We present the first detailed information on egg laying, egg temperature (T_egg), and development of attentiveness and incubation in Western Bluebirds (Sialia mexicana). We used miniature infrared CCD video cameras to record egg laying at three nest boxes and validate attentiveness measurements derived from T_egg in seven other nests. Females entered the nest box on egg-laying days between 0522 and 1037 hrs PST and laid an egg on average 11.4 ± 4.3 min later (n  =  11). Warm weather early in egg laying often elevated T_egg above physiological zero (27° C), despite low parental attendance. Females often began roosting in the nest box 2–3 nights before clutch completion, and most birds began steady night-time incubation with the penultimate egg. Attentiveness, mean T_egg, and number of minutes T_egg exceeded 27° C reached their highest values 1 day or night after clutch completion. Western Bluebirds exhibit gradual onset to incubation that is attributable to vagaries of weather and varying patterns of parental attentiveness.