Onset of incubation was studied in three Hermit Thrushes (Catharus guttatus) and one American Robin (Turdus migratorius), all with four-egg clutches, at a high altitude site in the Sierra Nevada, California, USA. Behavior of laying females at the nest was measured from continuous recordings of internal egg temperatures of first-laid eggs. Full nocturnal nest attentiveness began immediately with the first egg. Daytime attentiveness increased steadily during laying as foraging time away from the nest decreased. On-off bouts by tending females in daytime increased in frequency and decreased in duration until the last egg was laid. Time on the nest could not be directly equated to occurrence of incubation because eggs were not uniformly warmed to exceed the temperature threshold required for embryonic development (physiological zero). Incubation began, both day and night, after laying of the second egg. It increased steadily thereafter with percentage of daytime devoted to incubation lagging well behind that of nighttime. Nest attentiveness and incubation temperatures reached maxima about the time of clutch completion and were continued during following days.