 birds reproduce within electromagnetic fields ( emfs ) from transmission lines . 
 melatonin influences physiologic and behavioral processes that are critical to survival , and melatonin has been equivocally suppressed by emfs in mammalian species . 
 we examined whether emfs affect photophasic plasma melatonin in reproducing adult and fledgling american kestrels ( falco sparverius ) , and whether melatonin was correlated with body mass to explain previously reported results . 
 captive kestrel pairs were bred under control or emf conditions for one ( short - term ) or two ( long - term ) breeding seasons . 
 emf exposure had an overall effect on plasma melatonin in male kestrels , with plasma levels suppressed at 42 days and elevated at 70 days of emf exposure . 
 the similarity in melatonin levels between emf males at 42 days and controls at 70 days suggests a seasonal phase - shift of the melatonin profile caused by emf exposure . 
 melatonin was also suppressed in long - term fledglings , but not in short - term fledglings or adult females . 
 melatonin levels in adult males were higher than in adult females , possibly explaining the sexually dimorphic response to emfs . 
 melatonin and body mass were not associated in american kestrels . 
 it is likely that the results are relevant to wild raptors nesting within emfs.imagesfigure 1 
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