BLACKFLY-INDUCED MORTALITY OF NESTLING RED-TAILED HAWKS

We documented blackfly infestations (Simulium canonicolum) at 42 Red-t ailed Hawk (Buteo jamaicensis) nests in Wyoming. Blackflies caused mor tality at 6 of 42 (14%) nests where young hatched (13 of 87 nestlings) and were the only known cause of nestling mortality. The onset of inf estations occurred when nestlings were 3 to 20 days old and usually la sted until nestlings died or fledged. Age of nestlings at mortality ra nged from 9 to 43 days. Levels of blackfly infestation were highly var iable among nests and were affected by weather. The cumulative effects of infestations on nestlings, including physical harassment, Leucocyt ozoon (a blood protozoan transmitted by blackflies) infection, and dir ect loss of blood and body fluids from biting flies, apparently result ed in mortalities through sustained physiological damage, trauma assoc iated with early nest departure, or both. Because blackfly infestation s can be detected only at close range, are ephemeral at nests, and can cause mortality of nestlings over a wide range of ages, the presence of blackflies and their influence on reproduction probably are undetec ted during most raptor productivity surveys.