Does contemporary premature feather loss among common tern chicks in Lake Ontario reflect persistent pollutants, enigmatic diseases or novel pathogens?

We observed premature feather loss (PFL) among common terns Sterna hirundo at a small colony in northern Lake Ontario, Canada in July 2014. This condition is characterized by affected chicks losing all their wing, tail, head and body feathers several weeks after hatching. Rarely observed in wild birds, to our knowledge PFL in terns has not been recorded since 1974 (despite the banding of tens of thousands of tern chicks across North America since then). In July 2014, we observed PFL in chicks at between 2 and 4 weeks of age. The extent of feather loss was more extreme than in previous reports but was not accompanied by other aberrant developmental or physical deformities. Complete feather loss occurred over a period of a few days but all affected chicks quickly began to grow replacement feathers and all but one most likely fledged 10-20 days after normal fledging age. Feather samples, both shed feathers and re-growing live feathers, were collected from both affected chicks and normal individuals. One subsequently dead PFL chick was collected. Samples are awaiting further analysis. There was striking temporal association between the onset of PFL and persistent strong southwesterly winds that caused extensive mixing of near-shore, surface water with cool, deep lake waters. To our current knowledge it seems most probable that the PFL we observed in 2014 was caused by pathogens (viruses, bacteria, algal toxins) welling up from these deep waters along the shoreline but a direct link has not yet been made. The re-emergence of PFL in common terns may indicate acute health risks for birds and other wildlife in the Lake Ontario region and may also have potential for human health risks.