.
Neurological symptoms can develop as the virus progresses. Incoordination is the most frequently reported, where chickens stumble, walk in circles, or fall over. Some birds develop tremors or hold their heads in abnormal positions, twisted to one side or tilted backward. These signs indicate the virus has affected the nervous system and typically appear in the later stages of infection.
Many chickens with HPAI show no behavioral warning at all. Sudden death without any prior symptoms is one of the hallmark features of highly pathogenic strains. A flock owner may find multiple dead birds in the coop with no indication that anything was wrong the day before.
Changes in Egg Production
Egg production drops sharply in infected hens. Research on one avian influenza strain found that production fell from about 95% to roughly 60% within a week of infection. But the change isn’t just about quantity. The eggs themselves look different.
Shells become noticeably thinner, and many eggs come out soft-shelled or completely misshapen. Under magnification, infected hens’ eggshells show more pores, surface cracks, and rough, bumpy textures compared to normal eggs. The internal shell membrane also deteriorates, with the normally tight fiber layer becoming loose and fractured. These changes happen because the virus disrupts the hen’s ability to deposit calcium properly in the shell gland. If you’re suddenly finding eggs that feel fragile, look wrinkled, or collapse when you pick them up, that’s a significant warning sign, especially if multiple hens are affected at once.
Low-Pathogenicity vs. High-Pathogenicity Strains
Not all bird flu looks the same. Low-pathogenicity avian influenza (LPAI) causes mild illness or sometimes no visible symptoms at all. You might see slightly ruffled feathers and a modest dip in egg production, but the birds generally recover. LPAI can circulate through a flock without causing obvious distress, which makes it harder to detect without testing.
High-pathogenicity avian influenza (HPAI), including the H5N1 strain responsible for recent outbreaks, is far more severe. It causes the dramatic swelling, discoloration, neurological symptoms, and high mortality described above. In studies of H5N1 strains, nearly all experimentally infected chickens died, with death occurring in some cases within days of exposure. Some older strains took up to eight days on average, but more recent highly pathogenic strains can kill faster.
The critical concern with LPAI is that these milder viruses can mutate into high-pathogenicity forms over time, which is why even subtle flock-wide changes in egg production or energy levels warrant attention.
Why It’s Hard to Diagnose by Sight Alone
Several other poultry diseases look almost identical to bird flu. Newcastle disease, one of the most common mimics, causes the same sudden deaths, respiratory distress, and neurological symptoms. Infectious bronchitis, infectious laryngotracheitis, and fowl cholera can also produce overlapping signs like coughing, nasal discharge, and drops in egg production. Veterinary manuals are explicit that the visible signs of these diseases are not distinct enough to tell apart without laboratory testing.
This means that if your chickens are showing the symptoms described here, particularly swollen purple combs, sudden deaths, or rapid drops in egg production combined with respiratory distress, the only way to confirm bird flu is through testing. In the U.S., state veterinary labs can run these tests, and suspected HPAI cases should be reported to your state veterinarian or USDA. The visual signs tell you something is seriously wrong, but confirming the specific cause requires a swab and lab analysis.