Determination of breeding success of birds requires a variable amount of di
sturbance by human observers. The effects of human disturbance during resea
rch activities were assessed in a colony of South American Tems (Sterna hir
undinacea) by comparing hatching success between visited and unvisited nest
s during incubation. Predation be a mustelid, probably the Little Grison (G
alictis cuja), was the main cause of nest failure during incubation. Hatchi
ng success and daily nest survival rates during incubation did not differ b
etween visited and unvisited nests.