Contemporary premature feather loss (PFL) among common tern chicks in Lake Ontario: the return of an enigmatic developmental anomaly

In July 2014, we observed premature feather loss (PFL) among non-sibling, common tern Sterna hirundo chicks between 2 and 4 weeks of age at Gull Island in northern Lake Ontario, Canada. Rarely observed in wild birds, to our knowledge PFL has not been recorded in terns since 1974, despite the banding of tens of thousands of tern chicks across North America since then. The prevalence (5% of chicks) and 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 from all body areas (wing, tail, head and body) occurred over a period of a few days but all affected chicks appeared vigorous and quickly began to grow replacement feathers. All but one (recovered dead and submitted for post-mortem) most likely fledged 10-20 days after normal fledging age. Secondary covert feather samples were collected from PFL chicks (n=6; including shed feathers and re-growing live feathers) and normal individuals (n=8; plucked live feathers) and were analyzed for corticosterone concentrations. 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. We found no evidence of feather dystrophy, concurrent developmental abnormalities or nutritional shortfall among affected chicks. Thus, the PFL we observed among common terns in 2014 was largely of unknown origin but may have been caused by unidentified pathogens or toxins welling up from these deep waters along the shoreline. PFL was not observed among common terns at Gull Island in 2015, although we did observe similar feather loss in a herring gull Larus argentatus chick in that year. Comparison with sporadic records of PFL in other seabirds suggests that PFL may be a rare, but non-specific response to a range of potential stressors. Its reemergence in penguins, and now gulls and terns, may indicate widespread environmental changes that could lead to health risks for birds and other wildlife.