Whooping Cranes are highly endangered. To improve their recovery chances, a new migratory population was reintroduced into the wild in 2001, but their hatching success has been very low. A new study examines three possible hypotheses for this failure: harassment by black flies of nesting birds, effects of captive rearing, and inexperience of breeding birds. The overwhelming finding was that black fly harassment is the cause of poor hatching success: for example, when black fly numbers were reduced experimentally, breeding success doubled. In addition, as black fly numbers went up, especially of the Loon Blackfly (Simulium annulus), survival of chicks declined strongly. The other two hypotheses (effects of captivity and breeding inexperience) only weakly explained patterns in survival. Although black fly harassment was pinpointed as the key issue affecting hatching, other factors such as inexperience and the rearing environment still are suppressing the recovery of Whooping Cranes, so that black fly mitigation alone is insufficient for the population to grow at hope-for rates. The new paper was led by Jeb Barzen of the International Crane Foundation and coauthors included SAFS professor Sarah Converse. It appears in the journal The Condor: Ornithological Applications.