A test of the Skutch hypothesis: Does activity at nests increase nest predation risk?

Small clutches in tropical species of passerine birds may be a response to nest predation. The Skutch hypothesis predicts that nest predation increases with activity at nests, and activity at nests is assumed to increase with clutch size. This suggests that predation should be greatest when activity is greatest, all else being equal. If true, then nest predation should be greatest when adult birds are feeding nestlings rather than incubating eggs. This hypothesis was examined with a neotropical understory passerine bird, the Western Slaty Antshrike Thamnophilus atrinucha. Nest predation was the only important cause of nest failure. Mayfield daily survival rates were compared during the incubation and nestling stages. Activity at nests was observed and compared during those two stages. Anecdotally, we also report results of an attempt to identify predators and whether they fit the Skutch hypothesis. Activity, measured as adult arrival and departure frequency at nests, is greatest during the nestling stage. Nest predation rates, however, are equal during both stages of nesting. Activity is not associated with predation, and Skutch's hypothesis is not supported. Snakes, the predator considered by Skutch to be most important, may not be. Of 12 species of snakes found on or near the study area, one species ate eggs, and several ate young birds. Yet predation on nestlings is not greater than on eggs. On the other hand, small marsupial mammals are common and, as suggested by captures and discoveries in nests, may be the important predators. They are nocturnal so diurnal bird activity should not influence predation. Small arboreal marsupials and rodents are common throughout the neotropics. The research presented here, and the preliminary data describing potential nest predators, do not support Skutch's idea that predators are attracted to activity at nests.